Calculating Heat of Reactions

Calorimetry

Process used for collecting heat data Calorimeter – device used Insulated cup containing water Reaction takes place in the water Heat released in exothermic reaction raises the

water’s temp Heat absorbed in endothermic reaction lowers

the water’s temp

Calculating Heat

Heat released to or absorbed by the water in any reaction can be calculated by multiplying three factors

Q = c x m x Δt

Since it is water that is gaining or losing the heat in the calorimeter, c is the specific heat of water (4.18 J/goC)

Heat (ΔH) of a Reaction

ΔH is always the J given off per mole of reactant (J/mol)

ΔH is Q divided by the number of moles that reacts

For exothermic reactions, ΔH is negative For endothermic reactions, ΔH is positive

Example

In a calorimeter containing 100 g of water at 21ºC, a reaction is carried out in which 3.5 g of NH4NO3 decomposes into N2O and H2O. The temperature raises to 25ºC. Find the heat of reaction.

Example

The reaction below is carried out in a calorimeter

HCl + NaOH → NaCl + H2O

2.5g of NaOH reacts with excess HCl. The calorimeter contains 150g of water at 19oC initially and the temperature rises to 24oC during the reaction. Calculate the heat of the reaction.

Practice

1.5g of Na reacts with excess water according to the reaction below.

2Na + 2H2O → 2NaOH + H2

When the reaction is carried out in a calorimeter containing 125g of water at 18oC, the temperature rises to 28oC. Calculate the heat of the reaction.