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# Write an expression for the equilibrium constant of this reaction and calculate KcKc

## CONCENTRATION-TIME GRAPHS

Consider the following chemical equilibrium and graph and answer the questions that follow.

CO(g)+Cl2(g)CO(g)+Cl2(g) ⇌⇌ COCl2(g)COCl2(g)

1.  How much time was necessary for the system to reach equilibrium for the first time?
2.  How do the rates of the forward and reverse reactions compare at the following times:
• t = 55 ss
• t = 1717 ss
• t = 2323 ss
3.  Determine the equilibrium constants for the system at t = 1717 ss, 2525 ss, and 4545 ss.
4.  What happens at t = 2020 ss? Explain your answer by referring to Le Chatelier’s principle.
5.  What effect does the stress at t = 2020 ss have on KcKc?

### Check the axes so that you know what the variables are on this graph

The axes are labelled concentration and time. Therefore this is a concentration-time graph.

### How much time was necessary for the system to reach equilibrium?

The concentration of all three compounds becomes constant at t = 1515 ss. This means that the reaction has reached equilibrium.

### How can you determine which rate is faster from the concentration graphs?

If the concentrations of the reactants (COCO and Cl2Cl2) are steadily decreasing or the concentration of the product (COCl2COCl2) is increasing then the forward reaction is faster than the reverse reaction.

Similarly, if the concentrations of the reactants (COCO and Cl2Cl2) are steadily increasing or the concentration of the product (COCl2COCl2) is decreasing then the reverse reaction is faster than the forward reaction.

### Compare the forward and reverse reaction rates at t = 55 ss, t = 1717 ss and t = 2323 ss

At t = 55 ss and at t = 2323 ss the concentrations of the reactants are decreasing and the concentration of the product is increasing. Therefore the rate of the forward reaction is faster than the rate of the reverse reaction.

At t = 1717 ss the concentrations of the reactants and products are constant (unchanging). Therefore the reaction is in equilibrium and the rate of the forward reaction equals the rate of the reverse reaction.

### What are the concentrations of COCO, Cl2Cl2 and COCl2COCl2 at t = 1717 ss?

Reading off the graph you can see that:

[CO][CO] = 1,751,75 mol.dm−3mol.dm−3

[Cl2][Cl2] = 0,60,6 mol.dm−3mol.dm−3

[COCl2][COCl2] = 0,750,75 mol.dm−3mol.dm−3

### Write an expression for the equilibrium constant of this reaction and calculate KcKc.

Kc=[COCl2][CO][Cl2]Kc=[COCl2][CO][Cl2]

Kc=0,75(1,75)(0,6)=Kc=0,75(1,75)(0,6)= 0,710,71

### What happens at t = 2020 ss?

The concentration of COCO increases sharply. The concentrations of the other compounds do not change dramatically. Therefore COCO must have been added to the system.

After this addition of COCO there is a shift to reduce the amount of COCO, that is in the forward direction. Therefore the concentration of the product increases while the concentrations of the reactants decrease.