3.4 Le chateliers principle  (Page 3/4)

Reaction rates and equilibrium

1. The following reaction reaches equilibrium in a closed container: $CaC{O}_3\left(s\right)\rightleftharpoons CaO\left(s\right)+C{O}_2\left(g\right)$ The pressure of the system is increased by decreasing the volume of the container. How will the number of moles and the concentration of the CO ${}_2$ (g) have changed when a new equilibrium is reached at the same temperature?

   	moles of CO ${}_2$	[CO ${}_2$ ]
   A	decreased	decreased
   B	increased	increased
   C	decreased	stays the same
   D	decreased	increased

   (IEB Paper 2, 2003)
2. The following reaction has reached equilibrium in a closed container: $C\left(s\right)+H_{2}O\left(g\right)\rightleftharpoons CO\left(g\right)+H_2\left(g\right)$ $\Delta H$ >0 The pressure of the system is then decreased by increasing the volume of the container. How will the concentration of the H ${}_2$ (g) and the value of K ${}_c$ be affected when the new equilibrium is established? Assume that the temperature of the system remains unchanged.

   	[H ${}_2$ ]	K ${}_c$
   A	increases	increases
   B	increases	unchanged
   C	unchanged	unchanged
   D	decreases	unchanged

   (IEB Paper 2, 2004)
3. During a classroom experiment copper metal reacts with concentrated nitric acid to produce NO ${}_2$ gas, which is collected in a gas syringe. When enough gas has collected in the syringe, the delivery tube is clamped so that no gas can escape. The brown NO ${}_2$ gas collected reaches an equilibrium with colourless N ${}_2$ O ${}_4$ gas as represented by the following equation:
   $$2N{O}_2\left(g\right)\rightleftharpoons N_2{O}_4\left(g\right)$$
   Once this equilibrium has been established, there are 0.01 moles of NO ${}_2$ gas and 0.03 moles of N ${}_2$ O ${}_4$ gas present in the syringe.
   1. A learner, noticing that the colour of the gas mixture in the syringe is no longer changing, comments that all chemical reactions in the syringe must have stopped. Is this assumption correct? Explain.
   2. The gas in the syringe is cooled. The volume of the gas is kept constant during the cooling process. Will the gas be lighter or darker at the lower temperature? Explain your answer.
   3. The volume of the syringe is now reduced to 75 cm ${}^3$ by pushing the plunger in and holding it in the new position. There are 0.032 moles of N ${}_2$ O ${}_4$ gas present once the equilibrium has been re-established at the reduced volume (75 cm ${}^3$ ). Calculate the value of the equilibrium constant for this equilibrium. (IEB Paper 2, 2004)
4. Consider the following reaction, which takes place in a closed container: $\mathrm{A}\left(\mathrm{s}\right)+\mathrm{B}\left(\mathrm{g}\right)\to \mathrm{AB}\left(\mathrm{g}\right)$ $\Delta$ H $<$ 0 If you wanted to increase the rate of the reaction, which of the following would you do?
   1. decrease the concentration of B
   2. decrease the temperature of A
   3. grind A into a fine powder
   4. decrease the pressure
   (IEB Paper 2, 2002)
5. Gases X and Y are pumped into a 2 dm ${}^3$ container. When the container is sealed, 4 moles of gas X and 4 moles of gas Y are present. The following equilibrium is established: $2\mathrm{X}\left(\mathrm{g}\right)+3\mathrm{Y}\left(\mathrm{g}\right)\rightleftharpoons {\mathrm{X}}_2{\mathrm{Y}}_3$ The graph below shows the number of moles of gas X and gas X ${}_2$ Y ${}_3$ that are present from the time the container is sealed.

   

   1. How many moles of gas X ${}_2$ Y ${}_3$ are formed by the time the reaction reaches equilibrium at 30 seconds?
   2. Calculate the value of the equilibrium constant at t = 50 s.
   3. At 70 s the temperature is increased. Is the forward reaction endothermic or exothermic? Explain in terms of Le Chatelier's Principle.
   4. How will this increase in temperature affect the value of the equilibrium constant?