Thermochemistry• Heat Transfer and Specific Heat• Energy Changes in Chemical React

ions• Calculating ∆H

Heat

DefinitionThe energy transferred between samples of matter due to differences in their temperature.

Standard SI Unit Joule (J)

Other Common Unit

Calorie (cal)A calorie is the quantity of heat that raises the temperature of 1 g of pure water 1°C.

1 calorie = 4.18 J

Heat Transfer

Heat transfer – the transfer of energy, in the form of heat, from material at a higher temperature to a material at a lower temperature.

The three methods of transferring heat are conduction, convection and radiation.

Conduction - What is it?

Conduction is the transfer of heat by the direct contact of particles of matter.

Conduction can occur in all three states of matter.

Conduction - How does it happen?

1. Where two particles are in contact there are lots of collisions between the particles of each.

2. The particles with greater kinetic energy transfer some of that energy to the particles with less kinetic energy.

3. As energy is transferred, the temperature of the warmer object decreases and the temperature of the cooler object increases.

Examples of Conduction

Convection - What is it?

Convection is the transfer of heat by the actual motion of a fluid (liquid or gas) in the form of currents.

Convection does not occur in solids.

Convection currents are responsible for our weather. Warm air is less dense than cold air so warm air rises.

Examples of Convection

Radiation - What is it?

Radiation is heat transfer by electromagnetic waves.

Examples of Radiation

More Examples of Heat Transfer

Specific Heat

Specific Heat is the quantity of heat required to raise the temperature of one gram of a substance by one degree Celsius.

Formula for Calculating Specific

Heat

C p q

mass T( )

q mass T ( ) C p

The equation can be rearranged algebraically to solve for the amount of energy released or absorbed.

The specific heat of water is 4.18 J/g•°C or 1.00 cal/g•°C.

Cp = specific heat

q = energy released or absorbed

ΔT= change in temperature

Which metal will absorb the most energy?

Aluminum has a specific heat of 0.900 J/g•°C. Lead has a specific heat of 0.13 J/g•°C. If equal masses of both metals are heated to 100°C, which metal will absorb the most energy?

The aluminum will absorb the most energy because it has a higher specific heat.

1. The temperature of a piece of copper with a mass of 95.40 g changes from 25.0°C to 48.0°C when the metal absorbs 849 J of energy. What is the specific heat of copper?

C = 849 J

(95.40 g)(23.0 C) = 0.387

J

g Cp

q

mass T( )

C = 849 J

(95.40 g)(23.0 C) = 0.387

J

g Cp

q

mass T( )

2. The temperature of an unknown piece of metal with a mass of 50.00 g changes from 30.0°C to 60.0°C when the metal absorbs 400.0 J of energy. What is the specific heat of the metal?

C J

(50.00 g)(30.0 C)

J

g Cp

q

mass T( ).

400

0 267

C J

(50.00 g)(30.0 C)

J

g Cp

q

mass T( ).

400

0 267

C J

(50.00 g)(30.0 C)

J

g Cp

q

mass T( ).

400

0 267

3. How much energy is required to change the temperature of 10.00 g of water from 30.0°C to 55.0°C?

q mass T

( ) ) C = 10.00 g (25.0 C)(4.18 J

g C = 1045 J = 1050 J = 1.05 10 Jp

3

q mass T

( ) ) C = 10.00 g (25.0 C)(4.18 J

g C = 1045 J = 1050 J = 1.05 10 Jp

3

q mass T

( ) ) C = 10.00 g (25.0 C)(4.18 J

g C = 1045 J = 1050 J = 1.05 10 Jp

3

A coffee-cup calorimeter is commonly used to determine the heat of a reaction at constant pressure or to calculate the specific heat of a metal.

Calorimetry is used to determine the amount of heat released or absorbed during a chemical or physical change.

Energy Changes in Chemical Reactions

One indication that a chemical reaction has occurred is a change in energy.

This change in energy may be in the form of heat, light or sound.

Thermochemical Equations

A thermochemical equation is an equation that includes the heat change.

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l) + 1367 kJ

Heat of Reaction, ΔH

The heat of reaction (ΔH) (also called the enthalpy of reaction) is the quantity of energy absorbed or released as heat during a chemical reaction.

Heat of Reaction, ΔH

If ΔH is negative, the reaction is exothermic and energy is being released. However, when heat is shown as a “product” in an equation, it is expressed as an absolute value. Exothermic reactions are more common in nature.

CaO(s) + H2O(l) → Ca(OH)2(s) ΔH = -65.2 kJ

CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ

Heat of Reaction, ΔH

If ΔH is positive, the reaction is endothermic and energy is being absorbed.

2NaHCO3(s) → 2Na2CO3(s) + H2O(g) + CO2(g) ΔH = 129 kJ

2NaHCO3(s) + 129 kJ → 2Na2CO3(s) + H2O(g) + CO2(g)

For each equation listed below, determine the ΔH and type of reaction (endothermic or exothermic).

C(s) + O2(g) → CO2(g) + 393.51 kJ

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + 890.31 kJ

CaCO3(s) + 176 kJ → CaO(s) + CO2(g)

ΔH = -393.51 kJ; exothermic

ΔH = -890.31 kJ; exothermic

ΔH = +176 kJ; endothermic

Interpreting Reaction Progress Diagrams

Potential Energy of the Reactants Activation Energy of the Forward Reaction Potential Energy of the Activated Complex Heat of Reaction (∆H) Potential Energy of the Products Activation Energy of Reverse Reaction

ABC+ E C

DE

C C-E or B-D

Heat of Combustion (ΔHc)

The heat of combustion (ΔHc) is the energy released by the complete combustion of one mole of a substance.

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) ΔHc = -2219.2 kJ/mol

Note: In this reaction the heat of combustion and the heat of reaction would be the same since only one mole of C3H8 is being combusted.

Heat of Formation (ΔHf)

The heat of formation (ΔHf) is the energy absorbed or released in the formation of one mole a compound from its elements in their standard state.

H2(g) + ½O2(g) → H2O(l) ΔHf = -285.8 kJ/mol

What would be the heat of reaction for the reverse reaction?+285.8 kJ/mol

What would be the heat of reaction for the following reaction?

2H2(g) + O2(g) → 2H2O(l) 2(-285.8 kJ) = -571.6 kJ

Stoichiometrical Calculations1. Use the equation below to

calculate the kilojoules of heat required to decompose 2.24 mol NaHCO3.

2NaHCO3(s) + 129 kJ → 2Na2CO3(s) + H2O(g) +CO2(g)

Stoichiometrical Calculations2. Use the equation below to

determine the amount of heat released when 100.0 g of calcium oxide reacts with excess water.

CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ

Stoichiometrical Calculations3. The heat of combustion for

ethane (C2H4) is -1390 kJ/mol. Calculate the amount of heat produced when 4.79 g C2H4 reacts with excess oxygen.

C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) + 1390 kJ

Hess’s Law

There are several different ways in which the enthalpy change can be calculated from a reaction.

The first method we are going to look at is called Hess’s Law.

Hess’s Law states that the overall enthalpy change in a reaction is equal to the sum of the enthalpy changes for the individual steps in the process.

General Principles for Combining

Thermochemical Equations1. If a reaction is reversed, the sign of ∆H is also reversed.

2. If the coefficients in a balanced equation are multiplied by an integer, the value of ∆H is also multiplied by the same integer.