Energy Change during Formation and Breaking of Bonds

Energy Change during Formation and Breaking of Bonds
Bond breaking

• Usually chemical bonds of the reactant.
• Heat energy is absorbed.

Bond forming

• Usually new chemical bonds of the product.
• Heat energy is given out.

Relationship between energy change and the formation and breaking of bonds

• In a chemical reaction, if the heat energy absorbed in bond breaking is lower than the heat energy given out in bond forming, the reaction is an exothermic reaction.
• Example: ΔH (bond breaking) = +600 kJ, ΔH (bond forming) = -800 kJ, ΔH (heat of reaction) = [(+600) + (-800)] kJ= -200 kJ
• In a chemical reaction, if the heat energy absorbed in bond breaking is higher than the heat energy given out in bond forming, the reaction is an endothermic reaction.
• Example: ΔH (bond breaking) = +800 kJ, ΔH (bond forming) = -600 kJ, ΔH (heat of reaction) = [(+800) + (-600)] kJ= +200 kJ

Applications of Exothermic and Endothermic Reaction in Everyday Life
Hot pack

• Contains of anhydrous calcium chloride / anhydrous magnesium sulphate / wet iron powder and sodium chloride / calcium oxide.
• Uses: reduce swelling and mucles or joint sprain.

Cold pack or Ice pack

• Contains of ammonium nitrate / potassium nitrate / sodium thiosulphate.
• Uses: reduce swelling, muscles or joint sprain and reduce fever.

*Berry Important Notes
You must be able to

1. Calculate the number of moles of salt precipitated / metal displaced / water produced / fuel used;
2. Calculate the heat energy released (ΔH); and
3. Calculate the heat of precipitation / heat of displacement / heat of neutralization / heat of combustion.

Heat of Precipitation (Form 4, Chapter 8 Salts)

1. Heat of precipitation – the heat change when one mole of a precipitate is formed from their ions in aqueous solution.
2. Precipitation reaction = double decomposition which is used to prepare insoluble salts.
3. Heat change of a solution = mcθ Joule [m = mass of the solution (g), c = specific heat capacity of the solution (J g^-1˚C^-1), θ = temperature change in the solution (˚C)]
4. Heat change in a reaction, mcθ = n x ΔH
5. Heat of reaction / Heat of precipitation, ΔH = mcθ / n

Example 1:
Chemical reaction: Pb(NO₃)₂(aq) + 2KI(aq) –> PbI₂(s) + 2KNO₃(aq)
Ionic reaction: Pb²⁺(aq) + 2I^-(aq) –> PbI₂(s)
Heat of precipitation of PbI₂ = – Heat change / Number of moles of PbI₂
Example 2:
Chemical reaction: BaCl₂(aq) + Na₂SO₄(aq) –> BaSO₄(s) + 2NaCl(aq)
Ionic reaction: Ba²⁺(aq) + SO₄^2-(aq) –> BaSO₄(s)
Heat of precipitation of BaSO₄ = – Heat change / Number of moles of BaSO₄

Heat of Displacement (Form 4, Chapter 6 Electrochemistry & Form 5, Chapter 3 Oxidation and Reduction)

1. Heat of displacement – the heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal.
2. Heat change of the reaction mixture / Heat energy released / Heat given out in the reaction = mcθ Joule
3. Heat change in a reaction, mcθ = n x ΔH
4. Heat of reaction / Heat of displacement, ΔH = mcθ / n

Example 1:
Chemical equation: Mg(s) + FeCl₂(aq) –> MgCl₂(aq) + Fe(s)
Ionic equation: Mg(s) + Fe²⁺(aq) –> Mg²⁺(aq) + Fe(s)
Example 2:
Chemical equation: Zn(s) + CuSO₄(aq) –> ZnSO₄(aq) + Cu(s)
Ionic equation: Zn(s) + Cu²⁺(aq) –> Zn²⁺ (aq) + Cu(s)