Chemical equations and reactions
1. HOW IS A CHEMICAL REACTION DIFFERENT FROM AN EQUATION?
A chemical reaction usually describes the transformation of a substance(s) called reactants into product(s). A summary of this reaction is described as a chemical equation, which can be written using words or chemical symbols. A chemical reaction is irreversible, resulting in the formation of a new product. This describes a chemical change. Chemical reactions are usually accompanied by changes in mass, colour and temperature, evolution of gases and odours.
An equation is a representation of a reaction. An equation should show the reactants and products taking part in a chemical reaction, and should show in what ratio these products combine. The reactants are on the left side of the equation while the products are on the right-hand side. Sodium hydroxide + hydrochloric acid sodium chloride + water ... WORD
2. What are the different reaction types? Give examples of each.
Chemical reactions may fall under one or more of the following types: synthesis, decomposition, displacement, precipitation, neutralization, oxidation-reduction (redox) and reversible reactions. Synthesis or combination reactions refer to the direct combination of the reactants to form one product only. Decomposition reactions refer to the breaking down of a single reactant to form two or more products. Calcium carbonate-calcium-oxide + carbon dioxide Combustion or oxidation reactions occur when a substance is burnt in oxygen. Methane + oxygen-carbon dioxide + water vapour In displacement (single) reactions, an element which is more reactive displaces another element from a compound.
In precipitation (ionic) reactions, two substances in solution exchange ions forming an insoluble precipitate in the process. This is sometimes called a double-displacement reaction. Sodium chloride + lead nitrate = lead chloride + sodium nitrate NaCl (aq) + Pb(NO3)2 (aq) PbCl2 (s) + NaNO3 (aq)
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In ionic equations, only ions that change their charges (oxidation state) during the reaction are shown. The other ions that do not change are called spectator ions and do not appear in the equation. Pb2+ (aq) + 2Cl(aq) PbCl2 (s)... Ionic Equation Neutralization reactions describe the action of an acid and a base to produce salt and water. 2HNO3 (aq) + Ca(OH)2 (aq) Ca(NO3)2 (aq) + 2H2O (l) Nitric acid + calcium hydroxide = calcium nitrate + water Acid + Base Salt + Water
Redox reactions describe oxidation and reduction reactions occurring simultaneously in the same reaction. One substance is oxidized and another substance is reduced. Magnesium is oxidised by oxygen, while the oxygen is reduced by the magnesium.
Reversible reactions occur when the direction of the reaction can be changed depending on reactions conditions such as concentration, pressure and temperature. A reversible symbol is used to depict these reaction types.
3. What are the steps involved in writing and balancing an equation?
In writing an equation, the first step is to express it in word form. This ensures that all reactants and possible products are accounted for.
For example: Magnesium + oxygen –––– Magnesium oxide
The second step is to express the word equation in symbols. Here, the correct formula for each reactant and product must be given.
For example: Mg (s) + O2 (g) –––– MgO (s)
The final step is to check that the same number of atoms of a given element or their ions is on each side. When this is done, the equation is said to be balanced. State symbols are also added to the equation.
Symbols used are s (solid), l (liquid), g (gas) and aq (aqueous), which means dissolved in solution or water. For example: Mg (s) + O2 (g) ––––– MgO (s) Based on the equation: LHS: 1 Mg, O –––– RHS: 1 Mg, 1 O
So the equation is unbalanced. 2 To balance the equation, we add whole numbers before the formula of a compound only. This will ensure that the formula does not change even though the number of atoms in the formula can change. Adding the number 2 before the formula of Mg and MgO will balance the equation. Balanced Equation: 2Mg (s) + O2 (g) –––– 2MgO (s)