Chapter 17 Thermochemistry

Energy Transformations When fuel is burned in a car engine, chemical energy is released and is used to do work.

Measuring and Expressing Enthalpy Changes A burning match releases heat to its surroundings in all directions. Chemical energy → … Quick-write : Describe the energy conversions for this scenario. How much heat does this exothermic rxn release???

Energy Transformations Thermochemistry: study of heat changes (ΔH; change in heat content) that occur during chem rxns and changes in physical states. chemical potential energy (chemical energy) energy stored in the chem bonds of a sub

Calorimetry Calorimetry the precise measurement of the heat flow into or out of a system for chemical (e.g. burning) and physical processes (e.g. boiling, melting—phase changes).

Calorimetry In calorimetry, heat released by the system = heat absorbed by surroundings Conversely, heat absorbed by a system = heat released by surroundings

Calorimetry Calorimeter insulated device used to measure the absorption or release of heat in chem or physical processes.

Calorimetry Enthalpy (H) of the system. The heat content of a system cannot be measured directly with instruments. can measured enthalpy change (ΔH);

Calorimetry Calorimetry expts can be performed at a constant volume using a bomb calorimeter.

Thermochemical Equations A thermochemical eqn chem eqn that includes the enthalpy change (ΔH). exothermic In a chem eqn, the ΔH for the rxn can be written as either a reactant or a product.

Thermochemical Equations The heat of rxn is the ∆H. Exothermic Enthalpy change

Thermochemical Equations Exothermic Rxn

Thermochemical Equations Endothermic Rxn

Thermochemical Equations

Thermochemical Equations The (molar) heat of combustion is the heat of rxn for the complete burning of 1 mole of a sub.

Energy Transformations In what direction does heat flow?

Exothermic and Endothermic Process In an endothermic process, system gains heat as the surroundings cool down.(+ve) In an exothermic process, system loses heat as the surroundings heat up. (-ve)

Exothermic and Endothermic Process A system part of the universe on which you focus your attention. The surroundings include everything else in the universe. Law of conservation of energy states that in any chem or physical process, energy is neither created nor destroyed.

Exothermic and Endothermic Processes An endothermic process is one that absorbs heat from the surroundings.

Exothermic and Endothermic Processes An exothermic process is one that releases heat to its surroundings.

Units for Measuring Heat Flow 2 common units for heat flow calorie ; Calorie (food) joule.

Specific Heat Capacity The specific heat of a sub (pure) is the amt of heat it takes to raise the temp of 1 g of the sub 1°C. J/(g∙°C) q: qty of heat (J) m: mass of sub (g) ∆T: change in temperature (°C); ∆T = Tfinal – Tinitial

Heat Capacity and Specific Heat Water releases a lot of heat as it cools. During freezing weather, farmers protect citrus crops by spraying them with water.

Specific Heat

Heat Capacity and Specific Heat Because it is mostly water, the filling of a hot apple pie is much more likely to burn your tongue than the crust.

Calculate specific heat of a metal Problem 1: The temp of a 95.4-g piece of Cu increases from 25°C to 48.0°C when the Cu absorbs 849 J of heat. What is the specific heat of Cu ?

Calculate Amt of heat to … Problem 2: How much heat is required to raise the temp of 250.0g of Hg 52°C? (CHg = 0.14J/(g·°C).

Latent Heat Latent heat for phase change melting, freezing, condensation, vaporization, involved heat changes. the temperature of the sub remain constant; only phase change. Heat of fusion (ΔHfus), heat of solidification (ΔHsolid), heat of vaporization (ΔHvap), heat of condensation (Δhcond)

Latent Heats Latent heat (for phase change) abbreviation Sign of ΔH Heat of fusion ΔHfus +ve Heat of solidification ΔHsolid -ve Heat of vaporization ΔHvap Heat of condensation ΔHcond

Latent Heat Heat of fusion (ΔHfus) ΔH for melting (+ve; gain heat); e.g. ΔHfus of ice = 6.01 kJ/mol temp of sub remains constant e.g. ice at 0°C remains 0°C while melting though it keeps on absorbing heat refer to graph in the next slide.

Molar Heat of Fusion ΔHfus of ice = 6.01 kJ/mol Heat absorbed (+ve) Heat change (to melt 1 mole of ice) qty of Heat (kJ) = # moles of sub Heat absorbed (+ve)

Heats of Fusion and Solidification Molar heat of fusion (∆Hfus) amt of heat absorbed by 1 mole of a solid sub as it melts to a (l) at a constant temp. e.g. ΔHfus of ice = 6.01 kJ/mol Molar heat of solidification (∆Hsolid) amt heat lost when 1 mole of a (l) solidifies at a constant temp. e.g. ΔHsolid of water (l) = - 6.01 kJ/mol

Heats of Vaporization and Condensation

liquid water temp increasing vaporizing (e) liquid water temp increasing (d) (100°C) Temp increasing All ice 0°C Boiling water 100°C melting (c) (Liquid H2O + steam) All liquid water 0°C (b) (0°C) (Ice+water) 0°C (a) (Ice) temp increasing

Quick-write Why f.p. of water = 0°C and m.p. of ice is also 0°C?

Latent Heat Heat of vaporization (ΔHvap) ΔH is +ve Heat of fusion (ΔHfus) melting ΔH is +ve Heat of solidification (ΔHsolid) freezing ΔH is -ve Heat of vaporization (ΔHvap) ΔH is +ve Heat of condensation (ΔHcond) ΔH is -ve

Heats of Fusion and Solidification The qty of heat absorbed by a melting (s) is exactly the same as the qty of heat released when the (l) solidifies ∆Hfus = –∆Hsolid

Heats of Vaporization and Condensation How does the qty of heat absorbed by a vaporizing (l) compare to the qty of heat released when the vapor condenses?

Heats of Vaporization and Condensation Molar heat of vaporization (∆Hvap). amt of heat necessary to vaporize 1 mole of a given (l) at constant temp. Molar heat of condensation (∆Hcond). amt of heat released when 1 mol of vapor condenses at the normal b.p.

Heats of Vaporization and Condensation The qty of heat absorbed by a vaporizing (l) is exactly the same as the qty of heat released when the vapor condenses; ∆Hvap = –∆Hcond

Heats of Vaporization and Condensation

Heats of Vaporization and Condensation ΔH accompany changes in state.

Heat of Soln Heat of Solution During the formation of a soln, heat is either released or absorbed. Molar heat of soln (∆Hsoln) The enthalpy change caused by dissolution of 1 mole of sub.

Heat of Solution When NH4NO3 crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve.

Problem 3: The graph below shows a pure sub which is heated by a constant source of heat supplying 2000.0 j/min. Identify the area described in the questions below and complete the necessary calculations. UV = 0.36 min, VW = 3.6 min, WX = 3.6 min, XY = 19.4 min, YZ = 0.6 min

a. being warmed as a solid ___________ b a. being warmed as a solid ___________ b. being warmed as a liquid __________ c. being warmed as a gas ____________ d. changing from a solid to a liquid _____ e. changing from a liquid to a gas ______ f. What is its boiling temp? _________________ g. What is its melting temp? _________________ h. How many joules were needed to change the liquid to a gas? ____________ i. Where on the curve do the molecules have the highest KE? ______ j. If the sample weighs 10.0 g, what is its heat of vaporization in J/g? ______

Example 4 How many g of ice at 0°C could be melted by the addition of 5.75 kJ of heat? (ΔHfus of ice = 6.01 kJ/mol)

CST problem 1 The specific heat of Cu is about 0.4 J/(g·°C). How much heat is needed to change the temperature of a 30-g sample of Cu from 20.0°C to 60.0°C? A 1000 J B 720 J C 480 J D 240 J

CST problem 2 Which of these is an example of an exothermic chemical process? A evaporation of water B melting ice C photosynthesis of glucose D combustion of gasoline

The End

Example 5 How much energy is required to convert 300.0 g of water at 60°C completely to steam at 100.0 °C? (Given: ΔHvap of water = 0.9 kJ/mol; Cwater = 4.18 J/g· °C))

Calculating the Enthalpy Change in Solution Formation How much heat (in kJ) is released when 2.500 mol NaOH(s) is dissolved in water?

How many moles of NH4NO3(s) must be dissolved in water so that 88 How many moles of NH4NO3(s) must be dissolved in water so that 88.0 kJ of heat is absorbed from the water?

1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of A. formation. B. fusion. C. solidification. D. vaporization.

2. The heat of condensation of ethanol (C2H5OH) is 43. 5 kJ/mol 2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings A. stays the same. B. may increase or decrease. C. increases. D. decreases.

3. Calculate the amt of heat absorbed to liquefy 15 3. Calculate the amt of heat absorbed to liquefy 15.0 g of methanol (CH3OH) at its m.p. The molar ∆Hfus for methanol is 3.16 kJ/mol. A. 1.48 kJ B. 47.4 kJ C. 1.52  103 kJ D. 4.75 kJ

4. How much heat (in kJ) is released when 50 g of NH4NO3(s), 0 4. How much heat (in kJ) is released when 50 g of NH4NO3(s), 0.510 moles, are dissolved in water? Hsoln = 25.7 kJ/mol A. 12.85 kJ B. 13.1 kJ C. 25.7 kJ D. 1285 kJ