Energy and Reversible Reactions

C8.7 Student practical

Name...... Class...... Date......

Energy and reversible reactions

Specification references  C6.2.2 Energy changes and reversible reactions

Aims In this activity you will look at some reversible reactions and the energy changes that are involved.

Learning outcomes After completing this activity, you should be able to:  state whether a reversible reaction is exothermic or endothermic in the reverse direction if the forward direction is stated  write a word equation for the reversible reaction of dehydration/hydration of copper sulfate  explain why the energy change in a reversible reaction is exothermic in one direction and endothermic in the reverse direction  generate balanced symbol equations for reversible reactions from information provided  make predictive observations of familiar reversible reactions when information is supplied  explain in detail the energy changes in an equilibrium system  suggest and explain a simple laboratory test which could be completed using a reversible reaction.

Safety  Hydrated copper(II) sulfate: HARMFUL  Hydrated cobalt(II) chloride: TOXIC. Handle carefully.  Wear chemical splash proof eye protection.

Equipment  hydrated copper(II) sulfate  hydrated cobalt(II) chloride  spatula  dropping pipette  Bunsen burner and safety equipment  test-tube holder  boiling tube

Name...... Class...... Date......

Setting the scene When hydrated salts such as magnesium sulfate crystals are heated, the water of crystallisation is driven off, leaving the anhydrous salt. The hydrated form can be re-formed by adding water. heat

MgSO4.7H2O(s) MgSO4(s)  7H2O(l) cool HYDRATED ANHYDROUS This is an example of a reversible reaction. The forward reaction is endothermic because heat energy is taken in. The backward reaction is exothermic because heat energy is transferred to the surroundings. The amount of energy taken in is the same as the amount of energy transferred to the surroundings. In this activity, you can observe two such reactions: heating copper(II) sulfate crystals, and heating cobalt(II) chloride crystals.

Method

1 Add about half a spatula of copper(II) sulfate crystals (CuSO4.5H2O) into a boiling tube. 2 Using a test-tube holder, gently heat the blue powder in a Bunsen flame and observe the colour change. 3 Allow the solid to completely cool. 4 Using the dropping pipette, add a few drops of water and observe the colour change.

5 Repeat using cobalt(II) chloride crystals (CoCl2.6H2O) in place of the copper(II) sulfate. 6 Record your results in the form of a pictorial flow chart. Include relevant names and observations.

Name...... Class...... Date......

Questions 7 Explain what is meant by a a salt

(1 mark) b water of crystallisation

(1 mark) c a hydrated salt

(1 mark) d an anhydrous salt.

(1 mark)

8 a Write a word equation and symbol equation to show the equilibrium between hydrated and anhydrous copper(II) sulfate crystals.

(2 marks)

b Explain in terms of bonds broken and formed why the forward reaction is endothermic and the backward reaction is exothermic.

(2 marks)

9 The energy required to convert one mole of hydrated copper(II) sulfate to one mole of anhydrous copper sulfate is 78 kJ. a How much energy is transferred to the surroundings when one mole of anhydrous copper sulfate is converted back into one mole of hydrated copper(II) sulfate?

(1 mark)

Name...... Class...... Date...... b Draw a labelled energy profile diagram for this reaction. (3 marks)

10 a Explain why blue cobalt chloride paper is used as a test for water.

(2 marks)

b Explain why cobalt chloride paper is kept in a desiccator.

(1 mark)

11 When calcium carbonate is heated strongly in a closed system, an equilibrium is set up between calcium carbonate, calcium oxide, and carbon dioxide. a Write an equation, including state symbols, to show the reversible reaction.

(2 marks) b 178 kJ/mol is required for the forward reaction. What is the energy transfer in the reverse reaction?

(1 mark) c If calcium carbonate is heated in a boiling tube open to the atmosphere, the reaction goes to completion because the carbon dioxide can escape. Calculate the mass of carbon dioxide produced when 2.5 g of calcium carbonate is heated. (Mr CaCO3  100)

(4 marks)