The Acidic Environment
Contextual OutlineAcidic and basic environments exist
everywhere. The human body has a slightly acidic skin surface to
assist in disease control and digestion occurs in both acidic and
basic environments to assist the breakdown of the biopolymers
constituting food. Indeed, microorganisms found in the digestive
system are well adapted to acidic or basic environments. Many
industries use acidic and basic compounds for a wide range of
purposes and these compounds are found in daily use within the
home. Because of this, an awareness of the properties of acids and
bases is important for safe handling of materials. Currently,
concerns exist about the increased release of acidic and basic
substances into the environment and the impact of these substances
on the environment and the organisms within those environments.
This module increases students understanding of the history, nature
and practice of chemistry, the applications and uses of chemistry
and implications of chemistry for society and the environment.
1. Indicators were identified with the observation that the
colour of some flowers depends on soil compositionStudents learn
to: classify common substances as acidic, basic or neutralAcids
Change colour of indicator, litmus to red Are electrolytes Sour
taste React with reactive metals (H2 + salt) React with carbonates
and hydrogencarbonates (CO2, H2O, Salt) React with metal oxides
(H2O, salt) React with metal hydroxides (H2O, salt)Bases Change
colour of indicators, litmus to blue Are electrolytes React with
acids (H2O, Salt) React with amphoteric metals (H2) Dissolve
amphoteric metal hydroxidesAcidNeutralBase
HCl, used to clean bricksH2ONaOH, soap, polymers, drain
cleaners
H2SO4, fertilisers, car batteriesCONH3, fertilisers, household
cleaner
HNO3, pesticidesCa(OH)2, plaster and cement
CH3COOH, vinegarMg(OH)2, antacid
Citric Acid, fruits
Lactic Acid, milk and muscle tissue
identify that indicators such a litmus, phenolphthalein, methyl
orange and bromothymol blue can be used to determine the acidic or
basic nature of a material over a range and that the range is
identified by change in indicator colourIndicates such as litmus,
phenolphthalein, methyl orange and bromothymol blue can be used to
determine the acidic or basic nature of a material. All indicators
have their own range that is suitable to test. The range is
identified by change in indicator colour. No single indicator can
cover the entire 0-14 range.Universal indicator is a mixture of
indicators.
identify and describe some everyday uses of indicators including
the testing of soil acidity/basicityIndicators are used to test the
pH of soil, affecting growth of plants and can change the colour of
some flowers. It is also used for fish tanks and swimming pools,
where the pH of the water is important.
Students: perform a first-hand investigation to prepare and test
a natural indicator identify data and choose resources to further
information about the colour changes of a range of indicators solve
problems by applying information about the colour changes of
indicators to classify some household substances as acidic, neutral
or basic
2. While we usually think of the air around us as neutral, the
atmosphere naturally contains acidic oxides of carbon, nitrogen and
sulfur. The concentrations of these acidic oxides have been
increasing since the Industrial RevolutionStudents learn to:
identify oxides of non-metals which act as acids and describe the
conditions under which they act as acidsAcidic oxides react with
water to form an acid. Eg. CO2 mildly acidic, SO2 and NO2, Most
non-metal oxides except for CO, NO and N2O which are neutral.
analyse the position of these non-metals in the Periodic Table and
outline the relationship between position of elements in the
Periodic Table and acidity/basicity of oxidesThe metals in group I
& II form basic oxides. The basicity of these oxides increases
down the group. Most non-metals other than the noble gasses form
acidic oxides. The acidity of the oxide decreases down the group as
the elements become more metallic in character. define Le
Chateliers principleIf a chemical system at equilibrium experiences
a change in concentration, temperature, volume, or partial
pressure, then the equilibrium shifts to counteract the imposed
change and a new equilibrium is established.Remember for
equilibrium to be achieved, the system must be closed. This means
chemicals are not being added or removed.At equilibrium, there is
no change in macroscopic properties such as colour, temperature,
pressure of gases concentration of reactants and products. The only
movement is at the microscopic level and the movement of reactants
to products is equal to the movement of products to reactants.
identify factors which can affect the equilibrium in a reversible
reactionAfter time=0, the chemical reaction takes place, the conc
of reactants decreases and the conc of product increases with time.
After 4 hours, the reaction reached equilibrium. At equilibrium,
the rate of the forward reaction is the same as the rate of the
reverse reaction so the concentration of reactants and products
stay the same.Adding a CatalystThe catalyst speeds up both forward
and reverse reactions, allowing the reaction to reach equilibrium
faster. The position of the equilibrium doesnt change. At
equilibrium, the conc of reactants and products are the same as for
uncatalysed reaction.Changing the TemperatureIncreasing the
temperature of a reaction mixture speeds up both the forward and
reverse reactions.Exothermic(H2O(g) H2O(l) + energy)In exothermic
reaction, heat is product. Increasing the temperature of an
exothermic reaction favours the formation of reactants. The
equilibrium conc of reactants will be higher at higher temperature
and the equilibrium conce of products will be lower at higher
temperature.Endothermic(energy + H2O(l) = H2O(g))In endothermic
reaction, heat is reactant. Increasing the temperature of an
endothermic reaction favours the formation of products. The
equilibrium conc of products will be higher at higher temperature
and the equilibrium conc of reactants will be lower at higher
temperature.Change in Concentration(HCl + Mg -> MgCl2 + H2, 2H+
+ Mg Mg2+ + H2)When reactants are added, there would be an
instantaneous increase in the reactants followed by a readjustment
as more products are formed according to Le Chateliers Principle. A
new equilibrium position is reached where the conc of reactants and
conc of products are more than initially.When pressure or volume is
changed in gases, it reaches a new equilibrium position. When the
pressure is increased, the reaction moves to the side with less
moles and vice versa. If the no. of moles is the same, then the
equilibrium position remains the same. When the volume of the
system is halved, the concentration increases and a new position is
reached where both of conc are greater. When the volume is doubled,
the conc decreases by half and new equilibrium is reached that is
less than the previous one. describe the solubility of carbon
dioxide in water under various conditions as an equilibrium process
and explain in terms of Le Chateliers principleCO2 CO2(aq) + H2O(l)
H2CO3(aq) H+(aq) + HCO3-(aq)[exothermic]CO2 is acidic oxide,
dissolves in water to produce H2CO3. Soft drinks (carbonated) are
produced by dissolving CO2 in water at 400 500kpa. The water is
supersaturated. When can/bottle is sealed, system stays that way at
high CO2 gas pressure. At high pressure, equilibrium moves to the
right so more is dissolved, increasing acidity. When
uncapped/opened, the equilibrium moves to the left and CO2 comes
out of solution, the acidity decreases as less CO2 is dissolved.
When adding another acid into the system, the H+ ions of the new
acid increases the concentration of H+ ions in solution, pushing
the equilibrium left. Therefore, if acid added, then CO2 comes out
of solution ie. Goes flat faster.When the temperature is increased,
as it is an exothermic reaction, the equilibrium moves to the left
according to Le Chateliers principle to reduce the effect.
Therefore, as temperature is increased, more gas comes out of
solution. Similarly, if the system is cool, then more CO2 is
dissolved in solution. identify natural and industrial sources of
sulfur dioxide and oxides of nitrogenSulfur DioxideNatural sources
2/3 of SO2 come from natural sources such as volcanoes and hot
geothermal springs. Bacteria may decompose organic matter to
produce H2S which is oxidises to SO2.Industrial burning of fossil
fuels and extraction of metals from sulphide ores (smelting)Oxides
of Nitrogen3 main types: Nitrous oxide NO2Nitric oxide NONitrogen
dioxide NO2Natural sources lightning N2 (air)> NO2. Combustion
in high temperatures bushfires.Industrial sources power stations
(combustion and high temperature), cars, trucks. describe, using
equations, examples of chemical reactions which release sulfur
dioxide and chemical reactions which release oxides of
nitrogenSulfur DioxideBurning of coal for electricity. Coal
contains sulfur mainly as metallic sulfides (eg. FeS2). When burnt,
the sulfur combines with oxygen in the air to form SO2. Some coals
have a higher sulfur content than others. Australia has a lower
sulfur coal.S in compounds + O2 (g) -> SO2 (g)4FeS2 (s) + 11O2
(g) -> 2Fe2O3 (s) + 8SO2 (g)Crude oil and natural gas also
contain sulfur compounds most of which are extracted at the
refinery and used to produce sulphuric acid. However, some SO2 is
also produced when crude oil products and natural gas are
burnt.Metal sulfides are found naturally and used to extract
metals. The metal is roasted in air. This produces SO2 and the
metal or metal oxide. 2ZnS (s) + 3O2 (g) -> 2ZnO (s) + 2SO2
(g)Bacterial decomposition also produces H2S (rotten egg gas). This
oxidises to SO2.2H2S (g) + 3O2 (g) -> 2SO2 (g) + 2H2O(g)
Oxides of Nitrogen (NOx) nitric oxide (NO) a colourless gas, a
neutral oxide - nitrogen dioxide (NO2) a brown gas, an acidic
oxideLightning at very high temperature generated by lightning,
atmospheric oxygen and nitrogen react to produce nitric oxide which
then reacts slowly with more atmospheric oxygen to produce nitrogen
dioxide.N2 (g) + O2 (g) heat lightning-> 2NO(g)2NO(g) + O2 (g)
-> 2NO2 (g)Also in industries where high temperatures are
generated. About 86% of total NOx come from vehicle exhausts. NO is
formed when nitrogen and oxygen react at high temperatures. NO2 is
produced by the oxidation of NO. The rate of NO2 formation depends
on the concentration of NO in the air. Up to 10% of total NOx is
NO2.NOx is also derived from other sources, including industries,
electrical power production and oil refining. Nitrous oxide is
produced by certain bacteria. The amount of nitrous oxide produced
in this way has increased as nitrogenous fertilizers provide more
nitrogen. assess the evidence which indicates increases in
atmospheric concentration of oxides of sulphur and nitrogenNote
that in Australia NOx and SOx have remained fairly constant over
the past decade. It has also stabilised in Europe but is increasing
in Asia particularly China due to rapid
industrialisation.EvidenceAssess
The effects of acid rain, erosion of marble and limestone
buildings, forests and aquatic organisms. This has increased in
recent times.This is indirect and qualitative evidence. It is a
good indicator of levels of NOx and SOx in the air.
Increase of acidity in lakesNot good evidence. Indirect but
quantitative. Might be from other sources like soil, surface runoff
or organisms in the lake, not necessary NOx and SOx.
Higher atmospheric concentrations of SO2 and NOx measuredSolid,
direct, quantitative. Only started measuring in 1970s, nothing else
to compare this evidence to. Not good indicator of increased
levels.
Now find higher atmospheric concentrations of SO2 and NOx in
industrial areas.Direct and quantitative. But comparison not good
for increased levels over time. Difference between industrialised
areas and other areas not difference between time frames.
Heavy acidic smogs London 1952, Los Angeles and Tokyo in
1960sVisual pollution, able to be seen. Good evidence, direct and
quantitative. Smog wasnt there before, good comparison to times
past.
Overall assess:Up until the 1970s, the evidence are all indirect
and mostly qualitative. After the 1970s, we start directly
measuring the concentration of SOx and NOx, this is quantitative.
However, these new quantitative measurements cannot be compared to
the qualitative evidence, so the levels of NOx and SOx in the air
cannot be certain. calculate volumes of gases given masses of some
substances in reactions, and calculate masses of substances given
gaseous volumes, in reactions involving gases at 0C and 100kPa or
25C and 100kPa explain the formation and effects of acid rainRain
or precipitation that is more acidic than normal rain is referred
to as acid rain. Normal rain is slightly acidic because of carbonic
acid in the rain, formed with the CO2 in the air. Primarily,
moisture in the air react with SO2 and NOx to form H2SO4 and HNO3
acids. The presence of SO2 and NOx in the air can be caused by
natural events such as volcanic eruptions, forest fires, lightning
bolts and bacterial decomposition. Industries also contribute to
this, including power-generating plants, ore smelting, motor
vehicles and industrial furnaces.Acid rain can dissolve valuables
minerals in the soil. Severe acid rain can damage the protective
layer on leaves. Aquatic organisms are affected by the increased
acidity in their habitats. Acid rain can also damage man-made
buildings and monuments. The food chain can also be affected by
bioaccumulation of heavy metals washed into the river by acid rain.
Acidic water can dissolve metal piping.
Students: identify data, plan and perform a first-hand
investigation to decarbonate soft drink and gather data to measure
the mass changes involved and calculate the volume of gas released
at 25C and 100kPa analyse information from secondary sources to
summarise the industrial origins of sulphur dioxide and oxides of
nitrogen and evaluate reasons for concern about their release into
the environmentSome industrial sources of SO2 include the burning
of coal and natural gas, the refining of crude oil and smelting.
NO2 is generated in power stations and car engines, where high
temperature and combustion occurs. SO2 contribute greatly to the
quality of air. Excessive SO2 in the air can cause respiratory
problems. People with asthma can experience symptoms after exposed
to SO2 after 10 minutes. In others, SO2 can cause the inflammation
of the respiratory tract, induce coughing and muscus. Cardiac
problems can also be related to increased levels of SO2. These
problems are evident in that on days where the concentration of SO2
in the air is high, more people are admitted to hospitals for
cardiac and respiratory problems than days with low
concentrations.At a concentration of 200 g/m3, NO2 is a toxic gas.
In asthmatic children, high and constant exposure to NO2 is
believed to lead directly to increased chances of asthma attacks
and bronchitis. Also in children, excessive exposure to NO2 can
contribute to reduced lung function or the lung not developing
fully. SO2 and NO2 in the air can also react with the water
particles in the air to form sulfuric acid, which can precipitate
in the form of acid rain. Acid rain can cause damage to buildings,
water bodies with changed pH and increase corrosion in places. It
is damaging to the environment as the wax protective layers on
leaves of plants can be corroded away by acid rain, and also causes
deforestation. Changed pH in water bodies can pollute our water
sources and affect aquatic life. Small pH changes can cause
organism migration and large changes can cause death and
abnormalities in some organisms. Acid rain can also change soil
acidity, which affects plant growth. Important human monuments can
be corroded, especially sandstone structures.The release of SO2 and
NO2 into our environment is a large problem and raises concern. Its
effects range from affecting the human health, destroying natural
habitats, cause organism death and corrode important structures in
the world.
3. Acids occur in many foods, drinks and even within our
stomachsStudents learn to: define acids as proton donors and
describe the ionisation of acids in waterH atom contains 1 proton
and 1 electron. H+ is just a proton. A proton and a hydrogen ion
are the same and can be represented by H+. An acid is then a proton
donor. When acid molecule contacts water, it can ionise and donate
a proton to the water molecule.HCl(g) + H2O(l) -> H3O3(aq)
+Cl-(aq)Free H+ does not exist in aqueous solution, it forms
hydronium ion H3O+.When an acid molecule is placed in water, it can
ionise, releasing a proton and forming a negative ion. The proton,
H+, can attach to a water molecule, H2O, forming what is called a
hydrated hydrogen ion or hydronium ion, H3O+.Note base is a proton
acceptor. identify acids including acetic (ethanoic), citric (2-
hydroxypropane-1,2,3- tricarboxylic), hydrochloric and sulfuric
acidAcetic (ethanoic)
Citric (2- hydroxypropane-1,2,3- tricarboxylic)
describe the use of the pH scale in comparing acids and basesThe
pH scale is used to compare the concentration is H+ ions in
solutions of acids and bases. The pH scale normally extends from
0-14, where 0 is acidic, 7 is neutral and 14 is basic.If a strong
acid has a concentration greater than 1 mol/L, the pH is less than
zero. Similarly, a strong base solution with a concentration
greater than 1 mol/L has a pH greater than 14. Most solutions lie
between pH 0 to 14. There are several methods to estimate pH
including litmus paper, universal indicator (combination of
indicators) which is observed through colour-change. More
precisely, a pH metre is used or a data logger with a pH probe.
describe acids and their solutions with the appropriate use of the
terms strong, weak, concentrated and diluteConcentrated and dilute
acidsThese terms describe the amount of acid dissolved in the
solution.Concentrated acid an acid solution that has a high
concentration of solute and low solvent.Dilute acid an acid
solution that has a low concentration of solute and high
solvent.Strong and Weak AcidsThese terms describe the degree of
ionisation of the acid molecules.Strong acid an acid that is
completely ionised.Weak acid an acid that is not completely
ionised. identify pH as -log10 [H+] and explain that a change in pH
of 1 means a ten-fold change in [H+]pH = -log
[H+]pH01234567891011121314
H+10010-110-210-310-410-510-610-710-810-910-1010-1110-1210-1310-14
A change in pH of 1 means a change in H+ by a factor of 10.
compare the relative strengths of equal concentrations of citric,
acetic and hydrochloric acids and explain in terms of the degree of
ionisation of their moleculesBy comparing the hydrogen ion
concentration or the pH of acidic solutions of the same
concentration of the degree of ionisation can be calculated. Eg.
Acids of equal concentration (0.01molL-1)NameFormula[H+]pH
Hydrochloric AcidHCl0.0102.00
Ethanoic AcidCH3COOH4.1710-143.38
Citric AcidHOOCCH2COHCOOHCH2COOH2.7410-32.56
HCl has the greatest ion concentration and the greatest degree
of ionisation. It is the strongest acid. HCl and CH3COOH are
monoprotic while citric acid is triprotic. describe the difference
between a strong and a weak acid in terms of an equilibrium between
the intact molecule and its ionsA strong acid is one that ionises
completely. That is, the equilibrium lies to the right. The ions
are present in a much higher concentration than the intact
molecule. Eg. HCl, HBr, HNO3.A weak acid does not ionise
completely. Equilibrium lies to the left. Ions at a lower
concentration than the intact molecule. Eg. CH3COOH, citric acid,
H2CO3, H2S
Students: solve problems and perform a firsthand investigation
to use pH meters/probes and indicators to distinguish between
acidic, basic and neutral chemicals plan and perform a first-hand
investigation to measure the pH of identical concentrations of
strong and weak acids gather and process information from secondary
sources to write ionic equations to represent the ionisation of
acidsHydrobromicHBr(l) + H2O(l) -> H3O+(aq) + Br
(aq)NitricHNO3(l) + H2O(l) -> H3O+(aq) + NO3 (aq)
SulfuricH2SO4(aq) + H2O(l) -> H3O+(aq) + H+(aq) + SO42(aq)H3O+ +
HSO4 + H2O -> 2H3O+ + SO42Sulfuric acid is a diprotic acid
because each molecule can release up to two
protons.EthanoicCH3COOH(l) + H2O(l) -> H3O+(aq) +
CH3COO(aq)Phosphoric acid is called a triprotic acid because each
molecule can release up to three protons.H3PO4 + 3H2O -> 3H3O+ +
PO43 use available evidence to model the molecular nature of acids
and simulate the ionisation of strong and weak acidsModel kits and
pen caps. gather and process information from secondary sources to
explain the use of acids as food additivesAcids used to improve
taste, make the food sour, drinks and sweetsPreserve food bacteria
cant survive in acidic environment eg. Canned fruits and
vegetables, picklesPrevents spoilage by oxidation,
antioxidantDietary reasons-Vitamin CFood additives
NameFormulaFor
Acetic AcidCH3COOHPreserve food (pickling), flavouring
Citric AcidCH2COOHCOHCOOHCH2COOHFlavouring and preservative
(anti-oxidant)
Malic AcidCH2COOHCHOHCOOHFlavouring
Tataric AcidCHOHCOOHCHOHCOOHFlavouring, preservative,
antioxidant, rising agent
Lactic acidCH3CHOHCOOHProduction of dairy products
Phosphoric acidH3PO4Acidulation of soft drink, manufacture of
cheese
Propanoic acidCH3CH2COOHControls bacteria and mould growth
Ascorbic acid (vitamin C)CH2OHCHOH(C4H3O4)Antioxidant to prevent
spoilage, added to increase vitamin C in many foods
identify data, gather and process information from secondary
sources to identify examples of naturally occurring acids and bases
and their chemical compositionAcids
NameFormulaWhere occur
Citric acidCH2COOHCOHCOOHCH2COOHCitrus fruits, vegetables
Malic AcidCH2COOHCHOHCOOHApples, cherries
Tataric AcidCHOHCOOHCHOHCOOHGrapes and pineapples
Benzoic acidBenzene COOHPrunes, plums cranberries
Bases
NamesFormulaWhere occur
Calcium CarbonateCaCO3Limestone and marble
Calcium magnesium carbonateCaCO3MgCO3dolomite
process information from secondary sources to calculate pH of
strong acids given appropriate hydrogen ion concentrations
4. Because of the prevalence and importance of acids, they have
been used and studied for hundreds of years. Over time, the
definitions of acid and base have been refined outline the
historical development of ideas about acids including those of:
Lavoisier Davy ArrheniusScientist(s)Acid definitionBase
definitionNotesWhy the theory was modified
Lavoisier(1780s)Acids contained oxygen, which made them acidicIt
was observed that non-metal oxides reacted with water to form
acidic solutionsMetal oxides are not acidic and HCl did not have
oxygen.
Davy(1815)Acids all contained hydrogen.Bases react with metals
to form salts.Observed known acids contain H that could be replaced
by metal. 1830, 10 more acids with no H discovered.Doesnt explain
why substances without H are acidic.
ArrheniusAn acid produces H+ ions when dissolved in waterA base
produced OH- when dissolved in water Observed in electrolysis on
acids that H2 produced at cathode, therefore must contain ions.
Thought stronger acids ionised more than weak acids. This theory
explains neutralisation, the OH- and H+ forms water. Interpreted
property of acids.The theory is too restrictive and doesnt allow
for other solvent. Narrow definition of bases doesnt allow for
basic metal salts and acidic non-metal salts.Doesnt explain
amphoteric substances.
outline the Brnsted-Lowry theory of acids and basesIndependently
but simultaneously proposed that acids are proton donors and bases
are proton acceptors. Acid if it has greater tendency to give
protons than solvent. Base if has greater tendency to accept proton
than solvent.The Acidic Environment2013
10
HA + H2O => H3O+ + A-HCl(g) + H2O(l) => H3O+(aq) +
Cl-(aq)B + H2O => HB+ + OH-NH3(g) + H2O(l) => NH4+(aq) +
OH-
Includes effect of solvent, applies to non-aqueous solutions
too. Explains why some salts act as acids and others act as bases
and how some substances are amphoteric. In this theory, every acid
has a conjugate base. An acid or a base can be an ion. To be an
acid, substance must contain H atom.Eg. H2O(l) + H2O(l) =>
H3O+(aq + OH-(aq)Acidity doesnt depend on the structure of the
substance butt on its properties relative to those of the solvent
or other reactant in the solution. Hydrolysis of salts that produce
pHs different from 7 is acid/base reaction. describe the
relationship between an acid and its conjugate base and a base and
its conjugate acidConjugate means to be linked with something, so a
conjugate base is the base linked with the acid and vice versa. In
the Bronsted-Lowry theory, every acid has a conjugate base that has
the same formula but one less proton and therefore one less charge
than the acid. Every base has a conjugate acid that has one more
proton and one more change than its base. HCl + H2O => H3O+ +
Cl-NH3 + H2O NH4+ + OH-CH3COOH + H2O H3O+ + CH3COO-CO32- + H2O
HCO3- + OH- identify a range of salts which form acidic, basic or
neutral solutions and explain their acidic, neutral or basic
natureSalts are formed when acids react with bases. A salt is an
ionic compound formed from a cation other than H+ and an onion
other than OH- or O2-. The pH of aqueous salt solutions is not
always 7. Some salts form basic solutions and are called basic
salts. Other form acidic solutions and are acidic salts and those
that are neutral form neutral solutions. The Bronsted-Lowry theory
provides the explanation for this. It is because many of the anions
or cations can act as acids or bases. Hydrolysis is a
Bronsted-Lowry reaction of an ion with water to give excess H+ or
OH-. Basic Anions:Hydrolysis of basic anions:CH3COO-(aq) + H2O
CH3COOH + OH-CN- + H2O HCN + OH-HCO3- + H2O H2CO3 + OH-CO32- + H2O
HCO3- + OH-Basic anions are those that react with water to form
OH-. The presence of OH- results in basic solution. Basic anions
are from weak acids. Only small amount of basic anion reacts with
water because acid formed is stronger than water.
Acidic Anions: (typically contain H+ and are conjugate bases of
strong acids)HSO4- + H2O SO42- + H3O+HCO3- is basic as it more
readily accepts protons than donates them. It comes from a weak
acid.
No Basic Cations
Acidic Cations NH4+ + H2O NH3 + H3O+Equilibrium lies to the left
because H3O+ stronger acid than NH4+ and NH3 is stronger base than
H2O. Final solution is mildly acidic.
Generally:Strong acid + Strong base => Neutral Salt +
waterWeak acid + Strong base => Basic Salt + waterStrong acid +
Weak base => Acidic Salt + waterWeak acid + Weak base =>
Neutral salt + water identify conjugate acid/base
pairsAcidConjugate Base
HClCl-
HNO3NO3-
H2SO4HSO4-
HSO4-SO4-
NH4+NH3
CH3COOHCH3COO-
H2OOH-
BaseConjugate Acid
OH-H2O
H2OH3O+
CN-HCN
NH3NH4+
S2-HS-
CO32-HCO3-
Strong acids have weak conjugate bases. Moderately weak acids
have moderately weak conjugate bases. Weak acids have strong
conjugate bases.
Strong Acids: HCl, H2SO4, HNO3Weak Acids: CH3COOH, HNO2, H2CO3,
HFStrong Bases: NaOH, KOHWeak Bases: NH3, NH4OH identify
amphiprotic substances and construct equations to describe their
behaviour in acidic and basic solutions identify neutralisation as
a proton transfer reaction which is exothermic describe the correct
technique for conducting titrations and preparation of standard
solutionsA titration is an experimental procedure that mixes a
solution of known concentration and a solution of unknown
concentration until they neutralise. The concentration of the
second solution can then be calculated. It is a quantitative
volumetric analysis.Primary Standard SolutionRequirements: Have a
known formula and be very pure Should not absorb or lose moisture
from or to the air Soluble Relatively high formula mass to minimise
weighing errors HCl and H2SO4 unsuitable as conc. Varies from batch
to batch. NaOH unsuitable as absorbs water Na2CO3 used for basic
and Oxalic acid used for acidic1. Weigh out exact amount of solid
using electronic balance (heat to remove water)2. Place solid in a
beaker and mix with distilled water3. Pour solution into a
volumetric flask with a funnel4. Rinse beaker and funnel with
distilled water in a wash bottle so all solution goes into the
flask5. Fill the flask up to exactly 250ml6. Shake/invert the
volumetric flash so the mixture is well mixed7. Place the stopper
on the flask when storing to prevent evaporation
Titration1. Rinse a conical flask with distilled water, a bulb
pipette with primary standard and the burette with secondary
standard2. Draw out exactly 25ml of primary standard with pipette
and place in conical flask3. Add 3 drops of suitable indicator and
swirl4. Fill the burette with secondary standard and record initial
reading5. Drip secondary standard into the conical flask whilst
swirling the flask to mix6. Once the indicator changes colour, stop
the burette immediately7. Read and record the final burette
reading8. Calculate
IndicatorsStrong Acid + Strong Base use Bromothymol Blue (6.2
7.6)Strong Acid and Weak Base use Methyl Orange (3.1 4.4)Weak Acid
and Strong Base use Phenophthaline (8.3 10.0)Weak Acid and Weak
Base use Data Logger and change is too gradual to be seen
qualitatively describe the effect of buffers with reference to a
specific example in a natural systemA buffer is an aqueous solution
that resists a change in pH when a small amount of a strong acid or
strong base is added to it. It contains equal amounts of a weak
acid and its conjugate base and is made by mixing equal parts of a
weak acid and a salt of that acid or a weak base and a slat of that
base.Note Just a weak acid on its own does not make a buffer
because it will not dissociate enough to provide enough to provide
enough basic ion so it is a mixture of a weak acid and a salt of
that weak acid or a weak base and the salt of that weak.
Eg. Acetic acid and sodium acetate. pH around 4.6CH3COOH(aq) +
H2O(l) CH2COO- + H3O+ [eq1]
This is possible because the weak acid and base are at
equilibrium. When acid or H+ (H3O+) are added they react with the
weak base to form more of its conjugate acid and the equilibrium
shifts towards the weak acid right. Thus buffer resists the change
in pH by removing strong acid from the solution.When more base or
OH- are added then they react with the weak acid to form its
conjugate base and the equilibrium moves to the left. Thus buffer
has resisted the change in pH by removing strong base from the
solution.H3O+ + CH3COO- => CH3COOH(aq) + H2O(l) [eq2 acid added,
reverse of eq1]CH3COOH(aq) + OH- => CH3COO- + H2O(l) [eq3 base
added, this is forward reaction of eq1]Because acetic acid is a
weak acid, a change in the amount of acid present only producs a
very small change in the hydrogen ion concentration and therefore
there is only a small change in the pH.
Biological SystemsLiving things contain enzymes that are used to
control metabolic rates of reaction. Enzymes are pH sensitive and
so need to be at their optimum pH for metabolism. The pH in the
body is maintained by buffers.Specific Example (learn
equations)Blood is a buffered solution with a pH of 7.4 that
maintains the pH of the blood between 7.35 and 7.45.When CO2
dissolves in the blood it forms carbonic acid, which ionises to
form carbonate and H+. This happens as CO2 diffuses from the cells
into the blood.CO2 (g) + H2O(l) H2CO3 (aq)H2CO2 (aq) H+(aq) + HCO-2
(aq)If more CO2 is added then more H+ ions will form, lowering the
pH, but the equilibrium will move to the left to minimise the
change.When CO2 leave the blood at the surface of the lungs the pH
become higher but the equilibrium will move to the right to
minimise the change.Students: gather and process information from
secondary sources to trace developments in understanding and
describing acid/base reactions choose equipment and perform a
first-hand investigation to identify the pH of a range of salt
solutions perform a first-hand investigation and solve problems
using titrations and including the preparation of standard
solutions, and use available evidence to quantitatively and
qualitatively describe the reaction between selected acids and
bases perform a first-hand investigation to determine the
concentration of a domestic acidic substance using computer-based
technologiesAttached Sheet analyse information from secondary
sources to assess the use of neutralisation reactions as a safety
measure or to minimise damage in accidents or chemical spillsAcids
and bases are corrosive and if these chemicals are spilt, they must
be cleaned up quickly before damages are done to spilt surfaces.
Neutralisation is one way to minimise this damage. However,
neutralisation is an exothermic reaction that can generate a lot of
heat if a strong acid and a strong base is reacted together.
Firstly, the spill is contained and soaked up with sand or
vermiculite, it is then transported to a suitable location. For
safety reasons, acids and bases should be weak ones or diluted ones
before they are reacted together and a large amount of water needs
to be present when reacting them together. For acid spillages,
excess bases are added such as sodium carbonate:Na2CO3(s) +
2HCl(aq) => 2NaCl(aq) + H2O(l) + CO2(g)Sodium carbonate is a
good base to use for acid spills as it is cheap, easy to clean up
afterwards and does not pose a danger if used in excess. For spilt
bases, weak acids are often used such as vinegar or benzoic acid
and sometimes very dilute HCl or H2SO4NaHCO3(s) + NaOH(aq) =>
Na2CO3(aq) + H2O(l)Sodium hydrocarbonate is a very good neutraliser
for both acids and bases as it is amphiprotic. Sodium hydrogen
phosphate is also amphiprotic. Neutralisation reactions are
exothermic reactions that produce some amount of heat, therefore,
are not suitable to use for spillages onto the skin. If spilled
onto skin, flush with water.Safety precautions that use
neutralisation reactions include neutralising effluent discharges
by factories into sewers. The government has strict regulations on
the pH of effluents released into the sewer as a change of pH may
affect the bacterial breakdown in the sewers.
5. Esterification is a naturally occurring process which can be
performed in the laboratory describe the differences between the
alkanol and alkanoic acid functional groups in carbon compoundsA
functional group is an atom or group of atoms that reacts in a
characteristic way in different carbon compounds.Alkanols
(Alcohol)Alkanoic Acids
Functional GroupHydroxyl group(OH)Carboxyl Group(COOH)
General FormulaCnH2n+1OHROH where the R is the alkyl
groupCnH2n+1COOHRCOOH where R is the alkyl group
Acid/BaseNeutralNot readily lose OH or HAcidIt can donate a
proton, weak acid
SolublityOH group is polar, solubleForm hydrogen bonds in
waterCOOH is polar, soluble, H-bond
PolarityPolar due to OH groupMore polar than alkanol because of
COOH
MP and BPHigher MP or BP then similar alkane or alkene due to
H-Bonding.Note: Alkane, alkene have weak dispersion forceHigher MP
and BP than similar alkanol as it has more H-bonding.
identify the IUPAC nomenclature for describing the esters
produced by reactions of straight-chained alkanoic acids from C1 to
C8 and straight-chained primary alkanols from C1 to C8To name an
ester the alkanol name with the ending changed from anol to yl then
the acid name with the oic changed to oate explain the difference
in melting point and boiling point caused by straight-chained
alkanoic acid and straight-chained primary alkanol
structuresStraight-chained means no branches on the hydrocarbon
chainPrimary alkanol means the OH group is at the end of the
hydrocarbon chain
The high MP and BP in alkanols is due to H-boning between the O
in one molecule and the H of an OH in a nearby molecule.
The ability of the COOH group to be involved in two hydrogen
bonds gives an alkanoic acid an even higher boiling point than that
of a similar sized alkanol. Two h-bonds can occur between a pair of
alkanoic acid molecules.
identify esterification as the reaction between an acid and an
alkanol and describe, using equations, examples of esterificationAn
acid, containing the COOH group can react with an alkanol,
containing the OH group, to produce ester and water.
Eg Ethanoic acid and ethanol
The O in the water comes from the acidThe reaction is reversible
and comparable quantities of alkanol, acid, ester and water are
present at equilibrium. describe the purpose of using acid in
esterification for catalysis explain the need for refluxing during
esterificationEsterification is the reaction between an alkanol and
an alkanoic acid. (example of condensations reaction, water
condenses out) It is also an equilibrium reaction. It is a
moderately slow reaction at room temperature. To speed up the
reaction, a catalyst (concentrated H2SO4) is used and the reaction
is heated. The H2SO4 also acts as a dehydrating agent, absorbing
the water and forcing the equilibrium to the right, resulting in a
higher yield.Heating the reaction speeds up the reaction but the
volatile alcohol could escape. To prevent this, a condenser is
placed on top of the flask so that any volatile components are
cooled by the water in the condenser and runs back into the
reaction. This is refluxing. Refluxing also improves the safety of
the operation as the vapours are flammable.Because of the
volatility, the reaction is carried out in a water bath and often
on an electric hot plate. (if reflux is done correctly, there
should be no vapour escaping, dont put as answer)Boiling chips are
added for a greater surface area for the reason to boil, preventing
bumping.The flask is not stoppered to prevent a build-up of
pressure due to vapourisation. The reaction can be carried out in a
fume cupboard to prevent inhalation and safety glasses are worn.
The reaction can still take several hours to reach equilibrium.
outline some examples of the occurrence, production and uses of
estersEster3-methylbutyl ethanoate(or pentyl ethanoate)Benzyl
ethanoate
Where OccurAlso known as Banana oilNaturally in the banana
plant
Can be produced syntheticallyFound naturally in flowers
(jasmine, ylang-ylang)
Sweet aroma used in perfume, candlesSolvent in plastics, resin,
oils, lacquers
Equations(CH3)2CHCH2CH2OH + CH3COOH =>
CH3COOCH2CH2CH(CH3)2
3-methyl butan-1-ol + Ethanoic Acid => 3methylbutyl
ethanoate
[sulphuric acid is catalyst]C6H5CH2OH + CH3COOH =>
C6H5CH2OOCCH3
Benzyl Alcohol + Ethanoic Acid => Benzyl Acetate
[sulphuric acid catalyst]
UseBanana food flavouringJasmine/peach scented deodorants and
perfumes
Students: identify data, plan, select equipment and perform a
first-hand investigation to prepare an ester using reflux process
information from secondary sources to identify and describe the
uses of esters as flavours and perfumes in processed foods and
cosmeticsSee above