Chemistry 20 Final Review Bonding Unit Gases Unit Solutions, Acids and Bases Unit Stoichiometry Unit.

Chemistry 20 Final Review

Bonding UnitGases UnitSolutions, Acids and Bases UnitStoichiometry Unit

Bonding Unit Topics:

Lewis Structures:These are your electron diagrams for individual elements (showing valence e-)Ex.

Mg S

Intramolecular Forces:

Remember these are the forces WITHIN a moleculeWhat forces hold a molecule together?

ANS: BONDS

What type of bonds are there?ANS: COVALENT AND IONIC

Ionic Bond StructuresRemember how to draw electron dot diagrams for ionic compoundsThey DO NOT SHARE electrons – the metal looses its outer shell electrons and the non-metal gains to a full 8 e-Ex:

Covalent Bond Structures

These are the bonds holding MOLECULAR compounds togetherThey DO SHARE the electronsThese were also called Lewis StructuresThe element that goes in the middle is the one with the most BONDING e-Examples:

P

eg) PH3

H

H

H

P

HH

H

Remember double and triple bonds:

Each element except hydrogen needs 8 electrons around it and there should be NO LONE PAIRSThis is when double and triple bonds formEx. O O N N

Structural Diagrams and Shape Diagrams

When there are 2 shared electrons between two elements in a molecule draw a line to show this bond

Ex.

CH H

CH H

Remember the shapes and shape codesEx. tetrahedral, trigonal planar, pyramidal,

linear, bent

the code has two numbers: 1. the number of attached to the central atom 2. the number of on the central atom

atoms

lone pairs

eg) NH3(g)

H

H

HN

CH4

H

H H

H

C

3 - 1

pyramidal

4 - 0

tetrahedral

Code Shape Example

4 – 0

3 – 0

3 – 1

2 – 1

2 – 2

tetrahedral

trigonal planar

pyramidal

bent

bent

CH4

CH2O

NH3

HNO

H2O***all other codes arelinear

Molecules take on these shapes due to the VSEPR theory - valence shell electron pair repulsionmolecules adjust their shapes so that valence e- are as far away from each other as possible

Polar vs. NonpolarRemember electronegativities:

The number in each element box above the elementIt shows how badly an elements wants e-The higher the number, the stronger it pullsWhen two elements are bonded together and there is a difference in electroneg. then you have a polar bond• Ex. H – F (see next slide)

H – F

“arrow” points towards element with higher electronegativity (-)

“+” at the end that is +

Bond Dipole Arrows

+ -

you can use the difference in electronegativity between two atoms to determine the bond

mostly ionic

polar covalent

slightly polar covalent

non-polar covalent

3.3 1.7 0.5 0

Difference in Electronegativity

Polar vs. Nonpolar Molecules

tetrahedral: if all atoms attached have the same pull (in or out), if different atoms attached

trigonal planar: if all atoms attached have the same pull (in or out), if different atoms attached

pyramidal: as long as there is a difference in electronegativity between the atoms

bent:

linear: …look at electronegativity difference

nonpolarpolar

nonpolarpolar

polar

polar

polar or nonpolar

Examples 1. H2O 2. HCl

4. C2HI3. C2H2

O

H H

H Cl

HH C C IH C Cnp np

polarpolar

nonpolarpolar

Intermolecular ForcesThese are the forces that cause attraction BETWEEN moleculesThey are weaker then bonding within a moleculeThey are responsible for the bp and mp of compounds since when you boil/ melt a molecule you are ONLY breaking these forces BETWEEN moleculesThe three intermolecular forces we talked about the occur between MOLECULAR compounds

HB, DD, LD

DD - Dipole - DipoleThese attractions occur in POLAR molecular compouds ONLYThe slightly positive end of one molecule is attracted to the slightly negative end of another molecule

+

+

+

++

-

-

-

-

-

LD: London Dispersion ForcesThese attractive force occurs between ALL molecular compoundsIt is caused by electrons in atoms and molecules constantly being in motionSo sometimes one side of a molecule can have more electron then the other sideThis creates a temporary polar moleculeAn attraction then forms between the ends of these polar moleculesRemember you have stronger LD forces as the molecule becomes larger or has more electrons

HB: Hydrogen Bonding

These attractive forces occur in molecular compounds that H bonded to either N, O or FDraw the structural diagram of the molecular compound to make sure the H is actually bonded to the N, O or F

the hydrogen has such a low electroneg. in comparison to N, O and F so it has its electrons pulled so far away from it. This makes it able to be attracted not only to the pole

OHH

O

H H

O

HH

OH

H

but also to thelone pairs

Other melting/ boiling point’sRemember intermolecular forces only occur between molecular compounds and are weaker forces then intramolecular forces (bonds) So when melting molecular compounds only the LD, DD, and HB need to be overcomeMetals and ionic compounds are attracted to one another by the bonds holding them togetherMetallic structures and ionic compounds therefore have high bp/ mp due to having to overcome their intramolecular forces (bonding)

MP of Metals?

Metals are solid at room temp. because metal atoms have very strong forces between themI.e. metallic bondingSo in order to melt them you need to add LOTS of energy (high temp) to overcome these strong forces

metal cations

“sea” of delocalized electrons

Metallic Bond Model

Ionic Compounds

Ionic compounds are also attracted to one another by strong forces (not quite a strong as metallic though)I.e. ionic crystalsSo in order to melt them you need to add quite a bit of energy (high temp) to overcome these forces

ionic compounds have

they form so that are as as possible this is called a

crystal structure

3-D array of alternating positive and negative ions crystal lattice

Ionic Crystals

oppositely charged ionsclose together

Scale of Forces

very low

very high

Intermolecular Forces

(between)

Intramolecular Forces

(within) London Dispersion Dipole – Dipole Hydrogen Bonding

metallic ** wide range ionic

network covalent

eg) diamond, SiC, SiO2

covalent

LDDD

HB ioniccovalent

networkcovalent

Order of bp’sUsing the scale of forces you can order compounds based on their relative bp’sEx. From Highest to Lowest

Network covalent compound (ex. SiO2)Ionic compoundMolecular compound with HB, DD, LDMolecular compound with DD, LDMolecular compound with LD (if 2 molecular compounds have LD only then bigger molecule or molecule with more electrons has higher bp)

Gases Unit

Remember your formulas!!!!!

When to use what?Use Boyle’s Law when temp. is constant

P1V1 = P2V2

Use Charles’ Law when pressure is constant

V1/T1 = V2/T2

Use Combined Gas Law when all three variables change

P1V1/T1 = P2V2/T2

Use PV=nRT if given a mass or number of moles

Some othersIf you are only given info about pressure and tempurature and its in a sealed container then V1 = V2 so using Combined Gas Law cancel out the volumes to get left with:

P1V1/T1 = P2V2/T2

P1/T1 = P2/V2

Law of Combining VolumesYou can use a balanced equation and multiply by coefficients wanted/given to get the volume of one gas if you know the volume of another

What volume of oxygen is used up if 100 mL of steam is formed in a composition reaction?

O2(g) + 2H2(g) 2H2O(g)

100 mL x 1 2

x mL = 50.0 mL

x mL 100 mL

What are solving for

What you

are given

Convert 650 mmHg to kPa.

101.325 kPa = x

760 mmHg 650 mmHg x = 86.6… kPa

Ratio of known values

Ratio of what you are trying to find

Solutions, Acids and Bases

Remember your formula’s here too:

c=n/vn=m/MV1C1=VfCf

pH=-log[H30+]

pOH=-log[OH-][H30+]=10^-pH

[OH-]=10^-pOH

pH+pOH=14

Remember this is the dilution formula

Experiments

Remember in experiments there are always three variables:

Manipulated what you are changingResponding the response to the changeControlled what you keep the same

Ex: What effect does eating carrots have on eyesight?

Manipulated: amount of carrots eatenResponding: how well you can seeControlled: Same types of carrots, not eating any other food that could effect eyesight

Electrolytes?

Compounds that conduct electricity in water because they break apart into ions (ex. ionic compounds, acids)

Ex. NaCl Na+(aq) + Cl-(aq)

Molecular compounds DO NOT break down into ions so they are non-electrolytes

SolubilityThe ability to dissolveIf the solution is holding as many solutes as possible the solution is SATURATED and adding anymore solute will NOT be able to dissolveA saturated solution usually has a small amount of UNDISSOLVED solute at the bottom. This is in constant EQUILIBRIUM with the solute that is dissolved in the solution (they switch places with each other all the time)

Dissolved

UndissolvedEquilibrium

Standard SolutionRemember how to prepare a standard solutionUse formula’s n=m/M and c=n/v to get the mass you need for the certain volume and concentrationSteps:

Weigh out soluteDissolve in ~half amount of water in a beakerPour into final volume volumetric flaskFill flask, and invert to mix

DilutionWhen you have a solution that has too high of a concentration you can add water to dilute it (water it down so its not as strong)

Ex. You have 100mL of a 5.0 mol/L solution. You add 500mL of water. What is the new concentration?

Ci = 5.0 mol/L

Vi = 0.1L

Vf = 0.6L

Cf = ?

ViCi=VfCf

Solve for Cf

Dissociation and dissociation equationsThis is the same as ‘dissolving’When you have a compound and put it in water 4 situation to know:

It doesn’t dissolve• Ex. C25H52(s) C25H52(s)

It does dissolve and its ionic• Ex. Ca(OH)2(s) Ca2+(aq) + 2OH-(aq)

It does dissolve and its molecular• Ex. C12H22O11(s) C12H22O11(aq)

It does dissolve and its an acid (this is a special case because it is a molecular compound but it acts as an ionic compound)

• Ex. H2SO4(aq) 2H+(aq) + SO42-(aq)

Concentration of IonsIf asked to calculate the concentration of an ion in a solution 1st write the dissociation equation then treat it like a solution stoich. Question (no volumes are needed since they all have the same volume so you don’t need to calculate n first)

Ex. Calculate the ion concentrations when you have 0.500 mol/L H2SO4(aq) ?

g w w H2SO4(aq) 2H+(aq) + SO4

2-(aq)

c=0.500 mol/L c= ? c=?

c of H+is = 0.500 mol/L x 2/1 = 1.00 mol/L c of SO4

2- is = 0.500 mol/L x 1/1 = 0.500 mol/L

Remember your properties of acids/ bases

Acids Bases Neutral Substances

sour bitter

electrolytes electrolytes electrolytes, non-electrolytes bases acids

indicators indicators do not

H2(g)

eg)HCl(aq), H2SO4(aq)

eg)NaCl(aq), Pb(NO3)2(aq)

eg) Ba(OH)2(aq) NH3(aq)

less than 7

greater than 7

of 7

litmus - litmus - bromothymol blue - bromothymol blue

-

red blue blue yellow

taste taste

neutralize neutralize

react with react with

react with to produce

metals

phenolphthalein - phenolphthalein - colourless

pink

affect indicators the same way

pH pH pH

What is an acid?

The Arrhenius definition of an acid is that it has H+ at the beginning of the compound and is (aq)

Ex. HF(aq)

The modified Arrhenius definition of an acid is it reacts with water to form H3O+ ions

Ex. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq)

What is a base?

The Arrhenius definition of a base is that it has OH- ions at the end of an IONIC compound

Ex. NaOH(aq)

The modified Arrhenius definition of a base is that it reacts with water to form OH- ions.

Ex. Na2CO3(aq) + HOH(l) NaOH(aq) + H2CO3(aq) OH-

ions

Strong acids/ basesWeak acids and bases don’t 100% break down to form H3O+ ions and OH- ions (strong one’s DO)Strong acids are listed on the back of your periodic tableIf they are NOT on that list they are a weak acidStrong bases have OH- ions in them OR a metal with oxygen

Ex. NaOH and MgOEvery other base is a weak base

Ex. NH3 and Na2CO3

Monoprotic vs. PolyproticMonoprotic acids only have 1 H+ ion to give away (to water)

Ex. HCl, HFSimilarly monoprotic bases can only accept one H+ ion (from water) or has 1 OH- ion

Ex. NaOH, ions with only 1- charge (ex. F-)

Polyprotic acids have more than 1 H+ to give awayEx. H2SO4, H3PO4

Similarly polyprotic bases can accept more than 1 H+ ion

Ex. compound with ions that have more then 1- charge (ex. CO3

2-, PO43-)

Remember the pH scale and the pOH scale

pH scale is 0-140 = strong acid, 14 strong base

pOH is opposite0 = strong base, 14 strong acid

…and the calculations

Ex. what is pH if [H30+] = 0.05 mol/L

pH=-log[H30+]= - log[0.05]= 1.3 (remember SD – only 1 SD = 1 after decimal place on pH and pOH)

What is pOH if [H30+] is 0.90 mol/LpOH=-log[OH-]But we don’t know [OH-] pOH also = 14 – pH so lets find pH

…pH=-log[H30+]pH=-log[0.9]= 0.04575749…pOH = 14 - 0.045749…pOH = 13.95 (2 SD = 2 after decimal)

What is [OH-] if pOH = 6.7[OH-]=10^-pOH

[OH-]=10^-6.7

= 2 x 10-7 mol/L (1 after decimal = 1 SD)

Stoich!!!

Now put it all together

If dealing with MASS its gravimetric stoich…use n=m/M

If dealing with GASES its gas stoich…use n=m/M or PV=nRT (or if just volumes use volumes directly and x wanted/given)

If dealing with SOLUTIONS and CONCENTRATIONS its solution stoich…use n=m/M and c=n/v

LR vs. ERRemember that during a stoichiometric reaction there are always 2 reactants – one is being used all up during the reaction (LR) and one will have some leftovers (ER)You need to figure these out so you know how much product will be producedYou do this by calculating n of each reactant and then dividing them by their coefficient = the SMALLER # is the LRThe LR’s n (#of moles before you divided by the coefficient) is then what you use to calculate the product

Example 1  When 80.0 g copper and 25.0 g of sulphur react, which reactant is limiting and what is the maximum amount of copper(I) sulphide that can be produced?

16 Cu(s) + 1 S8(s) 8Cu2S(s)

x g M = 159.17 g/mol

m = 80.0 g M = 63.55 g/mol

n =

m = 25.0 g M = 256.56 g/mol

n/16 = 0.0786…mol n/1 = 0.0974… mol limiting excess

1.25…mol

= 0.629… mol

n = 80.0 g 63.55g/mol = 1.25… mol

n = 25.0 g 256.56g/mol = 0.0974… mol

8/16

m = (0.629…mol ) (159.17 g/mol) = 100.17… g  = 100 g

% Yield and % ErrorRemember these formulas:

Predicted = calculated amount from a stoich. calculation

Actual amount = amount you weigh after experiment

% error = actual – predicted x 100 predicted

% yield = actual x 100 predicted

% error = 93.5 g - 100 g x 100 100 g

= -6.50 %

% yield = 93.5 g x 100 100 g = 93.5 %

Calculate the % error and % yield for the following:

predicted mass of ppt = 100 gactual mass of ppt = 93.5 g

Titration'sThese where just a type of experiment in which you use solution stoichiometry and you find the volume of one of your reactants to calculate the concentration.In order to do a solution stoich. question, you need to know 3 variables in order to solve for the 4th

Ex. The 4: the c and v of one reactant, and the c and v of the other reactant (in order to solve the c of one you must know the other 3)In the question however you are only given 2 variables – the c and v of only one reactant – after the titration experiment you will have a volume to use giving you all 3

Ex. Calculate the concentration of HCl when it is

titrated into 10.0 mL of 0.50 mol/L Ca(OH)2? w g

2HCl(aq) + Ca(OH)2(aq) CaCl2(aq) + 2HOH(l)

c= x c=0.50 mol/L

v= find during v=10.0 mL titration n= cv = (0.50)(0.01) = 0.005 mol x 2/1 = 0.01

mol

c=n/v c = 0.01 mol/v Lets do the titration and see what v is.

Titration:

When doing this experiment you want to make sure you have the correct volume – so we do the experiment several times to make sure the volumes we are getting are all the same (or very close – within 0.2 mL of each other)This is why you have several trials.

Results:Trial 1 Trial 2 Trial 3 Trial 3

Initial Reading

0.0 mL 5.9 mL 11.2 mL

16.4 mL

Final Reading

5.9 mL 11.2 mL

16.4 mL

21.7 mL

Volume Used

5.9 mL 5.3 mL 5.2 mL 5.3 mL

These 3 are closest together so use them (they are within 0.2 mL of each other)

Exclude these results

5.3 + 5.2 + 5.3 = 5.3 mL

3

Ex. Calculate the concentration of HCl when it is

titrated into 10.0 mL of 0.50 mol/L NaOH? w g

2HCl(aq) + Ca(OH)2(aq) CaCl2(aq) + 2HOH(l) c= x c=0.50 mol/L

v= find during v=10.0 mL titration n= cv = (0.50)(0.01) = 0.005 mol x 2/1 = 0.01 mol c=n/v c = 0.01 mol/v Lets do the titration and see what v is… c = 0.01/0.0053 = 1.88679… = 1.9 mol/L

Lastly….

Remember titration curves and where the equivalence point is and what it means (when the acid and base fully react to create a neutral solution)

Strong Acid Titrated with Strong Base

pH

volume of titrant added (mL)

7

0

14

Endpoint

Strong Base Titrated with Strong Acid

pH

volume of titrant added (mL)

7

0

14

Endpoint