## Thursday, 26 February 2015

### 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.
 ○ ${\text{H}}_{2}{\text{SO}}_{4}\left(\text{concentrated}\right)\begin{array}{c}\to \\ \text{water}\end{array}\left({\text{H}}_{2}\right){\text{SO}}_{4}\left(\text{aq}\right)$

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
 ○ $\left({\text{NH}}_{4}\right){\text{SO}}_{4}\left(\text{s}\right)\begin{array}{c}\to \\ \text{water}\end{array}{\left({\text{NH}}_{4}\right)}_{2}{\text{SO}}_{4}\left(\text{aq}\right)$

Decomposition of carbonate salts by heat
 ○ ${\text{CaCO}}_{3}\left(s\right)\begin{array}{c}\to \\ \mathrm{∆}\end{array}\text{CaO}\left(\text{s}\right)+{\text{CO}}_{2}\left(\text{g}\right)$

Decomposition of nitrate salts by heat
 ○ $2\text{Zn}{\left({\text{NO}}_{3}\right)}_{2}\left(\text{s}\right)\begin{array}{c}\to \\ \mathrm{∆}\end{array}2\text{ZnO(s)}+4{\text{NO}}_{2}\left(\text{g}\right)+{\text{O}}_{2}\left(\text{g}\right)$
■ 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.