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| Question | Answer |
| What are the properties of a dynamic equilibrium? | Rate of forward reaction is the same as the rate of backward reaction Concentrations of reactants and products stay constant Dynamic equilibrium can only happen in a closed system |
| How is a dynamic equilibrium set up? |
As the reactants get used up, the forward reaction slows down. As more product is formed, the reverse reaction speeds up. Eventually, it'll reach a point where the rate of the forward reaction is equal to the rate of the backward reaction
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| What does Le Chatelier's principle state? | Le Chatelier's principle states that if there is a change in concentration, pressure or temperature, the position of equilibrium will move to help counteract the change |
| What happens to the position of equilibrium when you increase the concentration of a reactant? conc. of reactant > conc. of product | Equilibrium shifts to the RIGHT Equilibrium tries to get rid of the extra reactant by making more product |
| What happens to the position of equilibrium when you increase the concentration of a product? conc. of product > conc. of reactant | Equilibrium shifts to the LEFT Equilibrium tries to remove the extra product, making the reverse reaction go faster |
| What happens to the position of equilibrium when you increase the pressure? | Equilibrium shifts to the side with FEWER gas molecules |
| What happens to the position of equilibrium when you decrease the pressure? | Equilibrium shifts to the side with MORE gas molecules |
| What happens to the position of equilibrium when you increase the temperature? | Equilibrium shifts in the ENDOTHERMIC direction (+ΔH) to absorb the extra heat energy |
| What happens to the position of equilibrium when you decrease the temperature? | Equilibrium shifts in the EXOTHERMIC direction (-ΔH) to try to replace the heat energy that's been removed |
| In the following reaction, is the forward or reverse reaction exothermic? C₂H₄ + H₂O --> C₂H₅OH (ΔH = -46kJmol⁻¹) | FORWARD = exothermic REVERSE = endothermic |
| What happens to the position of equilibrium when you add a catalyst? | Nothing Catalysts have NO EFFECT on the position of equilibrium - they speed up both the forward and reverse reactions by the same amount Catalysts make the reaction reach equilibrium faster |
| TRUE OR FALSE: Catalysts increase the yield of the product formed by a reaction in equilibrium | FALSE They can't increase the yield (as the reaction is in equilibrium - both the forward and reverse rates of reaction are equally sped up) |
| What are the conditions for the production of ethanol? | 60-70atm 300°C Phosphoric acid catalyst |
| When producing ethanol, a low temperature should be used for a better yield. Explain why a lower temperature would increase the yield. C₂H₄ + H₂O --> C₂H₅OH (ΔH = -46kJmol⁻¹) | The forward reaction is exothermic so lower temperatures favour the forward reaction and you get a better yield. |
| In the production of ethanol, a low temperature should be used for a better yield. However, in industry the temperature is much higher (300°C). Why? | Lower temperatures mean a slower rate of reaction. Therefore 300°C is a compromise between maximum yield and a faster reaction |
| When producing ethanol, a high pressure should be used for a better yield. Explain why a high pressure would increase the yield. C₂H₄ (g) + H₂O (g) --> C₂H₅OH (g) | Higher pressures favour the forward reaction as when you increase the pressure, the position of equilibrium shifts towards the side with fewer moles (the right). |
| In the production of ethanol, a high pressure should be used for a better yield. However, in industry the pressure is lower (60-70 atm). Why? | High pressures are expensive to produce. Increasing the pressure can also cause side reactions to occur - so the 60-70atm is a compromise between maximum yield and expense |
| Only a small proportion of ethene reacts each time the gases pass through the catalyst. What do industries do to save money and raw materials? | The unreacted ethene is separated from the liquid ethanol and recycled back into the reactor. As a result, around 95% of the ethene is eventually converted to ethanol |
| The brown gas NO₂ exists in equilibrium with the colourless gas N₂O₄. 2NO₂ ⇌ N₂O₄ (ΔH = -57.2 kJmol⁻¹) Outline an experiment investigating the effect of changing temperature on the position of equilibrium. Describe the expected results. | 1. Place two sealed tubes containing the equilibrium mixture in water baths - one in a warm water bath and one in an ice bath 2. Observe the colours of the mixtures The tube in the warm water bath will change to a darker brown colour (as the equilibrium shifts to the left to absorb the extra heat by favouring the endothermic reaction) The tube in the ice bath will go a paler colour (as the equilibrium shifts to the right to replace the lost heat by favouring the exothermic reaction) |
| Iron (III) nitrate (yellow) and potassium thiocyanate (colourless) react together to form Fe(SCN)²⁺. Outline an experiment investigating the effect of changing concentration on the position of equilibrium. Describe the expected results. | 1. Number 4 test tubes 2. For test tube 1, add just the equilibrium mixture 3. For test tube 2, add a few drops of iron (III) nitrate solution to the mixture 4. For test tube 3, add a few drops of potassium thiocyanate 5. For test tube 4, add a few drops of iron (III) thiocyanate Test tube 1 = reddish Test tube 2 = deep red Test tube 3 = deep red Test tube 4 = yellow |
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