1. Copyright  2011 Pearson Canada Inc. 7 - 1

2. Slide 2 of 57 Determining the specific heat of lead – Example 7-2 illustrated FIGURE 7-3 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7

3. Copyright  2011 Pearson Canada Inc. 7 - 3

4. Slide 4 of 57 Heat Capacity Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7 q = mcTmcT CTCT

5. Slide 5 of 57 Law of conservation of energy In interactions between a system and its surroundings the total energy remains constant— energy is neither created nor destroyed. Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7 q system + q surroundings = 0 q system = -q surroundings

6. Slide 6 of 57 Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7 7-2 Heat Thermal Energy – Kinetic energy associated with random molecular motion. – In general proportional to temperature. – An intensive property. Heat and Work – q and w. – Energy changes.

7. Slide 7 of 57 7-3 Heats of Reaction and Calorimetry Chemical energy. – Contributes to the internal energy of a system. Heat of reaction, q rxn. – The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature. In practice, a small temperature difference may be permitted/required. General Chemistry: Chapter 7

8. Calorimetry – Unfortunate Name? Energy changes associated with both physical and chemical processes are measured with calorimeters. The q values obtained are either constant pressure measurements (q P for a constant pressure calorimeter – isobaric change) or constant volume measurements (q V for a constant volume calorimeter – isochoric change). q P and q V values can differ appreciably if the process involves gases.

9. Coffee Cups

10. “Coffee Cup” Calorimeters A coffee cup normally has a negligible heat capacity compared to the system being studied. A coffee cup, or a more accurate device, affords us a constant pressure measurement of heat transfer or a ΔH value for a physical or chemical change. Such a calorimeter is suited for studies where the reactants do not need to be confined and great accuracy is not required.

11. Slide 11 of 57 The “Coffee-Cup” Calorimeter Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7 FIGURE 7-6 A Styrofoam “coffee-cup” calorimeter A simple calorimeter. Well insulated and therefore isolated. Measure temperature change. q rxn = -q cal

12. Coffee Cup Reactions Reactions occurring when two aqueous solutions are mixed are easily studied using a coffee cup calorimeter. For dilute aqueous solutions we can often assume that the solution density and specific heat capacity are very similar to the values for pure water. Possible reactions: HNO 3 (aq) + KOH(aq) → KCl(aq) + H 2 O(aq) AgNO 3 (aq) + HCl(aq) → AgCl(s) + HNO 3 (aq) MgO(s) +2 HCl(aq) → MgCl 2 (aq) + H 2 O(l) H 2 O(s,0 o C) + H 2 O(l,”hot”) → H 2 O(l,”moderate T”)

13. Coffee Cup & Limiting Reagents Example: Determination of the molar enthalpy of neutralization of strong acid and strong base (completely ionized acid and base). In an experiment 125 mL of 0.400 mol∙L -1 HCl(aq) were mixed with 175 mL of 0.280 mol∙L -1 NaOH(aq). The temperature of the solution formed after rapid mixing increased from 20.9 o C to 24.0 o C. Find the molar enthalpy of neutralization and write the total and net ionic equations for the reaction. Limiting reagent?

14. Bomb Calorimeters – Big Budget! Bomb calorimeters are big and expensive devices. They have large heat capacities and often contain water - which serves to keep the temperature rise resulting from highly exothermic reactions at a moderate level. Bomb calorimeters are suited for studying reactions where the starting materials might be volatile and, without a sealed calorimeter, both starting materials and heat might escape from the calorimeter.

15. Slide 15 of 57 Bomb Calorimetry Copyright © 2011 Pearson Canada Inc. General Chemistry: Chapter 7 FIGURE 7-5 A bomb calorimeter assembly q rxn = -q cal q cal = q bomb + q water + q wires +… Define the heat capacity of the calorimeter: q cal =  m i c i  T = C cal  T all i heat

16. Typical Bomb Calorimeter Reactions Bomb calorimeters are especially suited to the study of combustion reactions. In such studies an excess of oxygen is normally employed. Why? Typical reactions: Mg(s) + ½ O 2 (g) → MgO(s) C 9 H 20 (l) + 19 O 2 (g) → 9 CO 2 (g) + 10 H 2 O(l) CH 3 CH 2 COOH(l) + 5/2 O 2 (g) →3 CO 2 (g) + 3 H 2 O(l)

17. Bomb Calorimeter Example