ThermochemistryChapter 5

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Energy is the capacity to do work

• Thermal energy is the energy associated with the random motion of atoms and molecules

• Chemical energy is the energy stored within the bonds of chemical substances

• Nuclear energy is the energy stored within the collection of neutrons and protons in the atom

• Electrical energy is the energy associated with the flow of electrons

• Potential energy is the energy available by virtue of an object’s position

6.1

Heat is the transfer of thermal energy between two bodies that are at different temperatures.

Energy Changes in Chemical Reactions

Temperature is a measure of the thermal energy.

Temperature = Thermal Energy

900C400C

greater thermal energy6.2

Thermochemistry is the study of heat change in chemical reactions.

The system is the specific part of the universe that is of interest in the study.

open

mass & energyExchange:

closed

energy

isolated

nothing

SYSTEM

6.2

The First Law of Thermodynamics

Energy is conserved.

Internal Energy includes all kinetic and potential energy in a system.

ΔE = Efinal - Einitial

Relating ΔE to Heat and Work

ΔE = q + w

For q + means system gains heat

- means system loses heat

For w + means work don on the system

- means work done by the system

For ΔE + means net gain of energy by system

- means net loss of energy by the system

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.

Endothermic process is any process in which heat has to be supplied to the system from the surroundings.

2H2 (g) + O2 (g) 2H2O (l) + energy

H2O (g) H2O (l) + energy

energy + 2HgO (s) 2Hg (l) + O2 (g)

6.2

energy + H2O (s) H2O (l)

EnthalpyHeat flow in a chemical reaction when no work is done. (constant pressure systems)

ΔH = change in enthalpy

Calculation of Enthalpy(H) Stoichiometrically: (Using

known values and stoichiometry)

When potassium hydroxide is added to water, the following reaction takes place

KOH (s) KOH (aq) ΔH=-43 kJ/mol

When 14.0 g of KOH is added to water:

a)Does the beaker get warmer or colder?

b)Is the reaction endo or exothermic?

c)What is the enthalpy change for the reacton?

There are five ways to calculate enthalpy:

1.Stoichiometrically

2.From tables of standard values

3.Hess’s Law

4.Calorimetry

5.Bond Energies

Properties of Enthalpy

• The change in enthalpy equals the heat gained or lost at constant pressure

• The enthalpy change for a reverse reaction is equal to the forward reaction with the opposite sign

• If the reaction is multiplied; the value of ΔH is multiplied by the same number

• The enthalpy change for a reaction depends on the state of the reactants and products

The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius.

The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius.

C = ms

Heat (q) absorbed or released:

q = mst

q = Ct

t = tfinal - tinitial

6.4

How much heat is given off when an 869 g iron bar cools from 940C to 50C?

s of Fe = 0.444 J/g • 0C

t = tfinal – tinitial = 50C – 940C = -890C

q = mst = 869 g x 0.444 J/g • 0C x –890C = -34,000 J

6.4

Constant-Pressure Calorimetry

No heat enters or leaves!

qsys = qwater + qcal + qrxn

qsys = 0

qrxn = - (qwater + qcal)

qwater = mst

qcal = Ccalt

6.4

Reaction at Constant PH = qrxn

Bomb Calorimetry (Constant-Volume)

This device has a known heat capacity (Ccal) that is determined experimentally.

qrxn = -Ccal x ΔT

Example

A 1.800 g sample of phenol (C6H5OH) was burned in a bomb calorimeter whose total heat capacity is 11.66 J/oC. The temperature increased from 21.36 to 26.37 degrees celcius.

a)Write a balanced equation for the reaction.

b)What is the heat of combustion per gram of phenol? Per mole?

Answer

Hess’s Law

The ΔH for a chemical reaction carried out in a series of steps can be determined by summing the ΔH for each step.

Take the steps and rearrange them so that they will combine to give the overall equation for the reaction. (See example on the board)

Using Standard Enthalpies of Formation

Example