Posted: January 5th, 2016

# Using a value of (delta)T = + 48.5 Celsius as the temperature rise for the reaction between Zn and CuSO4 what is the value of q?

1. For the enthalpy of the reaction of Zn + CuSO4
The heat evolved in the calorimeter in Joules (J) is given by the following formula;
q = – (m cw + cb) (delta)T
Where m = mass of water in g. The mass of water is the same as the mass of copper sulphate solution (assume 1g = 1 cm3 of solution).
cw = heat capacity of water which is 4.18 J g-1 K-1
cb = heat capacity of the calorimeter which is 7.35 J K-1
(delta)T is the change in temperature in Kelvin as measured from the graph (the change in Celsius is the same as the change in Kelvin).
(a) Using a value of (delta)T = + 48.5 Celsius as the temperature rise for the reaction between Zn and CuSO4 what is the value of q?
(b) How many moles of Zn are present at the start of the reaction?
(c) How many moles of CuSO4 are present at the start of the reaction?
(d) Which of the two answers (b) and (c) should be used to calculate the enthalpy of reaction in kJmol-1 and why? A one sentence answer, please.
(e) What is the enthalpy of reaction in kJmol-1

2. For the enthalpy change of the solution of ammonium nitrate in water, the heat absorbed in this endothermic change can be calculated using the same formula as in Q1. Because the temperature drop is negative the value of q will come out as a positive number.
(a) Calculate the value of q using a value of (delta)T of -9 Celsius
(b) How many moles of ammonium nitrate were present?
(c) What is the enthalpy change in kJmol-1

3. Calcium carbonate (10 g) was heated in a sealed vessel which initially contained air at 1 atmosphere pressure. The heat transferred from the surroundings into the sealed vessel during the reaction was 17.7 kJ.

Assuming that the reaction below went to completion;
CaCO3 (s)  CaO (s) + CO2 (g)
Which one of the following statements is correct?
mol-1 means ‘per mole of CaCO3’

A The reaction was exothermic and had an enthalpy change of -177 kJmol-1
B The reaction was endothermic and had an enthalpy change of -177 kJmol-1
C The reaction was endothermic and the heat transferred was + 177 kJmol-1 but this was not the enthalpy of reaction because the system was not at constant pressure during the reaction.
D The reaction was exothermic and the heat transferred was – 177 kJmol-1 but this was not the enthalpy of reaction because the system was not at constant pressure during the reaction.
E The reaction was exothermic and the heat transferred was – 17.7 kJmol-1 but was not the enthalpy of reaction because the system was not at constant pressure during the reaction.

4. Indicate whether each of the following is exothermic or endothermic (delete as appropriate)
A The condensation of mercury vapour to form a liquid [EXO/ENDO]
B Sublimation of a solid to form a gas [EXO/ENDO]
C The combination of two hydrogen atoms to form a hydrogen molecule [EXO/ENDO]

5. Assuming that the rotting of vegetables is an exothermic process which of the following is true?
A The higher the activation energy for the reactions involved in rotting food the faster the food will rot.
B The higher the activation energy for the reactions involved in rotting food the slower the food will rot.
C The higher the activation energy for the reactions involved in rotting food the more heat will be given out.
D The higher the activation energy for the reactions involved in rotting food the less heat will be given out.
E None of the above

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