An exothermic reaction transfers thermal energy to the surroundings leading to an increase in the temperature of the surroundings.
An endothermic reaction takes in thermal energy from the surroundings leading to a decrease in the temperature of the surroundings.
The equation for exo. reactions is described as :
reactants $\rightarrow$ products + thermal energy
The reactants and products have energy. It is stored as chemical energy in their bonds. The total energy does not change, during a reaction. It is the same on each side of the arrow. The products must have less energy than the reactants do.
In an exothermic reaction, the products have less energy than the reactants do. Energy has been given out.
The equation for endo. reactions is described as :
reactants + thermal energy -> products
The total energy does not change, during the reaction. It is the sam eon each side of the arrow. So it follows that the products must have more energy than the reactants do.
In an endothermic reaction, the products have more energy than the reactants do. Energy has been taken in.
Activation energy is the minimum energy that colliding particles must have to react.The transfer of thermal energy during a reaction is called the enthalpy change , delta H of a reaction.
Delta H is negative for exo. reactions and positive for endo. reactions .
Bond breaking is an endo. process.
Enthalpy change : Positive. The bonds in the reactant particles must be broken, taking in energy.
Bond making is an exo. process.
Enthalpy change : Negative. New bonds are made, giving the product particles, which releases energy.
The overall change in enthalpy of the reaction is found like this:
enthalpy change for the reaction = energy in for bond breaking - energy out from bond making
Energy changes in reactions are given as kJ/mol .
The bond energy is the energy needed to break bonds, or released when the same bonds form. It is given in kJ/mol.
Equation: H2 + Cl2 -> 2HCl
Bond equation: H-H + Cl-Cl -> 2 H-Cl
Energy in to break each mole of bonds in the reactants (given in exam) :
1 x H-H, 436 kJ
1 x Cl-Cl, 242 kJ
Total energy in : 436 + 242 = 678 kJ
Energy out from the two moles of bonds that form to give the product:
2 x H-Cl which is 2 x 431 = 862 kJ
Enthalpy change $\delta H$ = energy in - energy out
678 kJ - 862 kJ = -184 kJ
It is negative , therefore it is an exo. reaction. 184 kJ of energy is given out when 1 mole of hydrogen and 1 mole of chlorine react.
Equation: 2NH3 -> N2 + 3H2
Energy in to break the two moles of bonds:
6 x N-H which is 6 x 391 = 2346 kJ
Energy out from the four moles of bonds forming:
1 x N≡N, 946 kJ
3 x H-H, 3 x 436 = 1308 kJ
Total energy out : 947 + 1308 = 2254 kJ
Enthalpy change delta H = energy in - energy out
2346 kJ - 2254 kJ = +92 kJ
It is positive, indicating that this is an endo. reaction. So 92 kJ of energy is taken in to decompose 2 moles of ammonia.