Chemical Equations
-A chemical reaction involves a change in the physical and chemical properties, the composition and the physical state of a substance.
– A chemical equation is a symbolic representation of the reactants and the products in terms of their chemical formulae.
– The law of conservation of mass states that the number of atoms of each element should be equal on both sides of a chemical equation.
– The steps in balancing an equation are:
• Determine the reactants and products.
• Count the number of atoms of each on both sides.
• Select the element that occurs at the minimum number of places.
• Change the coefficient of molecules of the reactants or the products.
• Continue balancing the atoms till all the elements have been balanced in the equation.
Types of Chemical Reactions
– The important types of chemical reactions are:
• Combination: A combination reaction has multiple reactants, but only one product.
• Decomposition: A decomposition reaction always has one reactant and many products.
Decomposition can occur as a result of the action of heat, electricity or light energy.
• Single displacement: In a single displacement reaction, one of the reactants is always an element.
• Acid-base or neutralisation: In an acid-base reaction, one of the products is always water.
• Double displacement: Where the two reactants are compounds, the reaction involves double displacement.
• Combustion: A combustion reaction cannot occur without oxygen.
• Oxidation: An oxidation reaction involves the addition of oxygen or the removal of hydrogen from a substance.
• Reduction: A reduction reaction involves the addition of hydrogen or the removal of oxygen from a substance.
– Oxidation and reduction reactions together are known as redox reactions.
– Decomposition reactions are the opposite of combination reactions.
– Based on whether energy is released or absorbed , a reaction can be of two types:
• Endothermic : Chemical reactions in which energy is absorbed.
• Exothermic: Chemical reactions in which energy is released.