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Page 1: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

THERMOCHEMISTRY

Page 2: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Energy

• The ability to do work or transfer heat.– Work: Energy used to cause an

object that has mass to move.– Heat: Energy used to cause the

temperature of an object to rise.

Page 3: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Definitions #1

Energy: The capacity to do work or produce heat

Potential Energy: Energy due to position or composition

Kinetic Energy: Energy due to the motion of the object

2

2

1mvKE

Page 4: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Definitions #2

Law of Conservation of Energy: Energy can neither be created nor destroyed, but can be converted between formsThe First Law of Thermodynamics: The total energy content of the universe is constant

Page 5: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

E = q + wE = change in internal energy of a system

q = heat flowing into or out of the system

-q if energy is leaving to the surroundings+q if energy is entering from the surroundings

w = work done by, or on, the system

-w if work is done by the system on the surroundings

+w if work is done on the system by the surroundings

Page 6: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Work problemsChapter 5

• 5.25• 5.27 A and B• 5.31 All

Page 7: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Calorimetry

The amount of heat absorbed or released during a physical or chemical change can be measured…

…usually by the change in temperature of a known quantity of water1 calorie is the heat required to raise the temperature of 1 gram of water by 1 C1 BTU is the heat required to raise the temperature of 1 pound of water by 1 F

Page 8: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

The Joule

The unit of heat used in modern thermochemistry is the Joule

2

2111

s

mkgmeternewtonjoule

1 joule = 4.184 calories

Page 9: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

A Bomb Calorimete

r

Page 10: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

A Cheaper

Calorimeter

Page 11: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Specific HeatThe amount of heat required to raise the temperature of one gram of substance by one degree Celsius.Substance Specific Heat (J/g·K)

Water (liquid) 4.18

Ethanol (liquid) 2.44

Water (solid) 2.06

Water (vapor) 1.87

Aluminum (solid) 0.897

Carbon (graphite,solid) 0.709

Iron (solid) 0.449

Copper (solid) 0.385

Mercury (liquid) 0.140

Lead (solid) 0.129

Gold (solid) 0.129

Page 12: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Calculations Involving Specific Heat

Tmsq

s = Specific Heat Capacity

q = Heat lost or gainedT = Temperature

change

OR

Tm

qs

Page 13: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Problems

• 5.53 a and B

Page 14: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

State Functions depend ONLY on the present state of the system

ENERGY IS A STATE FUNCTION

A person standing at the top of Mt. Everest has the same potential energy whether they got there by hiking up, or by falling down from a plane!

WORK IS NOT A STATE FUNCTION

WHY NOT???

Page 15: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

State Functions

Usually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem.

Page 16: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

State Functions• However, we do know that the

internal energy of a system is independent of the path by which the system achieved that state.– In the system below, the water could

have reached room temperature from either direction.

Page 17: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

State Functions

• Therefore, internal energy is a state function.

• It depends only on the present state of the system, not on the path by which the system arrived at that state.

• And so, E depends only on Einitial and Efinal.

Page 18: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

State Functions

• However, q and w are not state functions.

• Whether the battery is shorted out or is discharged by running the fan, its E is the same.– But q and w are

different in the two cases.

Page 19: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Work

When a process occurs in an open container, commonly the only work done is a change in volume of a gas pushing on the surroundings (or being pushed on by the surroundings).

Page 20: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

WorkWe can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston.

w = −PV

Page 21: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Work, Pressure, and Volume

VPw Expansion Compression+V (increase)

-V (decrease)

-w results +w results

Esystem decreases

Work has been done by the system on the surroundings

Esystem increases

Work has been done on the system by the surroundings

Page 22: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Energy Change in Chemical Processes

Endothermic:

Exothermic:

Reactions in which energy flows into the system as the reaction proceeds.

Reactions in which energy flows out of the system as the reaction proceeds.

+ qsystem - qsurroundings

- qsystem + qsurroundings

Page 23: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Endothermic Reactions

Page 24: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Exothermic Reactions

Page 25: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Enthalpy• If a process takes place at constant pressure (as the

majority of processes we study do) and the only work done is this pressure-volume work, we can account for heat flow during the process by measuring the enthalpy of the system.

• Enthalpy is the internal energy plus the product of pressure and volume:

H = E + PV

At constant pressure and volume the change in enthalpy is the heat gained or lost

H = q

Page 26: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Enthalpies of Reaction

The change in enthalpy, H, is the enthalpy of the products minus the enthalpy of the reactants:

H = Hproducts − Hreactants

Page 27: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Enthalpies of Reaction

This quantity, H, is called the enthalpy of reaction, or the heat of reaction.

Page 28: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Problems

5.57 A and B

Page 29: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law“In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or a series of steps.”

Page 30: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law

Page 31: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law Example Problem

Calculate H for the combustion of methane, CH4:

CH4 + 2O2 CO2 + 2H2O    Reaction Ho  

C + 2H2 CH4 -74.80 kJ

C + O2 CO2 -393.50 kJ

H2 + ½ O2 H2O -285.83 kJ

Step #1: CH4 must appear on the reactant side, so we reverse reaction #1 and change the sign on H.

CH4 C + 2H2 +74.80 kJ

Page 32: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law Example Problem

Calculate H for the combustion of methane, CH4:

CH4 + 2O2 CO2 + 2H2O    Reaction Ho  

C + 2H2 CH4 -74.80 kJ

C + O2 CO2 -393.50 kJ

H2 + ½ O2 H2O -285.83 kJ

CH4 C + 2H2 +74.80 kJ

Step #2: Keep reaction #2 unchanged, because CO2 belongs on the product side

C + O2 CO2 -393.50 kJ

Page 33: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law Example Problem

Calculate H for the combustion of methane, CH4:

CH4 + 2O2 CO2 + 2H2O    Reaction Ho  

C + 2H2 CH4 -74.80 kJ

C + O2 CO2 -393.50 kJ

H2 + ½ O2 H2O -285.83 kJ

CH4 C + 2H2 +74.80 kJ C + O2 CO2 -393.50 kJ

Step #3: Multiply reaction #2 by 2

2H2 + O2 2 H2O -571.66 kJ

Page 34: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Hess’s Law Example Problem

Calculate H for the combustion of methane, CH4:

CH4 + 2O2 CO2 + 2H2O    Reaction Ho  

C + 2H2 CH4 -74.80 kJ

C + O2 CO2 -393.50 kJ

H2 + ½ O2 H2O -285.83 kJ

CH4 C + 2H2 +74.80 kJ C + O2 CO2 -393.50 kJ

2H2 + O2 2 H2O -571.66 kJ

Step #4: Sum up reaction and H

CH4 + 2O2 CO2 + 2H2O -890.36 kJ

Page 35: THERMOCHEMISTRY. Energy The ability to do work or transfer heat.The ability to do work or transfer heat. –Work: Energy used to cause an object that has

Calculation of Heat of Reaction Calculate H for the combustion of

methane, CH4: CH4 + 2O2 CO2 + 2H2OHrxn = Hf(products) - Hf(reactants)

    Substance Hf  

CH4 -74.80 kJ

O2 0 kJ

CO2 -393.50 kJ

H2O -285.83 kJ

Hrxn = [-393.50kJ + 2(-285.83kJ)] – [-74.80kJ]

Hrxn = -890.36 kJ


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