Do now: Write equations for the following expressions:

Transcription

1 Do now: Write equations for the following expressions: ½ N 2(g) + 1 ½ H 2(g) NH 3(g) Δ f H (NH 3(g) ) = -46 kj.mol -1 ½ H 2(g) + ½ Cl 2(g) HCl (g) Δ f H (HCl (g) ) = -92 kj.mol -1 ½ N 2(g) + 2 H 2(g) + ½ Cl 2(g) NH 4 Cl (s) Δ f H (NH 4 Cl (s) ) = -314 kj.mol -1

2 Hess s Law We use Hess s Law to work out an unknown enthalpy of reaction. We do this by using other reactions where we do know the enthalpy change to work out the unknown enthalpy change. For example: 2014 Exam Q2 c unknown enthalpy, Δ r H known enthalpys, Δ r H

3 Hess s Law The enthalpy change for a reaction is independent of the way in which a reaction proceeds and depends only on the initial conditions of the reactants and the final conditions of products The enthalpy change for a reaction is independent of the route taken

4 Hess s Law example Calculate the heat of formation of CS 2(l) given that the heats of combustion of carbon, sulfur and carbon disulfide are -393, -297 and kj.mol -1 respectively Step 1: Write down the reaction for enthalpy you are trying to find C (s) + 2 S (s) CS 2 (l) Δ f H =? Step 2: Write down the reactions for enthalpy s you know C (s) + O 2(g) CO 2(g) Δ c H = -393 kj.mol -1 S (s) + O 2(g) SO 2(g) Δ c H = -297 kj.mol -1 CS 2(l) + 3 O 2(g) CO 2(g) + 2 SO 2(g) Δ c H = kj.mol -1

5 Hess s Law example Step 3: Rearrange the equations so that the products and reactants and correct amounts match the desired equation C (s) + 2 S (s) CS 2 (l) Δ f H =? C (s) + O 2(g) CO 2(g) Δ c H = -393 kj.mol -1 S (s) + O 2(g) SO 2(g) Δ c H = -297 kj.mol -1 x2 2 S (s) + 2 O 2(g) 2 SO 2(g) CS 2(l) + 3 O 2(g) CO 2(g) + 2 SO 2(g) Δ c H = -594 kj.mol -1 Δ c H = kj.mol -1 reverse CO 2(g) + 2 SO 2(g) CS 2(l) + 3 O 2(g) Δ c H = kj.mol -1 Step 4: Cancel Step 5: Calculate the desired enthalpy = +20 kj.mol -1

6 Another example From the following equations and ΔH values H 2(g) + Br 2(g) 2 HBr (g) Δ r H = kj.mol -1 2 H 2 + O 2(g)+ 2 H 2 O (g) Δ r H = kj.mol -1 Calculate Δ r H for the following reaction: 4 HBr (g) + O 2(g) 2 Br 2(l) + 2 H 2 O (g) Step 1: Write down the reaction for enthalpy you are trying to find Step 2: Write down the reactions for enthalpy s you know Step 3: Rearrange the equations so that the products and reactants and correct amounts match the desired equation Step 4: Cancel Step 5: Calculate the desired enthalpy

7 Do now: Write equations for the following expressions C (s) + O 2(g) CO 2(g) Δ c H (C (s) ) = kj.mol -1 H 2(g) + ½ O 2(g) H 2 O (l) Δ f H (H 2 O (l) ) = kj.mol -1 C 2 H 5 OH (l) + 3 ½ O 2(g) 2 CO 2(g) + 3 H 2 O (l) Δ c H (C 2 H 5 OH (s) ) = kj.mol -1

8 This is from the 2014 Exam Q3 d. Hess s Law 2 C (s) + 3 H 2(g) + ½ O 2(g) C 2 H 5 OH (l) Δ f H (C 2 H 5 OH (l) ) =? Achieved Merit Excellence

9 Your turn Calculate the heat of reaction for the water gas reaction: C (s) + H 2 O (g) CO (g) + H 2(g) Δ r H =? Δ f H (CO 2(g) ) = kj.mol -1 Δ f H (H 2 O (g) ) = kj.mol -1 Δ c H (CO (g) ) = kj.mol -1 Workbook pg 63, 64 Q1, Q2 and Q3 Q2: combustion equation: B 2 H 6(g) + 3 O 2(g) B 2 O 3(s) + 3 H 2 O (l)

10 Quick recap Calculate the heat of reaction for the esterification of ethanol with ethanoic acid: CH 3 COOH (l) + C 2 H 5 OH (l) CH 3 COOC 2 H 5(l) + H 2 O (l) Δ r H =? Δ f H (CH 3 COOH) = kj.mol -1, Δ f H (C 2 H 5 OH) = kj.mol -1, Δ f H (CH 3 COOC 2 H 5 ) = kj.mol -1, Δ f H (H 2 O) = kj.mol -1 Δ r H = -2 kj.mol -1

11 It gets easier If Δ f H is given for all products and reactants we can use the following equation to calculate Δ r H. Δ r H = Σ Δ f H (products) - Σ Δ f H (reactants) Remember that Δ f H of an element is 0 kj.mol -1. For example: Calculate the enthalpy of the following reaction 4 NH 3(g) + 5 O 2(g) 6 H 2 O (g) + 4 NO (g) Δ f H (NH 3(g) ) = kj.mol -1, Δ f H (NO (g) ) = kj.mol -1, Δ f H (H 2 O (g) ) = kj.mol -1 Δ r H = Σ Δ f H (products) - Σ Δ f H (reactants) Δ r H = (6 x x 90.25) (4 x x 0) Δ r H = ( ) ( ) Δ r H = = kj.mol -1

12 Your turn Calculate the enthalpy of the following reaction : Δ r H = Σ Δ f H (products) - Σ Δ f H (reactants) CaCO 3(s) CaO (s) + CO 2(g) Δ f H (CaO) = kj.mol -1, Δ f H (CO 2 ) = -393 kj.mol -1, Δ f H (CaCO 3 ) = kj.mol -1 Find Δ f H (NaHCO 3 ) given the following data: 2 NaHCO 3(s) Na 2 CO 3(s) + CO 2(g) + H 2 O (g) Δ r H = 129 kj.mol -1 Δ f H (Na 2 CO 3 ) = kj.mol -1, Δ f H (CO 2 ) = -393 kj.mol -1, Δ f H (H 2 O) = -242 kj.mol -1

13 Your turn Calculate Δ f H (C 3 H 8 ) given the following information Δ c H (CO 2 ) = -393 kj.mol -1, Δ c H (H 2 ) = -286 kj.mol -1, Δ c H (C 3 H 8 ) = kj.mol -1 Find Δ r H for the hydrogenation of ethyne to ethene C 2 H 2(g) + H 2(g) C 2 H 4(g) Δ c H (C 2 H 2 ) = kj.mol -1, Δ c H (C 2 H 4 ) = kj.mol -1, Δ f H (H 2 O) = -286 kj.mol -1

14 2014 Exam Q2 c