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.
H2SO4(concentrated)water(H2)SO4(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
(NH4)SO4(s)water(NH4)2SO4(aq)

Decomposition of carbonate salts by heat
CaCO3(s)CaO(s)+CO2(g)

Decomposition of nitrate salts by heat
2Zn(NO3)2(s)2ZnO(s)+4NO2(g)+O2(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.

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