The cause for different forms of reactions is the products generated, the changes that arise, the reactions involved, etc. There are different kinds of reactions:

• Combustion reaction: A combustion reaction is a reaction with an oxidizing fuel to give an oxidized substance. The oxidizer is a chemical that has to be burned by gasoline, usually oxygen.
• Decomposition reaction: A decomposition reaction is a reaction in which a single component is broken down into several components. Certain changes in the atmosphere’s energies have to be made, such as heat, light or electricity that breaks the bonds of the compound.
• Neutralization reaction: The neutralization reaction is essentially the acid and the base’s reaction, giving salt and water as products. The water molecule is a mixture of OH– ions and H+ ions. The products’ total pH when a strong acid and a strong base undergo a neutralization reaction would be 7.
• Redox reaction: A reduction-oxidation reaction is a reaction in which electrons are transferred between chemical compounds.
• Precipitation or Double-Displacement reaction: It is a form of displacement reaction in which two compounds react and, as a result, their anions and cations turn into two new products.
• Synthesis reaction: A synthesis reaction is one of the most common reactions in which several simple compounds interact to produce a complex product under certain physical conditions. The result is still going to be a compound.

Because energy is absorbed when chemical bonds are destroyed and released during their formation, chemical reactions nearly often require a change of energy between products and reactants. However, according to the Law of Energy Conservation, we know that the system’s overall energy must remain constant and that often the chemical reaction absorbs or releases energy in the form of heat, light, or both. The energy change in the chemical reaction is due to the differences in the amount of chemical energy deposited between ingredients and reactants. This accumulated chemical energy or heat content of the system is known as enthalpy.

It is a chemical reaction that starts with light absorbed as a source of energy. There are temporary peak states that get activated as the molecules absorb light, and there are significant changes in physical and chemical properties from the actual molecules.

Ideally, the photo-reaction mechanism should provide a thorough characterization of the key events as outlined in the photochemical reaction pathway classification. Quantum yield and rate constants are all related to photophysical and photochemical processes, along with details on the structure and fate of any reactive intermediates, their lifetimes and their reactivity.

An electrochemical reaction is a mechanism in which electrons, such as an electrolyte, move between a solid electrode and a substance. This flow induces an electrical current through the electrodes, which causes the reaction to release or absorb heat. Chemical reactions either consume or emit energy that may be in the form of electricity. Electrochemistry is a chemical division that talks about the transfer of chemical energy to electrical energy. It has many different uses in daily life. All kinds of batteries, from those used to power a torch to a calculator, depend on chemical reactions to produce energy. Electricity is used for plating objects with metals such as gold or chromium.

In this chapter, we learnt about the concepts of chemical changes and reactions. The chapter also shed some light on the nature and characteristics of chemical change.

1. Give an example of a photochemical reaction.
2. Photosynthesis is when plants convert carbon dioxide and water to glucose and oxygen using solar energy. E.g., the generation of human vitamin D by exposure to sunlight, the enzyme in the abdomen catalyzing a light-producing reaction in fireflies called bioluminescence.
4. What is the primary photochemical process?
5. Photolysis is the mechanism by which a photochemical reaction occurs. The subsequent effect of the absorption of light is the primary photochemical reaction. Secondary reactions are subsequent chemical changes.
7. What is a chemical reaction?
8. A chemical reaction breaks down the chemical bonds of the reactants (reacting compounds) to form new bonds and substances (products).
10. Is the energy involved in a chemical change?
11. Yes, regardless of the formation and breakdown of bonds, energy is involved in a chemical change. Energy is produced in exothermic reactions and absorbed in endothermic reactions.
13. What are the fundamentals of chemical reactions?

• They are a phase in which one or more compounds (reactants) are converted into separate substances known as products.
• They rearrange the constituent atoms of the reactants to produce distinct molecules as products.
• They are different from physical effects, including state changes, such as ice freezing to water and water evaporating to vapour. If a physical change happens, the material’s physical characteristics will change, but the chemical identity will stay the same.