Chemical Equations
Balancing chemical equations
A chemical equation is said to be balanced when the numbers of atoms on both sides of the equation are equal. When balancing an equation the following procedure should be used.
Place large numbers (coefficients) in front of formulae.
Work from left to right across the equation making sure that you have the same number of atoms of each element on each side of the equation. This is a trial and error approach.
Often it is easier to balance atoms other than oxygen and hydrogen first, hydrogens next, and oxygens last.
Check!
NOTE: Two things you cannot do when balancing an equation.
You cannot change a subscript
You cannot place a coefficient in the middle of a formula.
Information from Equations
The reactants involved in a reaction, and the products which are formed by the reaction.
The physical states of the reactants and products.
The amounts, (moles) of each species consumed and each species produced. In gaseous reactions they also give information about the relative volumes of gases consumed and produced.
Some chemical equations also indicate whether the reaction is endothermic (absorbs energy) or exothermic (releases energy).
However, equations do not indicate the rate at which the reaction will proceed.
Important principles to remember
Every chemical compound has a formula, which cannot be altered. Before beginning to balance an equation, check that each formula is correct. Never change a formula during the balancing of an equation. A chemical reaction must account for every atom that is used. This is an application of the Law of Conservation of Matter, which states that in a chemical reaction atoms are neither created nor destroyed. (The total mass of the reactants = the total mass of the products.)
Balancing is done by placing coefficients in front of the formulae to insure the same number of atoms of each element on both sides of the arrow. Fractions may be used.
, means ‘yields’ and indicates the direction of the reaction.
A ‘delta’ (D) can be placed above the arrow to show that heat has been added. In thermochemical equations the actual amount of energy involved can be included in the reaction. (Further work on this in Year 12)
A double arrow, shows that the reaction is reversible and can proceed in both directions. (Further work on equilibrium reactions in year 12)
Refer to the Activity Series of metals and non-metals before attempting to write equations for displacement reactions.
If a reactant or product is a solid, (s) is placed as a subscript after the formula. If a reactant or product is a gas, (g) is placed as a subscript after it. If a reactant or product is a liquid, (l) is placed as a subscript after it, and if a reactant or product is in aqueous (water) solution, (aq) is used as a subscript.
Some products are unstable and break down, (decompose), as they are produced during the reaction. It is useful if these substances are recognised, identified, and written as their products.
Some examples of common substances that spontaneously decompose
Carbonic acid H2CO3(aq) ®H2O(l) + CO2(g)
Ammonium hydroxide NH4OH(aq) ® NH3(g) + H2O(l) |
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