Exothermic and Endothermic Reactions

Section 5.1

The study of energy changes associated with chemical reactions

Most reactions absorb or evolve energy This energy can be in the form of heat,

light, or mechanical energy Amount of energy is measured in

joules, J, or usually kJ since the joule is a small amount

Thermochemistry

Enthalpy is the total energy of a system, also known as heat content and abbreviated as “H”

Some of the energy of a system is stored as chemical potential energy in chemical bonds

Bonds are broken and formed during chemical reactions

Bonds broken require an energy input and bonds formed release energy

Enthalpy

All reactions are accompanied by a change in the potential energy of the bonds

This is called the enthalpy change Enthalpy cannot be measured directly, but

the enthalpy change can be measured The symbol for the enthalpy change of

reaction is ΔH and the units are kJ mol-1

Enthalpy Change

ΔH is equal to the difference in enthalpy between the reactants and the products

This assumes that the reaction occurs with no change in temperature or pressure, or that these conditions are restored to their initial values

If this is true, then the ΔH is equal to the change in the potential energy of the chemical bonds

More about ΔH

Sometimes a reaction occurs in which the enthalpy of the products is greater than the enthalpy of the reactants

This means that the ΔH is positive Heat energy was absorbed from the

surroundings The surroundings either get colder, or

heat is supplied by an external source Reactants are more stable than the

products as the bonds made are weaker than the bonds broken

Endothermic Reactions

Sometimes a reaction occurs in which the enthalpy of the reactants is greater than the enthalpy of the products

This means that ΔH is negative The chemical reaction releases heat

energy to the surroundings Products are more stable than the

reactants as the bonds made are stronger than the bonds broken

Exothermic Reactions

Ea or Ae The minimum amount of energy

required for a chemical reaction to occur

Must be overcome for the reaction to “go”

Activation Energy

Energy Level Diagrams

Abbreviated as ΔH ө Has special conditions: 1 atm of pressure, or 101.3 kPa Solutions are at 1 mol dm-3

Temperature of 298 K (25° C) These conditions are known as

thermochemical standard conditions

Standard Enthalpy Change

Combustion reactions are exothermic (you should know this!)

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

Acid-base neutralization reactions are also exothermic (more about those later, but you should remember:

Do as you otta’ add acid to wata’ is due to this fact!) The LAB you did where you measured the change in enthalpy

More about Exothermic Reactions

When a chemical reaction is written with an associated change in enthalpy: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) ΔH = -890 kJ

mol-1

NH4NO3(s) + aq → NH4+

(aq) + NO3-(aq) ΔH = +25 kJ mol-1

You can easily tell which reaction is exothermic and which is endothermic from the ΔH

Thermochemical Equation