Equilibria and Redox Reactions - Unit 1, Section 6 Flashcards
what is dynamic equilibrium
when the forward reaction is going at the exact same rate as the backward reaction.
can only happen in a closed system which is at a constant temperature.
Le Chatelier’s principle
tells you that the position of equilibrium will change if a condition changes:
‘If a reaction at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will move to counteract the change’
changing concentration
if you increase the concentration of a reactant, the equilibrium tries to get rid of the extra reactant. It does this by making more product, so the equilibrium has shifted to the right.
if you increase the concentration of a product, the equilibrium will shift left as it tries to make more reactants.
decreasing the concentrations has the opposite effects.
changing pressure
only affects equilibria involving gases.
increasing the pressure shifts the equilibrium to the side with fewer gas molecules. This reduces the pressure.
decreasing the pressure shifts the equilibrium to the side with more gas molecules. this raises the pressure again.
changing temperature
increasing the temperature makes the equilibrium shift in the endothermic direction to absorb this heat.
decreasing the temperature shifts the equilibrium in the exothermic direction to replace the heat.
if the forward reaction is endothermic, the backward reaction will be exothermic and vice versa.
compromise conditions in industry - reaction between ethene and steam to produce ethanol
conditions =
- pressure of 60-70 atm
- temp of 300*C
- phosphoric acid catalyst
it is an exothermic reaction so lower temperatures favour the forward reaction. this means that at lower temperatures more ethene and steam are converted to ethanol and you get a better yield. However, lower temperatures mean a slower rate of reaction. So, 3008*C is a compromise between maximum yield and a faster reaction.
higher pressure favours the forward reaction as it moves to the side with fewer gas molecules. so a pressure of 60-70 atm is used, as high pressures mean faster rate of reaction but it is also expensive to use higher pressures so this is a compromise between max yield and minimum expense.
only a small amount of ethene reacts each time the gases pass through the reactor so the unreacted ethene is separated from the ethanol and recycled back into the reactor, to save money and raw materials.
Kc, the equilibrium constant
Kc = [D]d x [E]e / [A]a x [B]b
products/reactants
find out equilibrium concentrations before finding Kc
- find out how many moles of each reactant and product there are at equilibrium.
- calculate the molar concentrations of each reactant and product by dividing each number of moles by the volume of the reaction.
- substitute values into the expression for Kc and calculate it.
using Kc to find unknown equilibrium concentrations
- put all the values you know into the expression for Kc
- rearrange the equation and solve it to find the unknown values.
changing the temperature - effect on equilibrium concentration (Kc)
if you increase the temperature, the equilibrium shifts in the endothermic direction to absorb the heat.
decreasing the temperature removes heat energy. the equilibrium shifts in the exothermic direction to try to replace the heat.
if the change means more product is formed, Kc will rise.
if it means less product is formed, then Kc will decrease.
changing the concentration - effect on Kc
if the concentration of one thing in the equilibrium mixture changes then the concentrations of the others must change to keep the value of Kc the same.
adding a catalyst - effect on Kc
catalysts have no effect on the position of equilibrium or the value of Kc.
adding a catalyst will increase the rate of both the forward and backward reactions by the same amount, therefore the position of equilibrium will be the same, it will just be reached faster.
what is a redox reaction?
where reduction and oxidation happen simultaneously.
OILRIG
Oxidation
Is
Loss (of electrons)
Reduction
Is
Gain (of electrons)
what is an oxidising agent?
accepts electrons and gets reduced
