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the number of gas molecules

Consider illustration in Figure 8.2.

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(a) A decrease in the pressure of this reaction favours the reverse reaction (more gas molecules), the equilibrium shifts to the left. (b) An increase in the pressure of this reaction favours the forward reaction (fewer gas molecules), the equilibrium shifts to the right.

Figure 8.2 shows how changing the pressure of a system results in a shift in the equilibrium to counter that change. In the original system there are 12molecules in total: 6H2+2N2⇋4NH36H2+2N2⇋4NH3

If you decrease the pressure (shown by an increase in volume), the equilibrium will shift to increase the number of gas molecules. That shift is to the left and the number of H2H2 and N2N2 molecules will increase while the number of NH3NH3 molecules will decrease:

526494ffbf8fcd1bbb64d55647484a70.png

If you increase the pressure (shown by a decrease in volume), the equilibrium will shift to decrease the number of gas molecules. That shift is to the right and the number of H2H2 and N2N2 molecules will decrease while the number of NH3NH3 molecules will increase:

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Note that the total number of nitrogen and hydrogen atoms remains the same in all three situations. Equations (a) and (b) are not balanced equations.

Another example is the reaction between sulfur dioxide and oxygen:

2SO2(g)+O2(g)⇋2SO3(g)