I. Chemical Reactions that Involve Heat

Transcription

1 Unit 12 Energy

2 I. Chemical Reactions that Involve Heat Thermochemistry: study of changes in heat in chemical reactions. Endothermic: absorbs heat; temp. goes down Exothermic: releases heat; temp. goes up

3 sur sur Endothermic reaction sur sur sur sur Exothermic reaction sur sur

4 A. Exothermic reaction Camping stove C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) kJ The energy released in creating new bonds is greater than the energy needed to break the old bonds. Joule (J) is used to measure heat and energy.

5 B. Endothermic reaction C (s) + H 2 O (g) + 113kJ CO (g) + H 2(g) H 2 O (l) + 286kJ ½O 2(g) + H 2(g) More energy is needed to break the old bonds than is released in forming the new bonds. Energy is stored in new bonds.

6 II. Heat and Enthalpy Changes Enthalpy (H): almost the same thing as energy; when pressure is held constant, it is the heat absorbed or released in a chemical reaction. Total Energy = P.E. + K.E. of particles

7 A. Enthalpy Change ΔH = H products - H reactants

8 A. Enthalpy Change (ΔH) ΔH for endothermic reaction = positive ΔH for exothermic reaction = negative Amount of heat released or absorbed depends on the conditions. Standard state: temperature = 25ºC pressure = 1 atm. physical state = *pure form Carbon - *graphite, diamond

9 Standard enthalpy change: ΔHº - enthalpy change when products and reactants are in their standard state. Sign of ΔH Process Heat Positive endothermic absorbed Negative exothermic released

10 Enthalpy problems C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) ΔHº = -2043kJ C (s) + H 2 O (g) CO (g) + H 2(g) ΔHº = +113kJ 1 mol C & 1 mol H 2 O 113kJ of heat 2 mol C & 2 mol H 2 O? Works like stoichiometry.

11 Enthalpy problems C 3 H 8(g) + 5O 2(g) 3CO 2(g) + 4H 2 O (g) ΔHº = -2043kJ C (s) + H 2 O (g) CO (g) + H 2(g) ΔHº = +113kJ 1 mol C & 1 mol H 2 O 113kJ of heat 2 mol C & 2 mol H 2 O 226KJ Works like stoichiometry.

12 III. Hess s Law A way to find the change in enthalpy for a reaction without doing it directly. If a series of reactions are added together, the enthalpy change for the net reaction will be the sum of the enthalpy change for the individual steps.

13 Examples N 2(g) + O 2(g) 2NO (g) ΔH 1 = +181 kj +2NO (g) + O 2(g) 2NO 2(g) ΔH 2 = -113 kj N 2(g) + 2O 2(g) + 2NO (g) 2NO (g) + 2NO 2(g) ΔH net = ΔH 1 + ΔH 2 N 2(g) + 2O 2(g) 2NO 2(g) ΔH net = 181 kj + (-113 kj) = +68 kj

14 Rules 1. If the coefficients of an equation are multiplied by a factor, the enthalpy change for the reaction is multiplied by the same factor. 2. If an equation is reversed, the sign of ΔH changes also. H 2(g) + CO (g) C (s) + H 2 O (g) ΔHº= -113 kj

15 IV. Calorimetry Endothermic: heat absorbed; temp. goes down Exothermic: heat released; temp. goes up Study of heat flow and heat measurement Calorimetry experiments determine the heats (ΔH) of reactions by making accurate measurements of temperature changes produced in a calorimeter.

16 IV. Calorimetry Heat capacity: amount of heat needed to raise the temperature of an object by 1ºC; depends on mass and composition. Specific heat: amount of heat needed to raise the temperature of 1 gram of an object by 1ºC. Specific heat of water = J/g-ºC

17 IV. Calorimetry Calorie: amount of heat needed to raise the temperature of 1 gram of water by 1ºC. 1 calorie = J 1 kcal = 1 Cal. = 1000 cal Calorimeter: insulator can be simple like a foam cup

18 Figure 10.6: A coffee-cup calorimeter.

19 Bomb Calorimeter

20 Calculations q = (mass)(specific heat)(temp.change) q heat absorbed or released in a calorimeter. m mass C specific heat ΔT change in temp. T final - T initial

21 Calculations q rxn heat of the reaction q sur heat of the surroundings q rxn = -q sur q sur = mcδt Calculate ΔH for the dissolving of NaOH in 75.0g of water if mol of NaOH was used and the temperature changed from 19.8 ºC to 26.7 ºC.

22 Calculations NaOH (s) Na + (aq) + OH - (aq) ΔH =? q sur = (75.0 g)(4.184 J/g-ºC)( ºC) = J q rxn = J = kj ΔH = Energy/mole ΔH = kj/0.050 mol = -44 kj

23 V. What is Heat? A. The Caloric Theory Antoine Lavoisier proposed that heat was an invisible, weightless fluid that flows from hot to cold. Benjamin Thompson (Count Rumford) did an experiment where he rubbed metals together and produced an uninterrupted flow of heat. Proposed heat couldn t be a fluid or it would gradually produce less heat.

24 V. What is Heat? James Joule Did experiments that showed heat is a form of energy. Changed heat into mechanical energy.

25 V. What is Heat? B. Kinetic Theory- heat results from the motion and vibration of the particles of matter. Heat: the transfer of kinetic energy from a hotter object to a colder object.