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# Reversible Reactions Help!

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1. Please can someone explain to me how the temperature/pressure/catalyst affects the way the equilibrium moves? I find it really hard to understand so lots of different ways of explaining it would be useful !

Thank You!

(By the way my exam board is AQA)
2. (Original post by k1234j)
Please can someone explain to me how the temperature/pressure/catalyst affects the way the equilibrium moves? I find it really hard to understand so lots of different ways of explaining it would be useful !

Thank You!

(By the way my exam board is AQA)
Temperature:
In short, an increase in temperature will INCREASE the rate of the ENDOTHERMIC reaction, whilst a DECREASE in temperature will INCREASE the rate of the EXOTHERMIC reaction.

Why?

In general, an equilibrium will attempt to revert to the original conditions, even when we change them. Therefore, when we increase the temperature, the equilibrium will decrease the temperature in order to revert back to the original temperature and vice versa. The only way for any reaction to change the temperature is due to the fact that one of its reactions its endothermic (take in energy, lowering the temperature), whilst the other is exothermic (give out energy, increasing temperature.
Thus, when we increase temperature, it attempts to lower that temperature back to what it was originally. In order to do so it increases the rate of the ENDOTHERMIC reaction to cool down the conditions to what they were initially.
To find out which reaction is therefore increased, just figure out which reaction is endothermic, and which is exothermic. They will state (or you will have previously figured out) the enthalpy change of the reaction. That enthalpy change is the one for the FORWARD reaction, so the backwards reaction is just the opposite of the one stated.
Finally, the exothermic reaction has a NEGATIVE enthalpy change, and an endothermic reaction has a POSITIVE enthalpy change.
Therefore you can find out which reaction if favoured, either the forward or backward reaction. Whatever the favoured reaction points to is the one the equilibrium shifts to: thus an increase in the forward reaction will increase in an increase of product yield.

Example: X + Y ---(reversible reaction) Z [-200KJ/mol]
Therefore if we increase the temperature in this experiment, the following occurs:
Equilibrium attempts to revert temperature to original temperature by decreasing it.
Equilibrium increases endothermic reaction.
Backward reaction is positive, thus endothermic.
Therefore increase in backward reaction.
Therefore DECREASE in product yield.

Pressure:
In short, an increase in pressure will shift towards the side of the reaction with LESS MOLES, a decrease in pressure will shift towards the side of the reaction with MORE MOLES.

Why?

In general, an equilibrium will attempt to revert to the original conditions, even when we change them. Therefore, when we increase the pressure, the equilibrium will decrease the pressure in order to revert back to the original pressure and vice versa. The only way for any reaction to change the pressure is due to the fact that each side of the reaction have different MOLES of gas.
Thus, when we increase pressure, it attempts to lower that pressure back to what it was originally. In order to do so it shifts the reaction to the side with LESS moles. The opposite is true when we decrease the pressure. Whichever reaction points towards the favoured side/side the reaction has shifted to is the reaction which increases in rate.

Example: 2A + 2B ---(reversible reaction) 3C
Therefore if we increase the pressure in this experiment, the following occurs: the equilibrium attempts to revert pressure to original pressure by decreasing it.
Equilibrium shifts to side with LESS MOLES.
Products have less moles (products = 3, whilst reactants have 2+2=4).
Equilibrium shifts to the products side.
Therefore increase in forward reaction.
Therefore INCREASE in product yield.

Catalyst:
A catalyst has no effect on equilibrium.
Why?
Because a catalyst equally increases the rate of BOTH the forward and backward reactions, ensuring that the balance does not shift.
3. (Original post by k1234j)
Please can someone explain to me how the temperature/pressure/catalyst affects the way the equilibrium moves? I find it really hard to understand so lots of different ways of explaining it would be useful !

Thank You!

(By the way my exam board is AQA)
I have a different exam board (edexcel igcse) and I dont know if you learn about The Haber Process so I didn't include much about that but here's what I learnt:

The position of equilibrium is the relative amounts of reactants and products in the closed system.

TEMPERATURE:
• Higher temperature increases endothermic reaction as it uses up the heat so the reaction increases
• Lower temperature increases exothermic reaction as it is then able to give out more heat
(If the forward reaction is exothermic, you use a lower temperature to move position of equilibrium to the right of the equation to give more product)

PRESSURE:
• Higher pressure encourages reaction which produces fewer molecules of gas
• Lower pressure encourages reaction which produces more molecules of gas
(High pressure is used in the Haber Process, as there are less molecules (or moles) on the right side of the equation. This shifts the equilibrium to the right and produces more yield)

CATALYST:
Catalysts increase the rate of reaction as it lowers the activation energy (the amount of energy needed for the reaction to start). However, catalysts do not affect yield, they just speed the reaction up.

4. (Original post by NuggetNugget)
I have a different exam board (edexcel igcse) and I dont know if you learn about The Haber Process so I didn't include much about that but here's what I learnt:

The position of equilibrium is the relative amounts of reactants and products in the closed system.

TEMPERATURE:
• Higher temperature increases endothermic reaction as it uses up the heat so the reaction increases
• Lower temperature increases exothermic reaction as it is then able to give out more heat
(If the forward reaction is exothermic, you use a lower temperature to move position of equilibrium to the right of the equation to give more product)

PRESSURE:
• Higher pressure encourages reaction which produces fewer molecules of gas
• Lower pressure encourages reaction which produces more molecules of gas
(High pressure is used in the Haber Process, as there are less molecules (or moles) on the right side of the equation. This shifts the equilibrium to the right and produces more yield)

CATALYST:
Catalysts increase the rate of reaction as it lowers the activation energy (the amount of energy needed for the reaction to start). However, catalysts do not affect yield, they just speed the reaction up.

Thank You! I do have to learn the haber process as well thanks!

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