Chapter 5: Unit 11. Heat Energy and Stoichiometry

Heat Energy and Stoichiometry

The heat of reaction is also called enthalpy change and symbolized by∆H, the energy absorbed or released in a reaction. In an endothermic reaction, energy is absorbed. Sign of ∆H is positive

If written separately, on the reactant side if mentioned in the reaction. On the energy diagram products are on the higher energy level than the reactants. It means reactants bonds are stronger than products bond.

In an exothermic reaction, energy is released. Sign of ∆H is negative if written separately, on the product side if mentioned in the reaction. On the energy diagram products are on the lower energy level than the reactants. It means products bonds are stronger than reactants bonds.

N₂ + 3H₂ → 2 NH₃(g) + 92.5 kJ

2 NaHCO₃+ 129 kJ → Na₂CO₃ + H₂O + CO₂

The above two reactions are examples of exothermic and endothermic reaction.  The diagram below  is the enthalpy diagram for exo- and endothermic reaction.

A balanced chemical equation gives the enthalpy value. The magnitude of enthalpy is proportional to the amount of substance.

In example #1, if 222.4  g of N₂ reacts, how many kilojoules of heat will be released?

We can use the balanced equation coefficients to find the amount of heat.

222.4 g N2 * 1 mol N₂             *    -92.5 KJ                = -74.0 kJ

28.00 g N₂                 1 mol N₂

Similarly in endothermic reaction, energy is absorbed and similar calculation can be performed.

Questions:

Ans: 1. 201 kcal 2. Exothermic, 637 kcal released