Chemistry_Grade_10-12 (1).pdf

3 increasing the pressure favours the side of the

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3. Increasing the pressure favours the side of the equilibrium with the least number of gas molecules. This is shown in the balanced symbol equation. This rule applies in reactions with one or more gaseous reactants or products. 4. Decreasing the pressure favours the side of the equilibrium with the most number of gas molecules. This rule applies in reactions with one or more gaseous reactants or products. 5. If the concentration of a reactant (on the left) is increased, then some of it must change to the products (on the right) for equilibrium to be maintained. The equilib- rium position will shift to the right. 6. If the concentration of a reactant (on the left) is decreased, then some of the products (on the right) must change back to reactants for equilibrium to be maintained. The equilibrium position will shift to the left. 7. A catalyst does not affect the equilibrium position of a reaction. It only influences the rate of the reaction , in other words, how quickly equilibrium is reached. Worked Example 81: Reaction Rates 1 Question: 2 NO 2 ( g ) 2 NO ( g ) + O 2 ( g ) and Δ H > 0 How will the rate of the reverse reaction be affected by: 1. a decrease in temperature? 2. the addition of a catalyst? 3. the addition of more NO gas? Answer 312
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CHAPTER 16. REACTION RATES - GRADE 12 16.8 1. The rate of the forward reaction will increase since it is the forward reaction that is exothermix and therefore produces energy to balance the loss of energy from the decrease in temperature. The rate of the reverse reaction will decrease. 2. The rate of the reverse and the forward reaction will increase. 3. The rate of the reverse reaction will increase so that the extra NO gas is converted into NO 2 gas. Worked Example 82: Reaction Rates 2 Question: 1. Write a balanced equation for the exothermic reaction between Zn(s) and HCl. 2. Name 3 ways to increase the reaction rate between hydrochloric acid and zinc metal. Answer 1. Zn ( s ) + 2 HCl ( aq ) ZnCl 2 ( aq ) + H 2 ( g ) 2. A catalyst could be added, the zinc solid could be ground into a fine powder to increase its surface area, the HCl concentration could be increased or the reaction temperature could be increased. Exercise: Reaction rates and equilibrium 1. The following reaction reaches equilibrium in a closed container: CaCO 3 ( s ) CaO ( s ) + CO 2 ( g ) The pressure of the system is increased by decreasing the volume of the con- tainer. 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 ( s ) + H 2 O ( g ) CO ( g ) + H 2 ( g ) Δ 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.
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