3a: Acids and Alkalis - YK

Question 1

What colours do litmus paper, methyl orange, phenolphthalein, and UI go in different pH values?

Answer 1

Acid: Litmus paper - red; phenolphthalein - colourless; methyl orange - red; UI - red
Neutral: Litmus paper - purple; phenolphthalein - colourless; methyl orange - orange; UI - green
Alkali: Litmus paper - blue; phenolphthalein - pink; methyl orange - yellow; UI - purple

Question 2

Why does UI turn a different colour at different pH values?

Answer 2

It is made of a mixture of different indicators

Question 3

What ions do acids and alkalis produce?

Answer 3

Acids produce H+ ions, alkalis produce OH- ions

Question 4

What are the formulas for hydrochloric, sulfuric, and nitric acid, and sodium, potassium, and calcium hydroxide?

Answer 4

HCl, H2SO4, HNO3, NaOH, KOH, Ca(OH)2

Question 5

What is dissociation?

Answer 5

The separating of an acid or alkali into its positive and negative ions in water, e.g. hydrogen chloride gas dissolving in water to form H+ and Cl- ions, solid sodium hydroxide dissolving to form Na+ and OH- ions

Question 6

What makes an acid strong?

Answer 6

The acid completely dissociates into ions

Question 7

What is a base?

Answer 7

A substance which reacts with acids to form a salt and water (all metal oxides are bases)

Question 8

How is a soluble salt made?

Answer 8

Add excess of metal oxide base to acid, gently warm the mixture (to speed up reaction), filter to remove unreacted solid from the solution (to make sure prepared salt is pure), heat (to evaporate water and concentrate salt solution), then leave to evaporate water slowly for crystallisation to occur
E.g. tin (II) oxide with hydrochloric acid

Question 9

What is the difference between a base and an alkali?

Answer 9

Many bases are insoluble, an alkali is a base which is soluble in water (to form alkaline solutions)

Question 10

What are the general formulas for metal hydroxides in groups 1 and 2?

Answer 10

MOH (Metal hydroxide) in group 1, M(OH)2 in group 2

Question 11

What kind of ions do acids and alkalis produce?

Answer 11

Acids produce H+ ions e.g. HCl (aq) becomes H+ (aq) + Cl- (aq); alkalis produce OH- ions e.g. NaOH (aq) becomes Na+ (aq) + OH- (aq)

Question 12

How do the ions from acids and alkalis react in a neutralisation reaction?

Answer 12

H+ (aq) + OH- (aq) becomes H2O (l), anion from acid reacts with cation from alkali to form a salt

Question 13

What process is used to form dry soluble salts?

Answer 13

Crystallisation of a neutral solution (formed by mixing an acid and alkali in precise quantities through titration) heated to evaporate water

Question 14

What are the steps to make a pure, dry salt?

Answer 14

Carry out a titration and record the exact volume of acid needed to neutralise the alkali, use a burette to add the correct volume of acid to the alkali without indicator, slowly evaporate the water from the solution formed

Question 15

How is a titration carried out?

Answer 15

Acid is added from a burette with 0.1 cm cubed graduations (more precise and gradual than a pipette) to a fixed volume of alkali in a conical flask (alkali measured using pipette, more accurate than measuring cylinder), process of reaction is measured using a single indicator (e.g. phenolphthalein) so change from alkaline to neutral solution (end-point) is sharp (easy to see)

Question 16

What are the general formulas for acid reactions with metals and carbonates?

Answer 16

metal + acid –> salt + hydrogen

metal carbonate –> salt + water + carbon dioxide

Question 17

What is an ionic equation?

Answer 17

An equation for a reaction which only includes the ions that change in the reaction (ions that don’t change are spectator ions), e.g. Mg with HCl acid can be written as Mg (s) + 2H+ (aq) –> Mg2+ (aq) + H2 (g)

Question 18

What is a half equation?

Answer 18

An equation which shows the change of an ion in a reaction (oxidation or reduction), e.g. Mg with HCl acid is 2H+ (aq) + 2e –> H2 (g) and Mg (s) –> Mg2+ (aq) + 2e

Question 19

What is a precipitation reaction?

Answer 19

A reaction in which soluble substances in solutions cause an insoluble precipitate to form

Question 20

What are the solubility rules?

Answer 20

All common sodium, potassium, and ammonium salts are soluble, all nitrates are soluble, common chlorides are soluble (except silver), common sulphates are soluble (except barium and calcium), common carbonates and hydroxides are insoluble (except sodium, potassium, and ammonium)

Question 21

What are the steps to prepare a dry sample of an insoluble salt (and an example of the reaction)?

Answer 21

Mix the two solutions in a beaker; filter the mixture; pour distilled water over the precipitate in the funnel; carefully remove the filter paper with the precipitate and dry it
E.g. lead nitrate + sodium chloride –> lead chloride + sodium nitrate
Pb(NO3)2 (aq) + 2NaCl (aq) –> PbCl2 (s) + 2NaNO3 (aq)