Energy Relationships in Chemical Reactions

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

Thermochemical Definitions

System : That part of the Universe whose change we are going to measure.Surroundings : Every thing else that is relevant to the change is defined as the “surroundings”.Internal Energy : The sum of the kinetic and potential energy of all the particles in a system.Heat (q) : Is the energy transferred between a system and it’s surroundings as result in the differences in their temperatures only!Work (w) : The energy transferred when an object is moved by a force.

Therefore: E = q + w

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

Change in Enthalpy = H

Enthalpy is defined as the system’s internal energyplus the product of its pressure and volume.

H = E + PVFor a change in enthalpy:

H = E + PV Exothermic and Endothermic Reactions:

H = H final - H initial = H products - H reactants

Exothermic : H final H initial H 0

Endothermic : H final H initial H 0

For processes occurring at constant pressure the enthalpy change equals the heat gained or lost.

H = qp

enthalpy of reaction or heat of reaction. (Energy change + small correction factor.)

Constant-Pressure Calorimetry

No heat enters or leaves!

qsys = qwater + qcal + qrxn

qsys = 0

qrxn = - (qwater + qcal)qwater = msDt

qcal = CcalDt

6.4

Reaction at Constant PDH = qrxn

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 = msDt

q = CDtDt = 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

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

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

The standard enthalpy of reaction (DH0 ) is the enthalpy of a reaction carried out at 1 atm.

rxn

aA + bB cC + dD

DHorxn dDHo (D)fcDHo (C)f= [ + ] - bDHo (B)faDHo (A)f[ + ]

DHorxn nDHo (products)f= S mDHo (reactants)fS-

Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

(Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end.)

Hess’s Law of Heat Summation

The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps.

Example: Problem: Calculate the energy involved in the oxidation of elemental sulfur to sulfur trioxide from reactions: 1) S (s) + O2 (g) SO2 (g) H1 = -296.0 kJ

2) 2 SO2 (g) + O2 (g) 2 SO3 (g) H2 = -198.2 kJ

3) S (s) + 3/2 O2 (g) SO3 (g) H3 = ?

Hess’s Law of Heat Summation

The enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps.

Example: Problem: Calculate the energy involved in the oxidation of elemental sulfur to sulfur trioxide from reactions:2 X 1) S (s) + O2 (g) SO2 (g) 2H1 = -296.0 kJ X2+ 2) 2 SO2 (g) + O2 (g) 2 SO3 (g) H2 = -198.2 kJ

3) S (s) + 3/2 O2 (g) SO3 (g) H3 = ?

H3 = 2H1 + H2