# H 2 k c a increases increases b increases unchanged c

• JusticeIce1840
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[H 2 ] K c A increases increases B increases unchanged C unchanged unchanged D decreases unchanged 313

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16.8 CHAPTER 16. REACTION RATES - GRADE 12 (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: 2 NO 2 ( g ) N 2 O 4 ( g ) 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. (a) 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. (b) 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. (c) 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: A(s) + B(g) AB(g) Δ H < 0 If you wanted to increase the rate of the reaction, which of the following would you do? (a) decrease the concentration of B (b) decrease the temperature of A (c) grind A into a fine powder (d) 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: 2X(g) + 3Y(g) X 2 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. 30 time (s) 0,5 4 number 70 100 moles of 314
CHAPTER 16. REACTION RATES - GRADE 12 16.9 (a) How many moles of gas X 2 Y 3 are formed by the time the reaction reaches equilibrium at 30 seconds? (b) Calculate the value of the equilibrium constant at t = 50 s. (c) At 70 s the temperature is increased. Is the forward reaction endothermic or exothermic? Explain in terms of Le Chatelier’s Principle. (d) How will this increase in temperature affect the value of the equilibrium constant? 16.9 Industrial applications The Haber process is a good example of an industrial process which uses the equilibrium principles that have been discussed. The equation for the process is as follows: N 2 ( g ) + 3 H 2 ( g ) 2 NH 3 ( g ) + energy Since the reaction is exothermic , the forward reaction is favoured at low temperatures, and the reverse reaction at high temperatures. If the purpose of the Haber process is to produce ammonia, then the temperature must be maintained at a level that is low enough to ensure that the reaction continues in the forward direction.

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• Fall '10
• ALLISON

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