8 — acids and bases

Question 1

Acid

Answer 1

An acid is a substance that dissociates/ionises in water to produce Hydrogen ions, H+

Question 2

Why acids cannot be dissolved in organic solvents but can dissolve in water

Answer 2

When acid molecules r dissolved in organic solvents, there is no dissociation to give H+ ions.
Acid molecules dissociates/ionises in water to produce H+ ions. Properties of acid r due to the present of H+ ions in acid.

Question 3

Properties of acid

Answer 3

Sour taste
Produces ions when dissolved in water hence able to conduct electricity due to the presence of mobile ions
Turns blue litmus paper red

Question 4

Acid reactions

Answer 4

acid + reactive metal/carbonates/bases

Question 5

Acid w reactive metals

Answer 5

Acid + reactive metals -> salt + H2 (g)

Effervescence observed, metal dissolves.
Confirmation test:
1. Use a lighted splint
2. In the presence of H2 gas, lighted splint will extinguish w a ‘pop’ sound

Unreactive metals:
Lead
Copper
Silver
Gold

Question 6

Lead reaction w acid

Answer 6

Lead appears to react w HCl and H2SO4 but reaction stops halfway because the initial reaction betw lead and acid produces an insoluble layer of lead (II) chloride/sulfate which prevents further reaction from taking place.

Question 7

Acid reaction w bases

Answer 7

Acid + metal oxide ->. Salt + water
Acid + metal hydroxide -> salt + water

Solid base dissolves, resulting mixture feels warm to touch

Question 8

Acid reaction w carbonates

Answer 8

Acid + metal carbonate -> salt + CO2 (g) + H2O

Effervescence observed and metal carbonate dissolves.
Confirmation test:
1. Bubble gas into limewater (Ca(OH)2)
2. In the presence of CO2 (g), white precipitate (CaCO3) forms in limewater

Question 9

Neutralisation reactions

Answer 9

Water is the only product apart from the formation of a salt

Question 10

Strength of an acid

Answer 10

Strength of an acid refers to the extent of dissociation/ionisation of the acid when dissolved in water

Comparing HCl and CH3COOH of the same concentration, the no. Of H+ ions in each dm^3 of solution/concentration of H+ ions is higher in HCl than that in CH3COOH

Question 11

Strong and weak acids

Answer 11

A strong acid is an acid that completely dissociates/ionises in water t give a high concentration of hydrogen (H+) ions

Question 12

Weak acid

Answer 12

A weak acid is an acid that partially dissociates/ionises in water to give a low concentration of hydrogen (H+) ions

Question 13

Concentration

Answer 13

Concentration refers to the amount of solute dissolved in 1 dm^3 of water or solvent

strength of an acid is not affected by its concentration.

Question 14

Basicity of acids

Answer 14

Basicity of acids refer to the no. Of hydrogen ions that can be produced by the ionisation of one molecule of the acid when dissolved in water

— monobasic
— dibasic
— tribasic

Question 15

Base

Answer 15

A base is any metal oxide or metal hydroxide. They contain either the oxide (O^2-)ion of the hydroxide (OH^-) ion.

Acid + bases -> salt and water

Question 16

Alkali

Answer 16

An alkali is a soluble base that ionises/dissociates in water to produce hydroxide (OH^-) ions

Question 17

Strong and weak alkalis

Answer 17

A strong/weak alkali is an alkali that completely/partially ionises or dissociates in water to give a high/low concentration of hydroxide (OH-) ions

Question 18

Properties of alkalis

Answer 18

Bitter taste
Feels slippery and soapy
Produces ions when dissolved in water hence alkalis r able to conduct electricity
Turns red litmus paper blue

Question 19

Alkali reaction w acids

Answer 19

Acid + alkali -> salt + water

Question 20

Alkali reaction w ammonium salts

Answer 20

Alkali + ammonium salt -> salt + ammonia gas + water

Effervescence observed and pungent gas produced
Confirmation test:
1. Use a piece of moist/damp red litmus paper
2. In presence of ammonia gas, mist/damp red litmus paper turns blue

Question 21

Universal indicator

Answer 21

Universal indicator changes colour from green to _)

Strong acid: red
Weak acid: orange/yellow
Neutral: green
Weak alkali: blue
Strong alkali: purple/violet

Question 22

An ammonium chloride fertiliser was added to provide nutrients to the soil. Explain why the farmer should not add slaked lime to the soil after adding the fertiliser.

Answer 22

Calcium hydroxide in slaked lime will react w ammonium chloride to form ammonia gas, which results in the loss of nitrogen, an important nutrient for plants. The fertiliser thus will not provide nutrients to the soil.

Question 23

Controlling pH of soil

Answer 23

Plants need nitrogen, potassium and phosphorus as essential nutrients to grow.
Most plants grow best when the soil is neutral or slightly acidic.

PH of the soil may become unsuitable for plant growth due to too much fertiliser added, env pollution such as acid rain

Soil too acidic -> slaked lime
Too much slaked lime -> soil too alkaline

Question 24

Classification of oxides

Answer 24

-> basic oxides
- most metal oxides
- insoluble in water, only a few oxides eg NO and KO dissolve readily in water to form alkalis

-> amphoteric oxides
- metallic oxides that react w both acids and bases to form salt and water
- ZAP:
ZnO, Al2O3, PbO

-> acidic oxides
- non-metal oxides
- React w alkalis to form salt n water

-> neutral oxides
- insoluble in water
- show neither acidic nor basic properties
- non-metal
- CO NO H2O