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Chemistry 2.9
AS edexcel chemistry - chemical equilibria
| Question | Answer |
|---|---|
| What is a reversible reaction? | A reaction that goes in both directions at the same temperature, so it never reaches completion. A + B <-> C + D |
| What is equilibrium and what does this look like on a graph? | A system has reached equilibrium when the concentration of the reactants and products remain constant. Concentration on y axis, time on x axis, both lines for reactants and products are horizontal |
| What is dynamic equilibrium? | When the rates of the forward and backward reactions are equal so there is no further change in the concentrations of the reactants or products |
| What defines the position of equilibrium? | The ratio of product to reactant concentration: [concentration of products in mol/dm3] divided by [concentration of reactants in mol/dm3] |
| What is the equilibrium yield? | The percentage of reactant that is converted into product |
| What is Le Chatelier's principle? | When the external conditions of a system in equilibrium are changed, the position of equilibrium shifts to nullify the change |
| What are the conditions required for equilibrium? | Closed system e.g. sealed container |
| What happens when temperature increases? | An increase in temperature causes the position of equilibrium toshift in the endothermic direction, increasing equilibrium yeild of endothermic reaction. System reacts to reduce temperature by converting heat energy into chemical energy. |
| Continued | Rate of reaction increases (collision theory). Rate of reaction increases more in the endothermic direction than the exothermic direction because the activation energy is higher and reactions with high Ea are more sensitive to temperature changes |
| What happens when pressure is increased? | An increase in pressure causes the position of equilibrium to shift to the side with fewer gas moles. The system reacts to reduce the pressure by reducing the number of gas moles present. Faster rate of reaction, collision theory. Only for gases! |
| What happens when concentration of a species increases? | Only for reactions with solutions! The position of equilibrium shifts to produce less of that species. Increased rate of reaction, collision theory |
| What happens when a catalyst is added? | No affect on position of equilibrium, no change in yield. Increases rate of reaction equally in both directions by providing an alternative path with a lower Ea, allows equilibrium to be reached more quickly. Low pressure, temp, saves energy, money, fast |
| CH4 (g) + 6H2O (l) ⇌ [CH4(H2O)6] (s) ΔH = -ve. What are the best conditions to extract CH4? | High temperature because reaction in exothermic left to right, and high temperature would shift position of equilibrium to the left. Low pressure, 1 gas mole on left, 0 on right |
| Cr2O72- (aq) + H2O (l) ⇌ 2CrO42- (aq) + 2H+ (aq) . What happens with KOH is added? | OH- ions react with H+ ions to form H2O - concentration of H+ decreases and H2O increases, so equilibrium shifts to the right |
| What happens when chlorine is added to iodine in a U-tube? | Firstly reacts to form ICl which is a brown liquid with a brown vapour above it. Then ICl + Cl2 <-> ICl3. When U-tube is tilted, ICl3 decomposes so position of equilibrium shifts to the left |
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