acids and bases

What is ionisation

Breaking down of covalent molecule to form ions where water must be present

What is an acid

Substance that ionises in water to produce Hydrogen ions (H+) when dissolved in water

Hydrogen causes acidity

Properties of acid

• sour taste

• Turns blue litmus paper red

• React with alkalis, carbonates, and reactive metals

• Dissolve in water to form solutions that conduct electricity

Why is dry hydrogen chloride not acidic

Hydrogen atom is still covalently bonded to chlorine atom

Hence no H+ ions present, no acidic properties

What is an alkali

Substance that dissolves in water to form hydroxide ions (OH-) where water must be present

Hydroxide ions case alkalinity

Properties of alkalis

• bitter

• Slippery and soapy

• Turn red litmus paper blue

• Reacts with acids

• Dissolve in water to form solutions that conduct electricity

What is the pH scale

Measure of how acidic or alkaline a solution is

The lower the pH, the higher the the concentration of hydrogen atoms

Unreactive metals

Copper, silver, gold

Reaction of acid with reactive metals

Metal + acid —> salt + hydrogen (gas)

What happens to metals atoms and hydrogen ions during reaction

Metal lose electrons to form metal ions (specify)

Hydrogen ions gain electrons to form hydrogen atoms, atoms bond to form molecules (H2)

Ionic equation for acid and metal reaction

M + 2H+ —> M*+ + H2

How to test for hydrogen gas

Place lighted splint at the mouth of test tube

If hydrogen is present, lighted splint extinguishes with pop sound

Chemical equation for hydrogen test pop sound

2H2 + O2 —>2H2O (combustion)

General equation for acid + carbonate

Acid + carbonate —> salt + caarbon dioxide + water

Ionic equation for acid + carbonate reaction

2H+ + CO3²- —> CO2 + H2O

Test for carbon dioxide

Bubble the gas into limewater

If carbon dioxide is present, white precipitate is formed in limewater

Equation for test for carbon dioxide (white precipitate is formed)

Ca(OH)2 (aq) + CO2 (g) —> Ca CO3 (s) + H2O (l)

What are bases

Any metal oxides that contain either oxide ion (O²-) or hydroxide ion (OH-)

What is neutralisation

When acid reacts with base to form salt and water

General equation for acid and alkali reaction

Acid + alkali —> salt + water

Ionic equation for acid and alkali reaction

H+ + OH- —> H2O (applies only on alkalis/ insoluble base and not insoluble bases)

Graphs for excess alkali added to acid

Graph for excess acid added to alkali

Graph for excess insoluble base added to acid

General equation for alkali reaction with ammonium salt

Ammonium salt + alkali —> salt + water + ammonia

Ionic equation for alkali react with ammonium salt

NH4+ + OH- —> NH3 + H2O

How to determine pH of soil

1. Add water to dissolve soil

2. Filter to obtain filtrate

3. Add universal indicator and compare colour against chart ? Use pH meter

How to reduce excess acidity in soil

• add lime ( calcium oxide or calcium hydroxide or calcium carbonate) to the soil to react with the excess H+ present in the soil

• This increases pH of soil

Type of fertiliser plant need

Fertiliser containing nitrogen (N), Phosphorus (P), potassium (K)

Common fertilisers: ammonium sulfate, potassium phosphate, ammonium nitrate

Why can’t add ammonium compounds to freshly limed soil

Ammonium ions from fertiliser reacts with hydroxide ions in soil to form ammonia gas which escapes, there is loss of nitrogen nutrient

Ionic equation for ammonium salt react with alkali

NH4+ + OH- —> NH3 + H2O

Test for ammonia gas

Place moist red litmus paper at the mouth of test tube If hydrogen

If ammonia gas is present, moist red litmus paper turn blue

Why must red litmus paper be moist for ammonia gas test

So that ammonia can dissolve to produce OH- ions which is responsible for the alkalinity

What are some soluble and insoluble bases (metals)

All group 1 oxides/ hydroxides and Ca(OH)2 , Ba(OH)2

All other metal oxides are insoluble

What are some acidic non metal oxides

CO2, NO2, SO2

What are some neutral non metal oxides (do not react with acids and bases)

NO, CO, H2O

What are some amphoteric oxides (react with both acids and bases)

ZnO, Al2O3, PbO (ZAP)

Test for oxygen

Place glowing splint into test rube

If oxygen is present glowing splint relights

What is a strong acid/ alkali

Substance that ionises fully in water to produce hydrogen/ hydroxide ions when dissolved in water

Eg HCl, HNO3, H2SO4/ NaOH, KOH

What is a weak acid/ alkali

Substance which ionises partially to produce hydrogen/ hydroxide ions when dissolved in water

Eg ethanoic acid/ aqueous ammonia

Simple test to distinguish between strong and weak acids

Strong acid

• universal indicator greeen Ui turns red

• pH meter 1-2

• Electrical conductivity higher current (more mobile ions)

  Weak acids

• Green UI turns yellow/ orange

• pH meter 4- 6

• Electrical conductivity lower current ( less mobile ions)

Experiments to distinguish between strong and weak acids

• add reactive metal or metal carbonate to equal volumes to strong and weak acids of equal concentration

• Measure volume of gas produced in a fixed time

• Strong acid produce larger volume of gas than weak acid in the same time

What to consider when collecting gases

1. Solubility of gas on water

2. Density of gas compared to air (to compare density of air, compare Mr of the gas with Mr of air which is about 30)

How to collect gas slightly insoluble/ insoluble in water

Downward displacement of water method

How to collect gas less dense than air

Upward delivery method

How to collect gas more dense than air

Downward delivery method

How to measure volume of gas collected

How to dry acidic gases

• concentrated sulfuric acid

• Fused calcium chloride

How to dry alkali gas

• calcium oxide

• Fused calcium chloride

What is relative molecular mass (Mr)

Tells us how heavy/ light a substance is

What is relative atomic mass (Ar)

Refers to mass of single atom (Ar can be found in periodic table bottom number)

How to calculate Mr

Add up all the Ar of each atom present