Chemical Reactions & Equations

Chemical Reactions:

• change in colour, state, the evolution of gas, change in temperature

Chemical Equation:

• Shorter way, concise & useful instead of using words
• Representation: Reactions, Products, Arrow Head [include pressure, temperature, catalyst, etc.]
• Skeletal chemical equations: Mg + O2 -> MgO
• Balanced chemical equations: 2Mg + O2 -> 2MgO

Types of Chemical Reactions:

1. Combination
2. Decomposition
3. Displacement
4. Double Displacement
5. Redox
6. Exothermic
7. Endothermic

Combination:

CaO + H2O -> Ca(OH)2

Ca(OH)2 + CO2 -> CaCO3 + H2O

Decomposition:

1. Thermal: CaCO3 --heat--> CaO + CO2
2. Photochemical: AgCl --light--> Ag + Cl2
3. Electrolysis: H2O --electricity--> H2 + O2

Displacement:

Fe + CuSO4 --> Fe(SO)4 + Cu

Elements of higher reactivity displace elements of lesser reactivity. Fe > Cu

Double Displacement:

Na2(SO)4 + BaCl2 --> BaSO4 + NaCl (this is a precipitation reaction as well)

Redox: Reduction Oxidation

Reduction means the removal of Oxygen (or) addition of hydrogen

Oxidation means the addition of Oxygen (or) removal of hydrogen

2Cu + O2 --heat--> 2CuO [oxidation]

CuO + H2--> Cu + H2O [reduction CuO to Cu, oxidation H2 to H2O]

Exothermic:

Burning of natural gas

CH4 + O2 --> CO2 + H2O

Respiration

C6H12O6 + O2 --> CO2 + H2O + energy

Endothermic:

ZnCO3 + Heat absorbed --> ZnO + CO2

Effect of Chemical Reactions:

Corrosion:

• undesirable change that occurs in metals in the presence of moisture, acids, bases & air
• Fe + H2O --> Fe2O3.nH2O

Rancidity:

• undesirable change that takes place in oil-containing food
• Chips: flushing out with air prevents oxidation of fatty acids.

Tarnishing:

• Silver reacts with sulphides in the atmosphere (e.g. mercuric sulphide) to form a black layer of silver sulphide.
• Toothpaste to it to remove the tarnish regains its colour.