# Module 3 Lesson 6 – Hess’ Law and enthalpy cycles part 2.

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Module 3 Lesson 6 – Hess’ Law and enthalpy cycles part 2

Objectives Must Recall Hess’ law and construct an enthalpy cycle based on given data for enthalpy changes of combustion Should Extend the principle to calculate enthalpy changes of reaction from enthalpy changes of formation. Could Construct and use an unfamiliar enthalpy cycle to determine an enthalpy change of reaction.

Starter 1.Define ΔH f 2.What are standard conditions? 3.What is the value of ΔH f for any element?

Using ΔH f As with ΔH c many ΔHf values have also been measured and can be looked up in tables ΔH r = - Σ ΔH f(reactants) + Σ ΔH f(products) Reactants Products Elements

Example You are provided with the following enthalpy changes of formation Determine the enthalpy change for the following reaction: 2NO (g) + O 2 (g)  2NO 2 (g) Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

2NO(g) + O 2 (g)2NO 2 (g) Elements ΔH r = -(2 x 90) + (2 x 33) = -114 kJmol -1 Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

Example You are provided with the following enthalpy changes of formation Determine the enthalpy change for the following reaction: 4NH 3 (g) + 5O 2 (g)  4NO(g) + 6H 2 O(l) Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

4NH 3 (g) + 5O 2 (g)4NO(g) + 6H 2 O(l) Elements ΔH r = -(4 x -46) + (4 x 90) + (6 x -286) = -1172 kJmol -1 Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

Example You are provided with the following enthalpy changes of formation Determine the enthalpy change for the following reaction: 2H 2 O (l) + 4NO 2 (g) + O 2 (g)  4HNO 3 (l) Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

2H 2 O(l) + 4NO 2 (g) + O 2 (g) 4HNO 3 (l) Elements ΔH r = -(2 x -286) - (4 x 33) + (4 x -174) = -256 kJmol -1 Substan ce NO 2 (g)NH 3 (g)NO(g)H 2 O(l)HNO 3 (l) ΔH f / kJmol -1 33-4690-286-174

Other enthalpy cycles

Examination question Attempt these complete examination questions from January and June 2010 under test conditions to prepare for a mid-unit test next lesson.

Question 1

Question 2

Mark schemes Cover until you have answered

Mark scheme Q1 – cover until you have answered

Mark scheme

Mark scheme Q2

Mark scheme

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