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16 Cards in this Set

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What is Le Chatelier's principle?

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.

(Using Le Chatelier's Principle) Changing conecntration

If you increase the concentration of a reactant , the equilibrium tries to get rid of the extra reactant. This is done by making more product. So the equilibrium shifts to the right. If you increase product the opposite happens and the equilibrium shifts to the left. If you decrease the concentration the opposite happens.

(Using Le Chatelier's principle) Changing pressure

Changing the equilibrium 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.

(Using Le Chatelier's principle) Changing temperature

Increasing the temperature means adding heat. The equilibrium shifts in the endothermic (positive delta H) direction to absorb this heat. Decreasing the temperature removes heat. The equilibrium shifts in the exothermic (negative delta H) direction to try and replace the heat. If the forward reaction's endothermic, the reverse reaction will be exothermic, and vice versa.

The reaction of [Cu(H20)6]+2 with concentrated hydrochloric acid

When [Cu(H20)6]+2 reacts with HCl, a copper chloride complex [CuCl4]2-, forms. This is a reversible reaction, so at any point there will be a mixture of [Cu(H20)6]+2 and [CuCl4]-2 present in the reaction container. [Cu(H20)6]+2 is a light blue colour while [CuCl4]-2 is a greeny-yellow. You can therefore monitor the equilibrium position of this reaction by noting the colour change. If the the solution is blue, then the position of equilibrium must lie to the left nd there'll be more reactants than products. But, if the the solution is greeny-yellow, the equilibium position must lie to the right and there'll be more products.

Changing the concentration ([Cu(H20)6]+2 reaction)

If you have a test tube containing [Cu(H20)6]+2, you'll see it's a light blue colour. If you slowly add concentration HCl the solution will change to bluey-green as the equiibrium is established. The more HCl you add the more green the solution turns as more product is formed- this is because you are increasing the concentration of Cl- ions. The equilibrium shifts to the right to try and remove the excess Cl- ions from the solution. You can push the equilibrium back to to the right by adding distilled water. The equilibrium position moves to mop up all the extra H20 molecules you adding to the soultion by forming more [Cu(H20)6]+2. Turning the solution blue again

Changing the temperature ([Cu(H20)6]+2 reaction)

The forward reaction of the equilibrium is endothermic is you add heat to the reaction it will attempt to get rid of this extra heat by taking it in making more product so the reaction will turn more green. The opposite happens if you cool the mixture down. The equilibrium will shift to the left to favour the reverse reaction and try to make up for the loss of heat. This means the solution will turn more blue.

Industrial processes- compromises in industry

Ethanol can be made via a reversible reaction between ethene and steam.


The conditions are as follows:


~ 60-70 atmospheres


~ temperature of 300 degrees


~ a phosphoric acid catalyst


Because it's an exothermic reaction, lower temperatures favour the forward reaction. This means that at lower temperatures more ethene and steam are converted to ethanol- you get a better yield. But lower temperatures means a lower rate of reaction- it will take too long to get a really high yield if its done at a low temperature. So 300 degrees is a compromise between max yield and a faster reaction.


Higher pressure favours the forward reaction as the gas molecule ratio is 2:1. However it is expensive to produce high pressures. So 60-70 atmospheres is a compromise between maximum yield and minimum expense.


Only a small proportion of the ethene reacts each time the gases pass through the reactor. To save money and raw materials, the unreacted ethene is separated from the ethanol and recycled back into the reactor. So 95% of the ethene is eventually converted to ethanol.

Methanol reversible reaction

Just like ethnol production, the conditions are a compromise between keeping costs low and yield high. The conditions for this reactions are:


~a pressure of 50-100 atmospheres


~a temperature of 250 degrees


~a catalyst of a mixture of copper, zinc oxide and aluminium oxide


2H2(gas) + CO(g) <=> CH3OH(g) delta h= -90 kJ mol-1

The equilibrium constant, Kc

aA + bB <=> dD + eE



Kc = [D]d[E]e = Products


-----------------------------


[A]a[B]b Reactants



Square brackets,[ ], mean concentration in mol dm-3


The lower case letters are the number of moles of each substance

Calculationg equilibrium constant, Kc

Just stick the concentrations into the expression for Kc. To work out the units of Kc, put the units into the expression instead of the numbers


You might have to figure out some of the equilibrium concentrations before you can find Kc.


1) Find out how many moles of each reactant and product there are at equilibrium. You'll usually be given the number of moles at equilibrium for one of these reactants. You can then ue the balanced reaction equation to work out all the moles of the others.


2) Calculate the molar concentration of each reactant and product by diviing each number of moles by the volume in the question but you may have to convert it into different units. To work out molar concentrations you need the volume to be in dm3.

Using Kc

If you know the value of Kc, you can use it to find unknown equilibrium concentrations.


1) Put all the values you know into the expression for Kc


2) Rearrange the equation and solve it to find the unknown values

Factors affectng the equilibrium constant- TEMPERATURE

If the change means more product is formed, Kc will rise.


If it means less product is formed, then Kc will decrease.

Factors affecting the equilibrium constant- CONCENTRATION

The value of the equilibrium constant, Kc, is fixed at a given temperature. So if the concentration of one thing in the equilibrium mixture chanes then the concentrations of the others must change to keep the value of Kc the same.

Factors affecting the equilibrium constant- ADDING A CATALYST

Catalysts have no effect on the position of equilibrium or o the value of Kc. This is because a catalyst will increase the rate of both the forward and the backward reaction. As a result, the equilibrium position will be the same but the speed at which an equilibrium is reached will be faster. So catalysts can't increase the yield- but they do decrease the time taken to reach equilibrium.

Redox reactions- What are they?

A loss of electrons is called oxidation. A gain in electrons is called reduction. This processes happen simultanously- hence redox. A reducing agent donates electrons and gets oxidised.