Reversible reactions
In principle, all chemical reactions are A chemical reaction which can go both ways.. The A substance formed in a chemical reaction. could be changed back into the original A substance that reacts together with another substance to form products during a chemical reaction. using a suitable reaction. This is not obvious when a reaction 鈥榞oes to completion鈥, a situation in which 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.
- reactions in which a product escapes, usually a gas
It is more obvious in reactions that do not go to completion that the reaction is reversible. The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.
Two examples
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, Bunsen burner heats a bowl of hydrated copper(II) sulfate
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Dynamic equilibrium
When a reversible reaction happens in a A reaction vessel where no reactants or products can escape or are lost to the surroundings., such as a stoppered flask, it reaches a dynamic 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.. At equilibrium:
- the forward and backward reactions are still happening
- the forward and backward reactions have an equal rate of reaction
- 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 all the reacting substances remain constant and 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)