Acids and Alkalis Year 11. CONTENTS Acidity and alkalinity Indicators pH Acids General methods for making salts Making salts from metal oxides Making.

Acids and Alkalis

Year 11

CONTENTS

• Acidity and alkalinity

• Indicators

• pH

• Acids

• General methods for making salts

• Making salts from metal oxides

• Making salts from metal carbonates

• Making salts from metals

• Making salts from alkalis – by titration

• Making insoluble salts – by precipitation

• Questions

Acids• Sour• Turn litmus red• pH less than 7• Dissolve carbonate rocks• Corrode metals • Conduct electricity

Acids• Acids are substances that give H+ ions

when added to water.• Hydrogen ion is equal to a proton.• So acid is a proton donor

• Common Acids are:• Hydrochloric acid HCl• Sulphuric acid H2SO4

• Nitric acid HNO3

• Phosphoric acid H3PO4

• Ethanoic acid CH3COOH• Carbonic acid H2CO3

Bases• A substance which neutralises an acid is called a

Base

• If a base dissolves in water, it is also called an al-kali

• So alkalis are bases that dissolve in water.

• Sodium hydroxide NaOH base and alkali• Potassium hydroxide KOH base and alkali• Ammonia solution NH4OH base and alkali• Copper(II) oxide CuO is a base• Magensium oxide MgO is a base

• Metal oxides and hydroxides are generally bases• Ammonium hydroxide is a base (alkali)

Alkalis•Bitter•Turn litmus blue•pH greater than 7•Slippery• caustic

Alkalis•An alkali is a substance that gives OH- ions when added to water

• All alkalis dissolve in water.

Neutralisation reactions

• Acids and alkalis neutralise each other to form a salt and water.• Example:

Hydrochloric acid + Sodium hydroxide Sodium chloride + Water

HCl + NaOH NaCl + H2O

• Reactions of acids and alkalis are called neutralisation reactions

• Neutralisation reaction is expressed by:

H+ + OH- H2O

Neutralization examples

• Indigestion is caused by too much amounts of hydrochloric acid in the stomach.

• It is cured by indigestion tablets.• Indigestion tablets contain

magnesium oxide that neutralises the excess HCl.

Neutralization examples

• Fields with acidic soil can be improved (by neutralisation) by adding lime.

• The lime contains calcium hydroxide Ca(OH)2 which is an alkali

Neutralization examples

• Lakes affected by acid rain can be neutralized by adding lime.

Strong vs. weak• Strong = complete dissociation

• Example, when HCl is dissolved in water, all of it becomes H+ + Cl-

• HCl, H2SO4, HNO3 are strong acids

HCl H+ + Cl-

H2SO4 2H+ + SO4 2-

• Weak = incomplete dissociation• Acetic acid (vinegar) also called Ethanoic acid is a weak

acid.• CH3COOH CH3COO- + H+

• CH3COOH, small amounts dissociate into H+ and CH3COO-

• CH3-COOH, H2CO3, HF are weak acids (Ethanoic acid, Carbonic acid, Hydroiodic acid

• More dissociation stronger acid

Strong and weak bases

• Strong bases ionises com-pletely

• NaOH Na+ + OH-• NaOH, KOH, Ba(OH)2, Ca(OH)2• Weak bases ionises partially• Mg(OH)2, CuO, Al2O3, NH4OH• NH4OH NH4+ +

OH-• Strong acids have a small

pH(1-2) • strong bases have high pH(13-

14)• Neutral solutions have pH 7• Smaller the pH, stronger the

acid• Larger the pH, stronger the

base

Indicators• Indicators are substances that change colour when placed in an acid or an al-kali.• There are four important indicators:

• Methyl orange• Phenolphthalein• Litmus paper• Universal indicator.• Universal indicator (pH indicator) is

the best as it shows the pH of the so-lution so that we can know how strong is the acid or base

• Colour

Methyl orange

Phenolphthalein

Litmus paper

Universal indicator

HCl Red Colourless Red Red

NaOH

Orange/yellow

Pink Blue Purple/violet

• <— 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 —>

• strongly weakly neutral weakly strongly

acidic acidic alkaline alkaline• • HCl CH3COOH H2O NH3 NaOH

HYDROCHLORIC ETHANOIC AMMONIA SODIUM

ACID ACID HYDROXIDE

• Acidic solutions (acidic) pH < 7• Alkaline solutions (basic) pH > 7• Neutral solutions pH = 7

Measuring pH

• Indicator Add a few drops of universal (full-range) indicator to the solution and compare the colour against a chart.

• The colour depends on how strong is the acid or base• pH meters Electrical/electronic devices which measure the

conductivity of the solution.• You place the probe in any solution and read off the pH value

on a scale or digital read-out

REACTIONS OF ACIDS• NeutralisationAcid reacts with a base to form salt and water.Base can be a metal oxide, metal carbonate or metal hydrox-ide.Ammonium hydroxide is also a base.• Calcium oxide + Nitric acid Calcium nitrate + waterCaO + HNO3 Ca(NO3)2 + H2O• Copper(II) oxide + Hydrochloric acid Copper(II)chloride +

waterCuO + HCl CuCl2 + H2O• Sodium hydroxide + Sulphuric acid Sodium sulphate +

water2NaOH + H2SO4 Na2SO4 + 2H2O• Ammonium hydroxide + hydrochloric acid Ammonium chloride + waterNH4OH + HCl NH4Cl + H2O

• Acid reacts with metals to form salt and hydrogen gas• Most metals react with dilute acids to form salt and hydro-

gen• Copper, Silver, Gold etc do not react with acids because these metals are not reactive.• Magnesium + sulfuric acid Magnesium sulfate + Hydro-

gen Mg + H2SO4 MgSO4 + H2• Zinc + Hydrochloric acid Zinc chloride + Hydrogen Zn + HCl ZnCl2 + H2Acids react with carbonate to form salt, carbon dioxide and water• Sodium carbonate + Hydrochloric acid Sodium chloride + Carbon dioxide + water Na2CO3 + 2HCl NaCl + CO2 + H2O• Copper(II) carbonate + sulphuric acid Copper(II) sulphate + Carbon dioxide + water CuCO3 + H2SO4 CuSO4 + CO2 + H2O

Salts• Salts formed from an acid and a base.• First part of a salt is from the base (+). First part of

a salt is normally a metal ion or ammonium ion (NH4+)

• Second part from the acid (-)• For example, Sodium chloride is a salt.• Sodium part comes from a base like sodium hy-

droxide.• Chloride part comes from an acid (hydrochloric

acid)• Second part of the salt depends on the acid.• Hydrochloric acid HCl - chloride Cl (-)• Sulphuric acid H2SO4 - sulphate SO4 (2-)• Nitric acid HNO3 - nitrate NO3 (-)• Carbonic acid H2CO3 - carbonate CO3 (2-)• Phosphoric acd H3PO4 - phosphate PO4 (3-)

How to make salts?

• Acid react with metal• Example: Making Zinc sulphate.• Take sulphuric acid in a beaker. (bec. the salt is

sulphate)• Add excess zinc powder and warm the mixture.• Wait until no more bubbles come out.• Cool the mixture and filter to get rid of excess Zinc.• Take the filtrate in an evaporating dish, heat and evaporate until crystallisation point. • Cool it in room temperature; filter to get the crystals

of Zinc sulphate. Dry the crystals using filter paper or in an oven.

From an acid and an alkali (both soluble)

• Example: When hydrochloric acid and potassium hydroxide solution are mixed, we can make Potassium chloride salt.• KOH (aq) + HCl (aq) KCl (aq) + H2O (l)• First conduct a titration to know what volume of acid and

base exactly required for neutralisation. It needs an indicator.• Then mix acid and base in larger volume in the same ratio

you found in the titration for complete neutralisation. This time no indicator must be used so that salt will be pure.• Heat and evaporate the resulting solution until crystallisa-

tion point. • Cool it and filter to get the crystals of potassium chloride.• Dry the crystals using filter papers or in an oven.

Double decomposition or Precipit-ation.

( make an insoluble salt)

• This method is useful to make a salt which is insol-uble in water.

• For example to make calcium carbonate salt which is

insoluble. Mix solutions of sodium carbonate and calcium chloride; both soluble and the cal-cium carbonate precipitates.Na2CO3 (aq) + CaCl2 (aq) CaCO3 (s) + 2NaCl (aq) • When two soluble salt solutions when mixed, the

insoluble salt is precipitated out.• The mixture is then filtered and the solid salt is left

for drying.

Soluble and insoluble

• Following statements help you to identify if a chemical can dissolve in water or not.

• All compounds of SODIUM, POTASSIUM and AMMONIUM are soluble in water.• All NITRATE compounds are soluble.• All CHLORIDES are soluble except Silver chloride and Lead(II) chloride.• All SULPHATES are soluble except Barium sulphate and Lead(II) sulphate.• All CARBONATES are insoluble except sodium carbonate, potassium carbonate and ammonium carbonate.• All metal oxides and metal hydroxides are insoluble in wa-

ter except sodium, potassium and calcium oxide/hydrox-ide.

Soluble or insoluble???• Sodium carbonate Na2CO3 • Soluble• Copper(II) sulphate CuSO4 • Soluble• Lead(II) chloride PbCl2 • Insoluble• Silver sulphate Ag2SO4 • Soluble• Barium chloride BaCl2 • Soluble• Ammonium sulphate (NH4)2SO4 • Soluble• Iron(III) oxide Fe2O3 • Insoluble

• Copper(II) hydroxide Cu(OH)2 • Insoluble• Silver chloride AgCl • Insoluble• Barium sulphate BaSO4 • Insoluble• Lead(II) nitrate Pb(NO3)2 • Soluble• Sulphuric acid H2SO4 • Soluble• Potassium chloride KCl • Soluble• Calcium sulphate CaSO4 • Soluble• Magnesium carbonate MgCO3 • Insoluble

• THE END