# Thermochemistry [ Enthalpy and Calorimetry]

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Information about Thermochemistry [ Enthalpy and Calorimetry]
Education

Published on October 12, 2014

Author: riabuan

Source: slideshare.net

## Description

Learn the basics of Enthalpy, State functions and Calorimetry.

1. Thermochemical Equations

2. Thermochemical Equations 푁푎(푠) + 퐻2푂(푙) → 푁푎푂퐻(푎푞) + 1 2 퐻2(푔) Δ퐻푟푥푛 = −183.75푘퐽 A thermochemical equation is a balanced chemical reaction + the value of its enthalpy of reaction Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

3. Requirement 1. Balanced chemical equation 2. The value of ΔHrxn per 1 mole of the limiting reagent Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

4. 푁푎(푠) + 퐻2푂(푙) → 푁푎푂퐻(푎푞) + 1 2 퐻2(푔) Δ퐻푟푥푛 = −183.75푘퐽 This equation means that 183.75 kJ of heat is released when 1 mole of sodium solid reacts with water to give 1 mole of aqueous sodium hydroxide and ½ moles of hydrogen gas. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

5. Important Conventions 푁푎(푠) + 퐻2푂(푙) → 푁푎푂퐻(푎푞) + 1 2 퐻2(푔) Δ퐻푟푥푛 = −183.75푘퐽 Specify the states of the reactants and products. They are important in the determination of standard enthapy values. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

6. Important Conventions 푁푎(푠) + 퐻2푂(푙) → 푁푎푂퐻(푎푞) + 1 2 퐻2(푔) Δ퐻푟푥푛 = −183.75푘퐽 The mole of the limiting reactant is important in the determination of ΔH of the reaction. Convention follows a ΔHrxn per one mole of the LR Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

7. Thermochemical Standard States They are the most stable pure form of the substance. Standard Pressure = 1.00 atm Specified temperature = usually at 25 ⁰C (unless otherwise stated) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

8. Standard States 1. For pure substances (solid or liquid) – standard state is the pure solid or liquid e.g. oxygen, O2 = gas bromine, Br2 = gas Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

9. a. For gases, P = 1.00 atm b. For gaseous mixtures, partial pressure must be at P=1.00 atm c. For aqueous solutions, concentration = 1.00 M Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

10. Standard Enthalpy Change, ΔHrxn The value of enthalpy change after the complete conversion of the specified moles of reactants all at their standard states into the specified moles of the products all at the standard states as well. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

11. Standard Molar Enthalpies of Formation, ΔHf⁰ The name implies that there should be a formation reaction. ΔHf⁰ is the standard molar enthalpy of formation of a one mole of substance formed from its elements in their standard states. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

12. Nose bleeds Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

13. Formation of CO2(g) 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) Δ퐻푓° = −393 푘퐽 Carbon dioxide is formed by reacting oxygen, O2(g) with carbon, C(graphite). Both oxygen and carbon are in their standard states. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

14. ΔHf⁰ values 1. Zero for elements in their standard states Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

15. ΔHf⁰ values 1. Non-zero for compounds, ions and elements in their non-standard states Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

16. Ok, are you still following us here? Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

17. When you’re ready, we’ll continue with calculating ΔHf⁰ from a given reaction Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

18. 푥퐴 + 푦퐵 → 푚퐶 + 푝퐷 In general ΔHprocess = 푐표푒푓푓푖푐푖푒푛푡푠 × Δ퐻푓 °푝푟표푑푢푐푡푠 − 푐표푒푓푓푖푐푖푒푛푡푠 × Δ퐻푓°푟푒푎푐푡푎푛푡푠 Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

19. 푥퐴 + 푦퐵 → 푚퐶 + 푝퐷 For this reaction: ΔHprocess = (m ∙ ΔHC +p ∙ ΔHD) − (x ∙ ΔHA +y ∙ ΔHB) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

20. 푥퐴 + 푦퐵 → 푚퐶 + 푝퐷 PRODUCTS REACTANTS ΔHprocess = (m ∙ ΔHC +p ∙ ΔHD) − (x ∙ ΔHA +y ∙ ΔHB) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

21. Take time to absorb these. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

22. Formation of CO2(g) 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) Recall this equation from earlier. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

23. Let’s set-up the equation for its enthalpy of formation Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

24. 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) Let’s apply the general equation ΔHprocess = (m ∙ ΔHC +p ∙ ΔHD) − (x ∙ ΔHA +y ∙ ΔHB) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

25. 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) ΔHprocess °= ° )− (1 ∙ ΔHO2(g) (1 ∙ ΔHCO2(g) °+1 ∙ ΔHC(graphite) °) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

26. Now, where can you find the ΔHf⁰ values of the reactants / products? Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

27. Guess? Kidding. Consult your book or the internet for the values. Do not guess, ok? Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

28. Going back here. 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) Evaluate first. 1. O2 and C(graphite) are in their pure states, right? So ΔHf⁰ should be zero. 2. CO2(g) is a compound in its non-standard state. Recall, we want to synthesize it through this reaction. So ΔHf⁰ should be non-zero. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

29. So, calculating.. 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) ΔHprocess °= ° )− (1 ∙ ΔHO2(g) (1 ∙ ΔHCO2(g) °+1 ∙ ΔHC(graphite) °) ΔHprocess °= [1 ∙ (−393kJ/mol)]− (0+0) Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

30. Formation of CO2(g) 푂2(푔) + 퐶(푔푟푎푝ℎ푖푡푒) → 퐶푂2(푔) Δ퐻푓° = −393 푘퐽 Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

31. Try this on your own: 2퐻2(푔) + 푂2(푔푎푠) → 퐻2푂(l) Find the ΔHrxn °. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

32. Okay, let’s up the game for a bit. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

33. Exercise: Calculate the ΔHrxn ° for the following reaction using the following values for ΔHf ° H2O2(l) → H2O(l) + ½ O2(g) H2O2 (l) = -187.8 kJ / mol H2O (l) = −285.8 kJ/mol Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

34. Exercises: H2O2(l) → H2O(l) + ½ O2(g) ° + 1/2 ∙ ΔHf,O2 ΔH rxn°=(1 ∙ ΔHf,H2O °) − (1 ∙ °) ΔHf,H2O2 ΔH rxn°=(1 ∙ (−285.8) + 1/2 ∙ (0)) − (1 ∙ (−187.8)) ΔH rxn°= -98.0 kJ / mol Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

35. Let’s end here. Check out the exercises on my blog. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

36. Also, check the tutorial videos of problem solving on my blog. Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

37. Until the next post! Prepared: Kemikal Drills by Rea Abuan 2014 www.reaabuan.com/blog/

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