The enthalpy (heat content) of a substance is given the symbol H.

The heat of reaction is the energy lost or gained during a chemical reaction.

. Since the heat of reaction is equal to the difference in enthalpy between the products and reactants.

∆H = H(products) – H(reactants)

•For exothermic reactions, ∆ H = < 0 (a negative value)

•For endothermic reactions, ∆ H = >0 (a positive value)

•During a chemical reaction, the atoms in the reactants are rearranged into products with different chemical energies.

Exothermic reactions

These are balanced chemical reactions which display the energy lost ( negative/exothemic) and energy absorbed (positive/endothermic) at the end of the equation.

This is shown with the symbol delta H and is measured in J/mol or KJ/mol.


6CO2(g) + 6H2O(l) —> C6H12O6(aq) + 6O2(g) ∆H = +2803 kJ mol –1

For any reaction, ΔH is directly proportional to the amount of substance.

If twice as much of the reactants is used then the ΔH would also be doubled.

When working with thermochemical equations, you need to be aware that:

•The coefficients of the reactants indicate the amounts, in mole, of each substance that react to give the specified heat change.

•States of reactants and products must be specified, since energy changes occur with a change of state.

•If a reaction occurs in reverse, it has the same magnitude of ΔH but the opposite sign.


•Calculate the heat energy released when 50.00mL of 0.200M sodium hydroxide reacts with dilute excess hydrochloric acid.

H+(aq) + OH-(aq) à H2O(l); ΔH=57.2kJ mol-1

From the equation: 1 mol of NaOH releases 57.2kJ of energy.

n(NaOH) = 0.200 x 0.0500 = 0.0100mol

Let 0.0100mol of NaOH release x kJ

By proportion: x = 0.0100

57.2 1

x = 0.0100 x 57.2 = 0.52kJ

Activation Energy (EA)

The activation energy is the energy required to break bonds so that a reaction can start.

Energy changes during a reaction can be displayed in an energy profile diagram.

Exothermic reactions

When the activation energy is less than the energy released when the “new” bonds form there is an overall release of energy, (usually as heat released to the surroundings)

ΔH is negative and an exothermic reaction has taken place

Exothermic energy profile diagram courtesy of By User:Slashme – Re-drawn from en:Image:Ac_com.png, Public Domain,

When the activation energy is greater than the energy released when the “new” bonds form there is an overall input of energy (usually as heat absorbed from the surroundings)

ΔH is positive and an endothermic reaction has taken place.

Endothermic energy profile diagram courtesy of By Brazosport College – Own work, CC BY-SA 3.0,


•The higher the EA the harder it is to initiate a reaction.

• EA has no effect on whether a reaction is endothermic or exothermic.

•The EA for the reverse reaction is the difference between the products and the highest part of the energy profile.

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