This is part of the HSC Chemistry course under the topic Equilibrium Systems Show HSC Chemistry SyllabusInvestigate the effects of temperature, concentration, volume and/or pressure on a system at equilibrium and explain how Le Chatelier’s principle can be used to predict such effects, for example:
Explain the overall observations about equilibrium in terms of the collision theory (ACSCH094) Examine how activation energy and heat of reaction affect the position of equilibrium What is Le Chatelier's Principle and how does it apply to equilibria?This video will explore what Le Chatelier's Principle is and discuss its relevance regarding understanding how equilibrium positions are manipulated.
“If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change.” Concentration
Figure: addition of chemical increases its concentration and rate of collision with other molecules which in turn increases the reaction rate. This disturbs the equilibrium as rates of forward and reverse reactions are no longer equal.
Figure: reduction in volume increases pressure on gases and the collision rate between them. Solids and liquids are not affected by changes in pressure and volume (the effect is negligible compared to gases). Pressure/Volume
Figure: changes in pressure or volume directly impact the collision rate and thus, the rate of reaction. An increase in pressure increases reaction rate while a decrease in pressure decreases reaction rate.
$$2NO_{2(g)} \rightleftharpoons N_2O_{4(g)}$$
Figure: changes in pressure and volume have greater effect when there are more gas molecules. In this example, there are more reactants (blue), so changes in pressure exert greater effects on the forward reaction rate.
Example: The graph below is plotted for a reversible reaction. $$2A_{(g)}+B_{2(g)} \rightleftharpoons 2AB_{(g)}$$
What change has been imposed on the system? The reaction reached and remained in equilibrium for a while until a change has been imposed, increasing rates of forward and reverse reactions. The pressure of the system has increased. This increases the rate of both forward and reverse reaction. However, since the forward reaction has greater number of gas molecules, the forward reaction rate increases more. By Le Chatelier’s principle, the system will consume some reactants to form products. This reduces the forward reaction rate while increases the reverse reaction rate. TemperatureFigure: A rise in temperature causes more particles to have energy greater than the activation energy. This means reaction rate is greater due to more successful collisions. T2 > T1.
Figure: energy profile of an exothermic reaction. The reverse reaction is endothermic and has a larger activation energy barrier than the forward reaction.
$$2NO_{2(g)} \rightleftharpoons N_2O_{4(g)} \;\;\; \Delta H=-58 \; kJ mol^{-1}$$ The forward reaction is exothermic, so an increase in temperature will favour the reverse reaction rate more (endothermic). Conversely, when temperature decreases, the forward reaction is favoured more (exothermic). CatalystFigure: catalysts reduce the activation energy of both forward and reverse reactions.
Catalysts do not disturb chemical equilibria Figure: addition of catalyst at the 10-minute mark increases the rate of both forward and reverse reaction. The equilibrium is not disturbed.
Reactions reach equilibrium faster Figure: addition of a catalyst allows the equilibrium to be reached faster.
Application of KnowledgePractice Question 1The following graph shows the change in reaction rate with time for the following generic reaction. $$2AB_{(g)} \rightleftharpoons 2A_{(g)} + B_{2(g)}\;\;\; \Delta{H}>0$$ What change is happening in this question? Why cannot it not be a change in pressure?
Practice Question 2Industrial synthesis of ammonia uses nitrogen gas and hydrogen gas as reactants. The chemical equation for this process is: (a) What is happening at t1? How many chemical species change in concentration instantaneously? (b) What is happening at t2? How many chemical species change in concentration instantaneously? (c) What is happening at t3? Is the change in concentration instantaneous? Practice Question SolutionPractice Question 1 $$2AB_{(g)} \rightleftharpoons 2A_{(g)} + B_{2(g)}\;\;\; \Delta{H}>0$$ There are two possible changes that would have caused an increase in rate of forward and reverse reaction: (1) temperature and (2) pressure/volume. Since the reaction rates have increased, the change must be either an increase in temperature or increase in pressure (reduced volume). The forward reaction is endothermic so an increase in temperature would increase the forward reaction rate more than the reverse reaction rate. This is shown in the graph as, after the change, forward reaction rate is higher than the reverse reaction rate. Since equilibrium is disturbed, rates of forward and reverse reactions subsequently decreased and increased respectively to re-achieve equilibrium. Why is the change not pressure? The answer in this case cannot be an increase in pressure as there are more gas molecules on the product side than the reactant side. There are 3 moles of gases on the product side for every 2 moles on the reactant side. This means an increase in pressure would have increased the reverse reaction (3 moles of gases) more than the forward reaction (2 moles of gases). This was not observed in the graph. Practice Question 2 $$N_{2(g)}+3H_{2(g)} \rightleftharpoons 2NH_{3(g)}\;\;\; \Delta{H} = -92\, kJmol^{-1}$$ Before beginning the question, it is important to note that this is not a rate versus time graph but instead it is a concentration versus time graph. (a) At `t_1`, only `[N_2]` increased instantaneously which suggests this is not a change in pressure/volume as it would otherwise affect concentrations of all chemical species in the reaction. This is also not a change in temperature as changes in temperature do not change concentrations of species instantaneously. Therefore, the change at `t_1` is likely to be addition of nitrogen gas. (b) At `t_2`, concentrations of all chemical species of the reaction have increased instantaneously. This suggests an increase in pressure on the system (or reduction in volume of vessel). This can be confirmed by examining the equation of the reaction: $$N_{2(g)}+3H_{2(g)} \rightleftharpoons 2NH_{3(g)}\;\;\; \Delta{H} = -92\, kJmol^{-1}$$ Since there are more moles of gases on the reactant (left) side, rate of forward reaction is favoured by an increase in pressure. This causes greater quantities of `NH_3` (ammonia) to be produced, which is shown as a gradual change right after the pressure change at `t_2`. (c) No chemical species at have changed their concentrations instantaneously at `t_3`. This means the change is associated with temperature. The concentrations of `N_2` and `H_2` gradually increased, and that of `NH_3` decreased. This indicates that the temperature change favours the reverse reaction. Since the reverse reaction is endothermic, the change must an increase in temperature. Previous section: Collision Theory in Equilibria Next section: Practical Investigations of Le Chatelier's Principle |