4.1 Energy Changes in Chemical Reactions

Changes of energy
■ Energy changes

	►	Occur in almost all chemical reactions.
	►	Heat of reaction, ∆H is the amount of heat given out or absorbed during the reaction.
	►	Energy changes during a chemical reaction are expressed in joules (J) or kilojoules (kJ) (1 kJ = 1000 J).

■ Heat changes in chemical reactions can be categorised as two types:

	►	Exothermic reactions
	►	Endothermic reactions

■ This video contains information regarding energy changes in chemical reaction

Exothermic reactions
■ Exothermic reaction

	►	Chemical reaction in which heat energy is released to it surroundings.
	►	The released heat energy causes the rise of temperature in the surroundings.
	►	The value of ∆H for exothermic reaction is negative because of the heat energy released.
	►	The following animation shows the heat change during the exothermic reactions.

■ Example of exothermic reactions :

	►	Combustion of fuel in excess oxygen. ○ C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O
	►	Neutralisation reaction between acid and alkali. ○ NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
	►	Dilution of concentrated sulphuric acid. ○ ${\text{H}}_2{\text{SO}}_4(\text{concentrated})\begin{array}{c}\to \\ \text{water}\end{array}({\text{H}}_2){\text{SO}}_4(\text{aq})$
	►	Reaction between active metals and water. ○ 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)

■ This video contains information regarding exothermic reactions.

Endothermic reactions
■ Endothermic reaction

	►	Reaction in which heat energy is absorbed from the surroundings.
	►	Absorption of heat energy causes the temperature of the surrounding area to decrease.
	►	The value of ∆H for endothermic reaction is positive because of the heat energy absorbed. Therefore, the total energy content of the products is higher than the total energy of reactants.
	►	The following animation shows the heat change during the endothermic reactions.

■ Example of endothermic reactions :

	►	Dissolving ammonium nitrate in water. ○ NH4NO3(s) + H2O(l) → NH4NO3(aq)
	►	Dissolving ammonium sulphate in water ○ $({\text{NH}}_4){\text{SO}}_4(\text{s})\begin{array}{c}\to \\ \text{water}\end{array}{({\text{NH}}_4)}_2{\text{SO}}_4(\text{aq})$
	►	Decomposition of carbonate salts by heat ○ ${\text{CaCO}}_3(s)\begin{array}{c}\to \\ \mathrm{∆}\end{array}\text{CaO}(\text{s})+{\text{CO}}_2(\text{g})$
	►	Decomposition of nitrate salts by heat ○ $2\text{Zn}{({\text{NO}}_3)}_2(\text{s})\begin{array}{c}\to \\ \mathrm{∆}\end{array}2\text{ZnO(s)}+4{\text{NO}}_2(\text{g})+{\text{O}}_2(\text{g})$

■ This video contains information regarding endothermic reactions.

Energy level diagram
■ Energy level diagram

	►	A graphical representation of energy changes during a reaction.
	►	Show only ○ The total energy content of the reactants and the products of the reaction. ○ The energy change that occurs in a reaction, ∆H. ∆H is the difference between the energy content of the reactant and the energy content of the product.
	►	Does not show ○ The rate of reaction. ○ The conditions needed to start the reaction. ○ The equilibrium position (for the reverse reaction) reaction.

■ Exothermic Reaction

	►	The reactant energy is higher than the products energy. This is because reactants have a lot of stored chemical energy.
	►	Negative ∆H occurs.
	►	The reaction will result in a rise in temperature.

■ Endothermic Reaction

	►	The products have higher energy content. The reactants have lower energy content.
	►	Positive ∆H occurs.
	►	The reaction will result in a fall in temperature.

■ This video contains information on the energy level diagram

✍ Worked-example 4.1(a) Draw an energy level diagram for the following reactions: (a) Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s), ∆H = -210kJ mol-1 Solution: (b) 2HgO(s) → 2Hg(l) + O2(g), ∆H = +182kJ mol-1 Solution:

Energy change during formation and breaking of bonds
■ Chemical reaction involves

	►	Bond breaking (requires energy).
	►	The following animation shows the energy involves during the bond breaking and bond formation.

Exothermic and endothermic reaction in everyday life
■ Example of exothermic reaction in daily life

	►	Hot pack ○ A flexible bag containing special heat-holding chemicals that produce an exothermic reaction when mixed with water. ○ The salts that suitable to make hot packs dry calcium chloride, aluminium chloride, dry copper(II) sulphate. ○ Used for warming of parts of the body in cold climate and relieve pain.
	►	○ A flexible bag containing special heat-holding chemicals that produce an endothermic reaction when mixed with water. ○ The salts that suitable to make cold packs ammonium nitrate, ammonium chloride and sodium thiosulphate. ○ It used to relieve pain, swelling, and inflammation from injuries and other condition.

■ This video shows the way to make hot and cold pack in the laboratory.

⇲ For exercise(objective and subjective), download for free on Android OS.