Document page
chemistry igcse acapprinted 125 · PDF 125
145 pagesrevision 46934
printed 125 · PDF 125
Page image rendered from the stored file at 110 dpi; it sharpens as you zoom. Outlines are placed from each region’s stored position on the page (612 × 792 pt).
readable
Source of this text: embedded text layer. The better of the embedded text and OCR is kept.
125
CHEMICAL EQUILIBRIUM
Reversible reactions are reactions in which the conversion of reactants to products and the conversion of
products to reactants occur simultaneously.
They consist of backwards and forwards reactions, whose respective rates are affected by a variety of factors,
namely temperature, pressure and concentration. They are represented by the symbol ⇌
A reversible reactions is at equilibrium when:
(a) the rate of the forward reaction is equal to the rate of the reverse reaction, so products are being formed at the
same rate as reactants
(b) the concentrations of reactants and products are no longer changing, since they are both being formed at the
same rate
Factors Affecting the Equilibrium:
A general principle that might make understanding the following easier is that the equilibrium always moves to
minimise change. If the temperature rises, there is a tendency to automatically lower it, if the concentration of a
certain substance drops, there is a tendency to raise it back up. Below is the reaction to form ammonia, which will
be used as an example throughout and is included in the syllabus.
N2(g) + 3H2(g) ⇌ 2NH3(g)
Temperature: Higher temperature makes the equilibrium move in the direction of the endothermic reaction,
while lower temperature makes it shift in the direction of the exothermic reaction. For example, in the reaction to
form ammonia, the forward reaction is exothermic, creating heat. Therefore, a lower temperature will shift the
equilibrium forward, in the direction of the exothermic reaction. A higher temperature would shift it backwards,
in the direction of the reverse endothermic reaction.
Concentration: Higher concentration of the reactants makes the equilibrium shift forward, while higher
concentration of the products makes it shift backwards.
Pressure: This factor only applies if the reaction involves gases. In that case, increasing the pressure will cause the
equilibrium to shift in the direction with less moles of gas and decreasing the pressure will cause it to shift in the
direction with more moles of gas. For instance, in the ammonia reaction, there are four moles of gas on the