MI_H2_CHEM_P2_Answer Prelim

1 Planning (P) In metal displacement reactions, a more reactive metal will displace a less reactive metal from a solution. Copper, silver and gold appear as elements in the earth due to their poor reactivity with the environment. These metals are easy to extract. You are required to plan an experiment to determine the enthalpy change of a metal displacement reaction between zinc and copper( II) sulfate solution. Zinc reacts with aqueous copper(II) sulfate as shown by the equation below. Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s) The reaction is exothermic and spontaneous, requiring only a few minutes to go to completion. You are provided with the following chemicals  Zinc powder,  0.8 mol dm -3 copper(II) sulfate, CuSO 4 as well as apparatus commonly found in the laboratory. (a) State the observations when the reaction goes to completion. Blue CuSO4 solution decolourises and pink copper solid is deposited at the bottom of the solution. [1] (b) Using the information above, write a plan to describe how you would determine the enthalpy change of the reaction between zinc and copper(II) sulfate. Your plan must include  a suitable mass of zinc to be used, and your justifications for choosing this mass,  the volume of copper(II) sulfate solution to be used,  detailed procedures of the experiment you would perform. Include details of the apparatus you would use, and  the precautions you would take to minimise heat loss. [4] Assuming 25.0 cm 3 of CuSO4 (aq) used, Amount of CuSO 4 = (25/1000) x 0.8 = 0.0200 mol = amount of zinc required. Minimum mass of Zinc = 0.02 x 65.4 = 1.31 g

Procedure: 1. Weigh an empty weighing bottle and record its mass. 2. Place between 1.40 g and 1.50 g of zinc powder into the weighing bottle ( excess zinc is used to ensure complete displacement). Record all required weighings in an appropriate table. 3. Place a styrofoam cup in the 250 cm 3 beaker and measure , using a measuring cylinder, 25.0 cm3 of CuSO4 into the styrofoam cup. 4. Stir gently with the thermometer and record the initial temperature of the solution. 5. Add zinc from the weighing bottle to the plastic cup. Immediately stir with the thermometer and record the highest temperature reached. 6. Reweigh the weighing bottle with residual zinc powder , if any, and record the mass. Accept any other suitable method. To minimise heat loss, a lid should be added to the styrofoam cup to prevent heat from escaping to the surroundings. This would lead to a more reliable ∆T result. 1m for calculation of mass of zinc 1m for steps 1,2,6 1m for steps 3,4,5 1m for method to minimise heat loss (c) A student carried out the reaction between zinc powder and copper sulfate solution using a data logger with a temperature probe. The zinc powde r was added in at the third minute. He obtained a set of temperature readings at different time intervals. The student then plotted a graph o