Reversible Reactions

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A level Chemistry (3.2 Physical Chemistry) Flashcards on Reversible Reactions, created by Yinka F on 06/03/2018.
Yinka F
Flashcards by Yinka F, updated more than 1 year ago
Yinka F
Created by Yinka F over 6 years ago
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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
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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