Reversible reactions
In principle, all chemical reactions are A chemical reaction which can go both ways.. This means that the A substance formed in a chemical reaction. can be changed back into the original A substance that reacts together with another substance to form products during a chemical reaction.. This is not obvious when a reaction 'goes to completion', where very little or no reactants are left. Examples of reactions that go to completion are:
- Burning in a plentiful supply of oxygen or air. Complete combustion of a hydrocarbon produces water vapour and carbon dioxide. of a fuel
- many A reaction in which an insoluble solid is formed when certain solutions are mixed.
- The formation of bubbles of gas produced by a chemical reaction in a liquid. reactions in which a gas escapes
It is more obvious in reactions that do not go to completion that the reaction is reversible. This is the case when the reaction mixture contains both reactants and products.
Examples of reversible reactions
Ammonium chloride
Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again. This reversible reaction can be modelled as:
Ammonium chloride 鈬 ammonia + hydrogen chloride
NH4Cl(s) 鈬 NH3(g) + HCl(g)
The symbol 鈬 has two half arrowheads, one pointing in each direction. It is used in equations that model reversible reactions:
- the forward reaction is the one that goes to the right
- the backward reaction is the one that goes to the left
Question
Write the A chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. for the forward reaction in the breakdown of ammonium chloride.
NH4Cl(s) 鈫 NH3(g) + HCl(g)
Copper sulfate
Blue copper sulfate is described as Containing water of crystallisation, which can be removed by heating strongly.. The copper Electrically charged particle, formed when an atom or molecule gains or loses electrons. in its The regular arrangement of particles (atoms, molecules or ions) in a crystal. structure are surrounded by water A collection of two or more atoms held together by chemical bonds.. This water is driven off when blue hydrated copper sulfate is heated, leaving white A substance containing no water. copper sulfate. This reaction is reversible:
Hydrated copper sulfate 鈬 anhydrous copper sulfate + water
CuSO4.5H2O(s) 鈬 CuSO4(s) + 5H2O(l)
Image caption, A reversible reaction
1. Bunsen burner heats a bowl of hydrated copper sulfate
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Dynamic equilibrium
When a reversible reaction happens in a closed container, it can achieve a Describing the position attained in a reversible reaction when the proportions of reactants and products stay the same. The term 鈥榙ynamic鈥 emphasises that the forward and the backward reactions continue to occur (at the same rate).. At In chemical reactions, a situation where the forward and backward reactions happen at the same rate, and the concentrations of the substances stay the same.:
- the forward and backward reactions are still happening
- the rates of the forward and backward reactions are the same
- the The concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. of the reactants and products remain constant (they do not change)
Question
Nitrogen reacts with hydrogen to produce ammonia: N2(g) + 3H2(g) 鈬 2NH3(g)
Write the balanced equation for the backward reaction.
2NH3(g) 鈫 N2(g) + 3H2(g)
Note that equilibrium can only be achieved if none of the reactants or products can escape. For example if calcium carbonate is heated in a sealed container, breakdown occurs and the following equilibrium is made:
CaCO3(s) 鈬 CaO(s) + CO2(g)
If this reaction is carried out in an open test tube the breakdown goes to completion:
CaCO3(s) 鈫 CaO(s) + CO2(g)
This occurs as the carbon dioxide gas escapes and the backwards reaction cannot occur.