Chemical Reactions and Equations

Characteristics of Chemical Reactions: 

Some of the following findings can aid in determining when a chemical reaction occurs:

• The combustion reaction of candle wax is characterised by a shift in the state from solid to liquid and gas.

• Colour change: The chemical reaction between sulphur dioxide gas and refluxed dichromate solution results in a colour change from orange to green.

• The evolution of hydrogen gas is a characteristic of the chemical interaction between zinc and dilute sulphuric acid.

• Temperature change: The chemical reaction between quicklime water and slaked lime is characterised by a change in temperature (temperature starts increasing).

Types of Chemical Reactions

Combination reaction:

• Reactions in which two or more reactants combine to generate a single product are known as combination reactions.

A + B → AB

Decomposition reactions: 

• Reactions in which one component decomposes into two or more compounds or elements are known as decomposition reactions. The reaction is the polar opposite of a combination reaction.

Combination reaction: A + B → AB

Decomposition reaction: AB → A + B

The types of decomposition reactions are as follows:

1. Thermal decomposition – It is the breakdown of a substance when it is heated.

2. Electrolysis – It is the decomposition of a substance caused by the passage of electricity.

3. Photolysis – Also known as photo breakdown, it is the decomposition of a substance caused by sunlight.

Displacement and double displacement reactions:

• A more reactive element disperses a less reactive element from a molecule in a chemical change is called a displacement reaction

AC + B = A + BC

It only happens when A reacts faster than B. If B is much more reactive than A, A will not be able to push C out of BC.

• Reactions in which ions are shared between two reactants to form new compounds are known as double displacement reactions.

AC + BD → AB + CD 

• Precipitation reactions: 

Reactions in which an aqueous solution of two salts is mixed to generate a precipitate.

• Neutralisation reaction:

A neutralisation reaction occurs when an acid combines with a base to produce salt and water through ion exchange. An example is demonstrated below –

HCl + NaOH → NaCl + H2O

Oxidation and Reduction Reactions:

• Oxidation: Reactions wherein oxygen is gained or hydrogen is lost.

Eg: Fe + O2 (g) → Fe2O3(g)

Eg: Ag(s) + H2S → Ag2S (g) + H2(g)

• Reduction: Actions in which oxygen is lost, or hydrogen is gained.

Eg: Fe2O3(g) + 3CO → 2Fe + 3CO2

Eg: CH3CHO + 2H → CH3CH2OH

• Redox reaction: A reaction where both oxidation and reduction reactions occur at the same time.

Eg: H2 + F2 → 2HF

Here, the oxidation half-reaction is: H2 → 2H+ + 2e– 

and the reduction half-reaction is:

 F2 + 2e– → 2F–

Eg: Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)

Zn is oxidised and Cu is reduced

Exothermic and Endothermic Reactions:

• When heat energy is absorbed in a reaction, it is called an endothermic reaction.

• When energy is released in the form of light or heat, it is called an exothermic reaction. Consider the process of respiration, which involves humans taking in oxygen and exhaling carbon dioxide.

• The results of the oxidation reaction can be seen in everyday life.

• Corrosion is the gradual transformation of metals into undesired compounds as a result of reactions with oxygen, water, acids, gases, and other elements in the atmosphere. Example: The corrosion of iron

• Painting, greasing and oiling, galvanization, and other methods to avoid rusting

• When food products, including fat and oil, are exposed to air for an extended period of time, they develop peroxidation, which affects the taste and odour.

• The oxidation of fat and oil in dietary materials is the cause.

• Antioxidants, pressure packaging, nitrogen replacement, food refrigeration, and other methods are used to avoid this.

Conclusion 

Any chemical reaction has two main components – reactants and products. Reactants are known as the substances that are known to react. Alternatively, the products are the substances that are known to form in a specific chemical reaction. One thing to remember in a chemical reaction is the minimum amount of energy that is required to activate the reactants to participate in the reaction and form the products. This minimum energy requirement is known as activation energy. As the reaction proceeds, the concentration of the reactants starts to decrease whereas the concentration of the products starts to increase.