Energy Changes in Chemical Reaction

  1. There is energy changes when a chemical reaction takes place.
  2. The energy is in the form of heat, could be released or absorbed during the chemical reaction.
  3. There are 2 types of reaction with change in heat energy:
    1. a. Exothermic reaction
    2. b. Endothermic reaction
  4. The chart below shows the difference between this 2 reactions.

Exothermic Reaction

  1. Exothermic reaction is the chemical reaction that releases heat to the surrounding.
  2. When energy is given off during a chemical reaction, the temperature of the surrounding will increase.


Example of Exothermic Reaction

Reaction Equation
Neutralization Diluted sulphuric acid is neutralized by sodium hydroxide aqueous

H2SO4 + 2NaOH → Na2SO4 + 2H2O
Acid + carbonate Reaction between hydrochloric acid and sodium carbonate

2HCl + Na2CO3 → 2NaCl + CO2 + H2O
Calcium oxide + water
CaO + H2O → Ca(OH)2
 
Water + dehydrated copper(II) sulphate
CuSO4 + xH2O → CuSO4xH2O
Dissolve NaOH and KOH in water
NaOH + water → Na+ + OH
KOH + water → K+ + OH
Reaction between alkali metal (Li/Na/K) and water Sodium + Water

2Na + 2H2O → 2NaOH + H2
 
Reaction between reactive metal (Mg/Al/Zn/Fe) and diluted acid Magnesium + Nitric acid

Mg + 2HNO3 → Mg(NO3)2 + H2
Combustion Combustion of methane

CH4 + 2O2 → CO2 + 2H2O
Others
  • Rusting
  • Dilution of concentrated sulphuric acid/ concentrated nitric acid or others acid in water.
  • Crystallization of sodium tiosulphate.
  • Change of state from gaseous state to liquid state and liquid state to solid state.

Endothermic Reaction

  1. Endothermic reaction is a chemical reaction that absorbs heat from surroundings.
  2. When energy is taken from the surrounding during a chemical reaction, the temperature of the surrounding will decrease.



Example of Endothermic Reaction

Decomposition of carbonate metal by heat Decomposition of calcium carbonate by heat

CaCO3 → CaO + CO2
Decomposition of nitrate metal by heat Decomposition of Lead Nitrate by Heat

2Pb(NO3)2 → 2PbO + 4NO2 + O2
Dissolve potassium nitrate, ammonium nitrate or ammonium chloride in water.
KNO3 + Water → K+ + NO3
NH4NO3 + Water → NH4+ + NO3
NH4Cl + Water → NH4+ + Cl-
Reaction of heat on hydrated copper(II) sulphate.
CuSO4xH2O → CuSO4 + xH2O
Others
  • Change of physical state from solid to liquid or from liquid to gas.


Energy Level Diagram

  1. When a chemical reaction occurs, certain amount of heat is given off or absorb.
  2. The energy change in the chemical reaction can be presented by an energy level diagram.
  3. An energy level diagram it is a graph that shows the energy change in a chemical reaction.

Exothermic Reaction

  1. Figure below shows the general energy level diagram for exothermic reaction.
  2. We can see that the energy decreases after reaction. This is because energy is given off during an exothermic reaction.

Example

Zn + 2HCl →  ZnCl2 + H2
∆H =   126 kJ mol-1

The energy level diagram of the reaction above is as below:

Endothermic reaction
  1. Figure below shows the general energy level diagram for endothermic reaction.
  2. The energy increases after reaction. This is because energy is absorbed during an endothermic reaction.

Example

CuO + H2 → Cu + H2O
H= +130.5 kJ mol-1

The energy level diagram of the reaction above is as below:

Relationship between Energy Change and Breaking/Formation of Bonds

Breaking and Formation of Chemical Bond

  1. During a reaction, energy must be supplied to break bonds in the reactants, and energy is give out when the bonds in the products form.
  2. The amount of energy that absorbed or released depends on the strength of the bond.
  3. If the amount of energy released during the process of creating bond is higher than the amount of energy that taken in during process of breaking bond, the reaction is an exothermic reaction.
  4. If the amount of energy been absorbed to break the bond is higher than the amount of energy been released during the formation of chemical bond, the reaction is an endothermic reaction.

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