Chemistry 100

Enthalpy and Hess’s Law

Energy Changes in Chemical Reactions

Let’s take a typical reactionCH4 (g) + O2 (g) CO2 (g) + 2 H2O (l)

This is an example of ‘burning a fossil fuel.’ This reaction releases energy!

What about this reaction? KCl (s) KCl (aq)

This reaction requires energy!

Energy Changes

Almost all chemical and physical processes are accompanied by the loss or gain of energy from the system.

System

energy

surroundings

Some Definitions

Exothermic reaction ® heat is released from the system to the surroundings e.g., the combustion of methane

CH4 (g) + O2 (g) CO2 (g) + 2 H2O (l)

Endothermic reaction ® heat is supplied to the system by the surroundings. e.g., dissolving potassium chloride in water.

KCl (s) KCl (aq)

Enthalpy

Enthalpy H º the heat content of a substance.

pq H

Enthalpy change - DH thermal energy (heat) evolved or absorbed under constant pressure conditions).

Exothermic vs. Endothermic Processes

· Exothermic process ® heat content of system decreases, the system enthalpy decreases (i.e., DH < 0)

· Endothermic process ® energy of the system is increased (i.e. DH > 0).

The Enthalpy Change in a Chemical Reaction

We want to know how much energy is released when we burn ethane! C2H6 (g) + 7/2 O2 (g) 2 CO2 (g) + 3 H2O

(l) We wish to know the difference in the

heat content of the reactants vs. the products. H npH (products) - nr H (reactants)

np and nr represent the number of moles of products and reactants, respectively.

The Enthalpy Change (cont’d)

For the ethane combustion reaction1 C2H6 (g) + 7/2 O2 (g) 2 CO2 (g) + 3 H2O

(l)

H npH (products) - nr H (reactants)

= 3 H[H2O (l)] + 2 H[CO2 (g)] - (7/2 H[O2(g)] +

1 H[C2H6 (g)] )

Conventions for Writing Thermochemical Equations

For exothermic reactions, H <0; for endothermic reactions, H > 0.

H values are given for processes occurring at 25C and 1 atm pressure STTP - standard thermodynamic

temperature and pressure.

Conventions (cont’d)

The physical state of each substance participating in the process must be stated s solid l liquid g gas aq aqueous solutions

Conventions (cont’d)

The stoichiometric coefficients = the number of moles of each substance involved in the transformation

Fractional stoichiometric coefficients are permitted in thermochemical

equations.

Conventions (cont’d)

Multiply or divide a chemical equation by a factor, the H value must also be multiplied or divided by that factor Enthalpy is an extensive property.

When an equation is reversed, the sign of the H value is changed but its magnitude stays the same.

DH Values and Hess’s Law

Calculating enthalpy changes for physical and chemical transformations. tabulated values of reaction enthalpies

(largely measured with a calorimeter). indirect Method – Hess’s Law.

Hess’s Law

· Hess’s Law - enthalpy changes for sequences of reactions.· Enthalpy change accompanying a

reaction is the same whether the reaction occurs in a single step or in many steps.