Reactions of Acids

1. Give the names of the products formed in the following two chemical reactions.
•  zinc metal added to sulfuric acid         •  iron hydroxide added to nitric acid

Answer: zinc sulfate and hydrogen gas ; iron nitrate and water

2. Name the separation process used to:

a) separate excess solid metal oxide from a solution of a soluble salt

Answer: filtration

b) form crystals of a soluble salt from a solution of that salt

Answer: crystallisation

3. Describe the differences between an acid and an alkali.

Answer: An acid releases hydrogen ions in solution whilst an alkali releases hydroxide ions in solution. Acids have a pH less than 7, alkalis have a pH greater than 7.

4. Describe how the colour of universal indicator added to hydrochloric acid will change as sodium hydroxide is added until it is in excess.

Answer: Universal indicator will start off red. As sodium hydroxide is added to the hydrochloric acid the colour will change to orange, then green, then to blue and finally to purple.

5. Describe the difference between:

a) a strong acid and a weak acid

Answer: A strong acid completely ionises when dissolved in water, but a weak acid only partially ionises when dissolved in water.

b) a dilute acid and a concentrated acid

Answer: A dilute acid has fewer acid molecules dissolved in a given volume of water, whilst a concentrated acid has more acid molecules dissolved in a given volume of water.

6. Zinc chloride is a soluble salt that can be formed by reacting solid zinc carbonate with an acid.

a) Zinc chloride contains the ions Zn2+ and Cl. Give the formula of zinc chloride.

Answer: ZnCl2

b) Give the word equation for the reaction of zinc carbonate to form zinc chloride.

Answer: Zinc carbonate + hydrochloric acid  →  zinc chloride + carbon dioxide + water

c) Name the particle in the acid that makes it acidic.

Answer: The hydrogen ion/H+ ion

d) Describe a method for producing pure, dry crystals of zinc chloride from solid zinc carbonate and your chosen acid.

Answer: Add hydrochloric acid to a beaker and add the zinc carbonate until it is in excess. Once the reaction / fizzing has stopped filter out the excess zinc carbonate using a filter paper and funnel. Pour the solution of zinc chloride into an evaporating basin. Heat the solution until crystals start to form. Leave the solution to cool, filter out the crystals and allow them to dry.

7. When magnesium reacts with hydrochloric acid the following reaction occurs: Mg + 2HCl  →  MgCl2 + H2
This reaction is a redox reaction involving both oxidation and reduction.

a) Explain why magnesium is oxidised during this reaction.

Answer: Magnesium atoms lose two electrons to form Mg2+ ions / Mg  →  Mg2+ + 2e. Oxidation is the loss of electrons.

b) Explain why hydrogen is reduced during this reaction.

Answer: Hydrogen ions gain electrons to form hydrogen gas / 2H+ + 2e  →  H2 . Reduction is the gain of electrons.

c) 1.25 g of magnesium were reacted with an excess of hydrochloric acid. Calculate the mass of hydrogen gas formed. Relative atomic mass (Ar) Mg = 24  and  Relative formula mass (Mr) H2 = 2

Answer: mol Mg = mass ÷ Ar = 1.25 ÷ 24 = 0.05208 mol ;
mol H2 = mol Mg (1:1 ratio in the balanced equation) ;
mass H2 = mol × Mr = 0.05208 × 2 = 0.1042 g

d) Use the answer above to calculate the volume of hydrogen gas produced at room temperature and pressure, when 1.25 g of magnesium react. The volume that one mole of any gas at room temperature and pressure is 24.0 dm3.

Answer: volume = mol × 24.0 = 1.25 dm3

8. A student decided to monitor the changes in pH of the neutralisation reaction between nitric acid, HNO3 , and sodium hydroxide, NaOH, using universal indicator solution.

a) Name the products of the reaction between nitric acid and sodium hydroxide.

Answer: sodium nitrate and water

b) Give the name of the particle that gives a solution a pH less than 7.

Answer: Hydrogen ion / H+ ion

c) Give the name of the particle that gives a solution a pH greater than 7.

Answer: Hydroxide ion / OH ion

d) Give the ionic equation for the neutralisation reaction between nitric acid and sodium hydroxide.

Answer: H+ + OH  →  H2O  or  H3O+ + OH  →  2H2O

e) During the neutralisation reaction the student observed a change in colour of the universal indicator from red, to orange, to green and finally to purple. Explain these colour changes. Give your answer in terms of pH and the concentrations of hydrogen and hydroxide ions in the reaction mixture.

Answer: At the start the pH = 1-2 and the colour is red, as the concentration of H+ ions is at its greatest (no OH has been added). As hydroxide ions are added they react with the hydrogen ions to form water / H+ + OH  →  H2O. The concentration of H+ ions decreases so the pH increases and the colour changes to orange. Eventually all the H+ ions have been used up / neutralised and the concentration of H+ and OH are equal. pH = 7 / the solution is neutral, and the colour is green. As more hydroxide ions are added in excess their concentration increases and the solution becomes alkaline. The colour changes to purple.

9. A student was asked to carry out a titration experiment to determine the concentration of some dilute potassium hydroxide using a solution of sulfuric acid with known concentration, and some phenolphthalein indicator. The reaction between potassium hydroxide and sulfuric acid is: 2KOH(aq) + H2SO4(aq)  →  K2SO4(aq) + 2H2O(l). Phenolphthalein indicator is pink in alkaline solutions and colourless in acidic solutions. The student used 25.0 cm3 of a 0.135 mol/dm3 solution of sulfuric acid in their titration experiment. The table below shows the results.

a) Describe a method for how the student could carry out a titration to determine the concentration of the potassium hydroxide solution. Do not include the calculations needed to determine the concentration.

Answer: Add sulfuric acid to a burette. Use a glass pipette to measure out 25.0 cm3 of potassium hydroxide solution into a conical flask. Add 3 drops of the indicator to the potassium hydroxide (it will turn pink). Add the acid to the alkali and swirl until the indicator changes colour / goes colourless. Record the volume added. Repeat and find the mean volume of sulfuric acid added.

b) Calculate the concentration of the potassium hydroxide solution in mol/dm3. Give your answer to three significant figures. Use only the student’s concordant results. Concordant results are those within 0.10 cm3 of each other.

Answer: Correct choice of titrations 2, 3 and 5 ; Average titration volume = 21.77 cm3 ;
mol H2SO4 = concentration × volume = 0.135 × (25.0 ÷ 1000) = 0.003375 mol ;
mol KOH = 2 × mol H2SO4 (from 2:1 ratio in balanced equation) = 0.00675 mol ;
concentration KOH = mol ÷ volume = 0.00675 ÷ (21.77 ÷ 1000) = 0.3101 mol/dm3 = 0.310 mol/dm3

10. A student was given unlabelled samples of two acids labelled acid A and acid B. The student was told that one acid is hydrochloric acid, HCl, and the other is ethanoic acid, CH3COOH, and that the concentration of both acids was 0.10 mol/dm3. The student carried out a series of experiments to determine which acid was a strong acid and which was a weak acid. The table below shows the results.

a) Determine which acid, A or B, is hydrochloric acid.

Answer: Acid B is hydrochloric acid.

b) Determine how many times more concentrated the hydrogen ions are in acid B compared to acid A.

Answer: 100 times more concentrated. (for every 1 unit fall on pH scale, concentration increases by a factor of 10)

c) Explain why both acids have the same concentration but different pH values.

Answer: Acid A (or ethanoic acid) is a weak acid that only partially ionises when dissolved in water. Acid B  (or hydrochloric acid) is a strong acid that completely ionises in water. This means the concentration of H+ ions in the weak acid is less than the concentration in the strong acid, so the pH is higher (or vice versa).