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Topic 10/9: Energetics

Hazard Code A

The enthalpy change of a displacement reaction

Zinc is more reactive than copper, so when zinc is added to copper(II) sulphate solution, copper is displaced. This reaction is associated with an enthalpy change and enthalpy is converted into heat energy. The heat energy warms up the surroundings (which in this case is the water in which the reaction takes place). Requirements: 25 cm measuring cylinder watch glass stop clock
2.5g of zinc powder 0-100C thermometer

Procedure: a) Measure out 25 cm of 1.0M copper (II) sulphate solution in a measuring cylinder and pour it into an expanded polystyrene cup. b) Stir with a thermometer, start a stop clock and measure the temperature every 30 seconds. c) Meanwhile weigh out about 2.5g of zinc powder onto a watch glass. Record the mass. d) At three minutes (when the temperature of the solution is constant) add all of the zinc powder to the cup and stir carefully with the thermometer. Continue to stir and record the temperature every 30 seconds until the temperature has passed its maximum and has been falling for two minutes. e) Note any changes to the appearance of the solution and the solid and then dispose of the contents of the cup into the container provided.

25cm3 of 1.0M copper (II) sulphate solution

expanded polystyrene cup

Safety: No particular hazards

Results Record your results (and any additional observations) in your rough book using a table based on the template below. When you have finished the experiment and cleared away your apparatus, copy your results and the completed table into your exercise book after this sheet. Mass of zinc = Time (mins) Temp (C) 0.0 g 0.5 1.0 1.5 2.0 2.5 3.0 Add Zn Temperature rise (T) = Maximum temp - Temp before zinc was added = C 3.5

Analysis Answer the following questions after the results in your exercise book, using full sentences and showing your working for calculations. 1. 2. 3. 4. 5. 6. 7. Calculate the number of moles of copper(II) sulphate in 25cm of 1.0M copper (II) sulphate solution. Calculate the number of moles of zinc in the mass you weighed out. [Ar(Zn) = 65] Write a balanced equation for the reaction of zinc with copper(II) sulphate (CuSO4) solution. How many moles of zinc would react with one mole of copper(II) sulphate? By considering your answers to (1), (2) and (4), state which reagent is in excess. How many moles of each substance will actually have reacted? The energy required to raise the temperature of 1g (or 1cm ) of water by 1C is 4.2J. Calculate the energy 3 required to raise the temperature of 25cm of your solution by T using the equation: 3 Energy absorbed by solution = (mass of solution) x 4.2 x T [assume that mass (in g) = volume (in cm )] Where has this energy come from? Calculate the enthalpy change (H) for this displacement reaction in kJ mol . Remember that your answer to (7) will be in J (not kJ) and that you used much less than 1 mole of reactant.
-1 -1 3 3

8. 9.

10. Is the reaction exothermic or endothermic. Will the sign of H be positive or negative? 11. The accepted value of H for this reaction is -219 kJ mol . Give two reasons for the difference between your answer and this value. 12. Suggest how you could improve the experiment to obtain a more accurate value.