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Energy Changes Accompanying Physical and Chemical Reactions | Enthalpy Change

When substances undergo change (physical or chemical), energies are involved. These energies are usually heat change, also known as Enthalpy Change, and can either be evolution (given off) or absorption (taking in).

Heat change can be measured in the laboratory using the calorimeter. If heat is evolved, the change or reaction is regarded as exothermic; if heat is absorbed, the change is endothermic. The reason for the heat change is that, a change (either physical or chemical) involves:

The breaking of bonds in the reactants - this requires energy.

The formation of new bonds (for chemical change) or formation of forces of attraction (for physical change) between the reactant particles.

The sum of the heat energies of the above processes gives the enthalpy change, ∆H.

The heat change accompanying a chemical change can also be determined by taking the difference between the heat of formation of products and the heat of formation of the reactants.

I.e. ∆H = H_{f (products)} – H_{f (reactants)}

Simple Calculations involving Reactions

Example: calculate the standard enthalpy change (∆H^Ɵ) for the reaction:

CH_4(g)+ 4CuO_(s) → CO_2(g) + 2H₂O(l) + 4Cu(s)

Given that H^Ɵ_f for CH_4(g) = -74.96 kJ mol^-1

H^Ɵ_f for CuO(s) = -157.54 kJ mol^-1

H^Ɵ_f for CO_2(g) = -394.07 kJ mol^-1

H^Ɵ_f for H₂O_(l) = -286.26 kJ mol^-1

Solution: ∆H^Ɵ = H^Ɵ_f (products) – H^Ɵ_f (reactants)

H^Ɵ_f(products) = H^Ɵ_f(CO₂) + H^Ɵ_f(2H₂O) + H^Ɵ_f(4Cu)

                    = - 394.07 + 2(-286.26) + 4(0)

                    = - 394.07 - 572.52 + 0

                    = - 966.59 kJ

Note: the standard heat of formation of elements is zero.

Therefore, H^Ɵ_f of 4Cu(s) = 0

H^Ɵ_f (reactants) = H^Ɵ_f (CH₄) + H^Ɵ_f (4CuO)

                      = - 74.96 + 4(-157.54)

                      = - 74.96 - 630.16

                      = - 705.12 kJ

Therefore, ∆H^Ɵ = - 966.59 – (- 705.12) kJ = - 966.59 + 705.12 = - 261.47 kJ