1. Chapter 17 Thermochemistry and Kinetics

2. Thermochemistry – study of transfer of energy as heat that accompanies chemical reactions and physical changes Heat and Temperature - Temperature – measure of the average kinetic energy of particles (increase T, increase KE; object feels hotter) - Joules – SI unit of heat (and energy) kJ is also used commonly EX. 980 kJ  980,000 J

3. - Heat – the energy transferred between samples of matter because of a difference in their temperatures - Energy moves from warmer to cooler objects - Energy absorbed or released in a reaction (or change) is measure in a calorimeter. - Calorimeters are sealed and placed in H 2 O. The T change of H 2 O is used to determine the energy change inside the calorimeter.

4. Heat Capacity and Specific Heat - Objects have different capacities for absorbing heat (energy). - Specific heat – the amount of energy required to raise the temperature of one gram of a substance by 1 o C. - Units are J/g o C - Table on pg. 513 - Water has a HIGH specific heat (4.184 J/g o C)

5. - That means water can absorb (or release) a lot of energy before its T will begin to change. - Even on a hot day, lake water can still feel cold. q = c x m x ΔT q = energy c = specific heat m = mass ΔT = change in T (subtract the T’s)

6. Ex. A 4.0 g sample of glass heated from 1 o C to 41 o C, and was found to have absorbed 32 J of energy. What is the specific heat of the glass? q = c x m x ΔT ΔT = 41 o C – 1 o C = 40 o C 32 J = c (4.0 g)(40 o C) 32 J = c (160 g o C) c = 0.2 J/g o C

7. Your Turn! Determine the specific heat of a material if a 35 g sample absorbed 48J as it was heated from 293K to 313K. If 980 kJ of energy are added to 6.2 L of water at 291 K, what will the final temperature of the water be?

8. Heat of Reaction – amount of energy absorbed or released during a chemical reaction - When energy is released (a product) in a reaction, it is an exothermic reaction ex. 2 H 2 + O 2  2 H 2 O + 483.6 kJ If 4 moles of H 2 O formed then 967.2 kJ would be released (2 x 483.6)

9. - When energy is absorbed (added to the reactants) it is an endothermic reaction. - Energy absorbed or released during a reaction is represented by ΔH rxn - H stands for enthalpy – heat content of a system - ΔH is the change in heat during the reaction: ΔH = H products – H reactants

10. ΔH is positive for endothermic reactions (heat added to reactants) ex. 2 H 2 + O 2  H 2 O ΔH = +483.6 kJ ΔH is negative for exothermic reactions (reactants losing heat) 2 H 2 O  2 H 2 + O 2 ΔH = -483.6 kJ

12. Heat of Formation – energy absorbed or released when 1 mole of a compound is formed from its elements - Designated as ΔH f o Ex. What would ΔH f o be for H 2 O(g)? 2 H 2 + O 2  2 H 2 O ΔH = -483.6 kJ That’s for 2 moles of H 2 O; find ΔH for 1 mole: -483.6 kJ = - 241.8 kJ = ΔH f o 2

13. Heat of Combustion – energy released as heat by complete combustion of 1 mole of a substance (remember combustion is adding O 2 ) ex. C 3 H 8 + 5O 2  3CO 2 + 4H 2 O ΔH o comb = -2219.2 kJ Combusting one mole of C 3 H 8 releases 2219.2 kJ (because ΔH o comb is negative, it releases energy – exothermic)

14. Heat of combustion is defined in terms of one mole of reactant, whereas the heat of formation is defined in terms of one mole of product.

15. 1.Identify each of the following reactions as exothermic or endothermic. CH4(s) + 2O2(g) → CO2(g) + 2H2O(l) ΔH° = -890 kJ 2HCl(g) → H2(g) + Cl2(g) ΔH° = 185 kJ 4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(l)ΔH° = -1169 kJ

16. Rate of Reactions Kinetics – study of reaction rates Rate Influencing Factors - for reactions to occur, particles must come into contact with each other (collide) in a favorable way. - anything that changes the frequency of collisions or the efficiency of collisions will change the rate of the reaction

17. 1) Surface Area - In homogeneous mixtures (dissolved) particles mix and collide freely so those reactions occur rapidly - Heterogeneous mixtures can only react where the 2 phases are in contact with each other; so the surface area of a solid is a factor; Increase surface area, increase rate

18. 2) Temperature - Increasing T increases KE of particles (move faster); therefore there will be more collisions and rate will increase - Decreasing T decreases KE and number of collisions, so rate decreases 3) Concentration - The greater the concentration of a substance, the more particles there will be; the more particles, the more collisions and increase in rate

19. - Lowering the concentration has the opposite effect on the rate. 4) Presence of Catalysts - Catalyst – changes the rate of a reaction without being used up (speeds up the reaction without interfering) - They are written over the arrow MnO 2 2 H 2 O 2 ----- > 2 H 2 O + O 2