Chemical Equations Section 3.5

Chemical Equations A chemical equation is a way of representing a chemical change. It shows reactants and products. To balance an equation means to change the numbers of each molecule involved, so that the same number of atoms of each element appear on the reactants side and on the products side. Chemical equations balance on an atomic level, not molecular. You cannot change the formula of a substance, i.e. if the equation has NH3 you cannot change this you can only put a number in front of it, 2NH3, increasing the number of N’s and the number of H’s. Never change the subscripts (small numbers). It is possible to write balanced equations for lots of reactions but that does not mean that the reaction actually takes place.

Formation of Water (Oxygen does not exist as single atoms) (This reaction does not take place)

Balancing an Equation

Balance the following

Balancing Redox Equations In working out what is oxidised and what is reduced in a reaction, it is important to remember that oxidation numbers are not a charge. Write the oxidation numbers below each atom to which it applies, as shown in the examples. Oxidation is an increase in oxidation number. Reduction is a decrease in oxidation number.

Assign oxidation numbers to all the atoms in the equation. Write down half equations of what is oxidised and reduced. Attach subscripts to atoms oxidised and reduced and balance the half equations. Balance the half equations. Attach species that were attached to the oxidised and reduced atoms. Include all the original species and complete the balancing by inspection.

Calculations using Chemical Equations A balanced equation for a chemical reaction gives the relative amounts of each reactant and each product involved in the reaction. If the amount of one substance is known, based on the molar ratios in the equation, the amounts (masses, particles or volume)of other substances can be calculated. Make sure to always work in moles.