1. Unit 3 PPA 1 Verification of Hess’s Law

2. Verification of Hess’s Law (Unit 3 PPA 1) The aim of the experiment is to show that the enthalpy change for a reaction is independent of the reaction pathway. The diagram below shows how potassium chloride can be formed from potassium hydroxide by two different reaction pathways. KOH(aq) 1  H 1 2  H 2  H 3 KOH(s) KCl(aq) 3 (1) Name the chemicals shown by the numbers 1,2 and 3. (1) 1 is water, 2 and 3 are hydrochloric acid. (2) What is the relationship between  H 1,  H 2 and  H 3 if the enthalpy of a reaction is independent of the reaction pathway? (2)  H 3 =  H 1 +  H 2

3. (3) Complete the list of measurements that would have to be made in order to calculate  H 2. (i) Mass of potassium hydroxide (ii) ……………………………………………………………………………………………………………………………………….. (iii) ……………………………………………………………………………………………………………………………….…….. (iv) …………………………………………………………………………………………………………………………………….. Mass of mixed solution (potassium hydroxide solution and hydrochloric acid) Average temperature of solutions (KOH(aq) and HCl(aq) before mixing. Highest temperature of mixed solutions. The diagram below shows how potassium chloride can be formed from potassium hydroxide by two different reaction pathways. KOH(aq) 1  H 1 2  H 2  H 3 KOH(s) KCl(aq) 3 (4)In the determination  H 3 of the following results were obtained:- When 1.4g of potassium hydroxide is added to 50cm 3 of 1 mol l -1 hydrochloric acid the temperature rose from 21.5 o C to 27.7 o C. Calculate the value for the enthalpy change,  H 3. (4)  H = - cm  T = - 4.18 x 0.05 x 6.2 = - 1.296 kJ