Chapter 17

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Chapter 17

Thermochemistry

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Energy Transformations

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

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Measuring and Expressing Enthalpy Changes

• A burning match releases heat to its surroundings in all directions.

• Chemical energy → …

How much heat does this exothermic rxn release???

Quick-write : Describe the energy conversions for this scenario.

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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

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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).

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Calorimetry

In calorimetry,

heat released by the system =

heat absorbed by surroundings

Conversely,

heat absorbed by a system = heat released by surroundings

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Calorimetry

Calorimeter• insulated

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

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Calorimetry

Enthalpy (H) of the system.

The heat content of a system cannot be measured directly with

instruments. can measured enthalpy change (ΔH);

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Calorimetry

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

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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.

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• The heat of rxn is the ∆H.

Enthalpy change

Exothermic

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Exothermic Rxn

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Endothermic Rxn

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

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In what direction does heat flow?

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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)

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Exothermic and Endothermic Process

A system• part of the universe on which you focus your

attention.

Law of conservation of energy• states that in any chem or physical process, energy is neither created nor destroyed.

The surroundings• include everything else in the universe.

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Exothermic and Endothermic Processes

An endothermic process is one that absorbs heat from the surroundings.

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Exothermic and Endothermic Processes

An exothermic process is one that releases heat to its surroundings.

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Units for Measuring Heat Flow

2 common units for heat flow calorie ; Calorie (food) joule.

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Specific Heat CapacityThe 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.

Tm

qC

q: qty of heat (J)m: mass of sub (g)∆T: change in temperature (°C); ∆T = Tfinal – Tinitial

J/(g °C)∙

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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.

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Specific Heat

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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.

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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 ?

Tm

qC

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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).

Tm

qC

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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)

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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 +ve

Heat of condensation ΔHcond -ve

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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.

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Molar Heat of Fusion

ΔHfus of ice = 6.01 kJ/mol

Heat change (to melt 1 mole of ice)

qty of Heat (kJ)# moles of sub

=

Heat absorbed (+ve)

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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

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Heats of Vaporization and Condensation

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(Ice+water) 0°C

All liquid water 0°C

All ice 0°C

(Ice) temp increasing

melting

vaporizing

(a)

(b)

(c)

(d)(e)

Boiling water 100°C

(100°C)

(0°C)

liquid water temp increasing

Temp increasing

(Liquid H2O + steam)

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Quick-write

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

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Latent HeatHeat 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

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Heats of Fusion and Solidification

∆Hfus = –∆Hsolid

The qty of heat absorbed by a melting (s) is exactly the same as the qty of heat released when the (l) solidifies

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

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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.

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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

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ΔH accompany changes in state.

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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.

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Heat of Solution

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

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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

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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? ______

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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)

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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

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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

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The End

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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))

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

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

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

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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.

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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

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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

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Chapter 17