1. (a) IN YOUR OWN WORDS, explain how Hess\' Law (of constant heat summation) ap

Question

1. (a) IN YOUR OWN WORDS, explain how Hess' Law (of constant heat summation) applies to the 3 chemical reactions that will be studied in this experiment. (Note: answers copied from any source will NOT receive credit) (b) Is energy released or absorbed for an exothermic reaction? (c) What is the sign (positive or negative) for the AH of an exothermic reaction? (d) Use the heat capacity of the calorimeter given and the equation in the middle of p. 38 to calculate the heat gained by the calorimeter if the T for a reaction was 5.86 °C.

Explanation / Answer

Hess law of constant heat summation says that Energy of a reaction is same whether you bring it in single steps or multiple steps.

This can be achieved by simple mathematical manupulations

For Example:

CS2(l) + 3 O2(g) CO2(g) + 2 SO2(g)

Given:
C(s) + O2(g) CO2(g); Hf = -393.5 kJ/mol
S(s) + O2(g) SO2(g); Hf = -296.8 kJ/mol
C(s) + 2 S(s) CS2(l); Hf = 87.9 kJ/mol

We need one CO2 and the first reaction has one CO2 on the product side.

C(s) + O2(g) CO2(g), Hf = -393.5 kJ/mol

This gives us the CO2 we need on the product side and one of the O2 moles we need on the reactant side.

To get two more O2 moles, use the second equation and multiply it by two. Remember to multiply the Hf by two as well.

2 S(s) + 2 O2(g) 2 SO2(g), Hf = 2(-326.8 kJ/mol)

Now we have two extra S and one extra C molecule on the reactant side we don't need. The third reaction also has two S and one C on the reactant side. Reverse this reaction to bring the molecules to the product side. Remember to change the sign on Hf.

CS2(l) C(s) + 2 S(s), Hf = -87.9 kJ/mol

When all three reactions are added, the extra two sulfur and one extra carbon atomsare cancelled out, leaving the target reaction. All that remains is adding up the values of Hf

H = -393.5 kJ/mol + 2(-296.8 kJ/mol) + (-87.9 kJ/mol)
H = -393.5 kJ/mol - 593.6 kJ/mol - 87.9 kJ/mol
H = -1075.0 kJ/mol

b) energy is released for exothermic reaction

energy is absorbed in endothermic reaction

c) exothermic heat of reaction is negative H = -ve

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