Le Chatelier’s principle states that as long as the temperature remains constant then the value of the equilibrium constant will also remain constant. However, Le Chatelier's Principle also states that the equilibrium system will react in order to remove extra heat, causing the reaction to proceed in the reverse direction, converting products back to reactants. This is because if an equilibrium system is subjected to change the position of the equilibrium will shift in order to minimize the effects of the change as much as possible. There are four factors that can affect the position of the equilibrium:
- Change in concentration
Adding or removing substances from chmical reactions affects the concentration of the substances in the equilibrium. As a result of this the value of the equilibrium expression no longer equals the value of the equilibrium constant. Le Chatelier’s principle allows us to predict whether the reaction will shift to the left or to the right. Increasing the concentrations of the reactants causes the equilibrium to shift to the right increasing the concentration of the products so that the concentration of the reactants decrease and resotre the equilibrium. In general if a substance is added then the equilibrium will move in the direction that will consume the substance added and if a substance is removed then it will move in the direction that can replace the removed substance
- Change in pressure
By changing the volume of a system of gases in equilibrium the pressure is affected. Again using Le Chatelier’s principle then we can predict how altering the pressure will affect the position of the equilibrium. By decreasing the number of moles in a gas the equilibrium shifts from left to right as decreasing the volume of the reaction mixture increases the pressure within the system. Therefore increasing the pressure in the system shifts the equilibrium to the side with the least volume, as it reduces the number of molecules and minimizes pressure within the equilibrium.
- Catalyst
Catalysts are known to affect the rate of a reaction. Due to this using a catalyst in an equilibrium reaction has the same effect on both the forward and reverse reactions in the equilibrium system. as a result there is no affect on the overall position of the equilibrium, the prescence of the catalyst only allows the system to reach the equilibrium quicker. As the catalyst does not effect the position of the equilibrium it does not effect the value of the equilibrium constant.
- Change in temperature
The effect that temperature can have on the equilibrium depends on the energy changes that take place in the chemical reaction. For exothermic reactions that are in the forward reaction, the equilibrium constant decreases with increasing temperature. However if the forward reaction is endothermic then the equilibrium increases with temperature. As we increase the temperature of a system we transfer energy to it so the equilibrium shifts to the left absorbing some of the added energy and therefore opposing the change. In a similar way when the temperature is decreased the equilibrium shifts to the right.
The equilibrium I am investigating is:
and I am going to be determining the affect of temperature on the value of the equilibrium constant in this reaction between Silver Nitrate and Iron(II) Sulphate.
Hypothesis:
The experiment taking place between the Silver Nitrate and the Iron (II) Sulphate is an exothermic one and therefore the value of the equilibrium constant will decrease as there will be an increase in temperature in the equilibrium system. This increase in heat pushes the system away from that of an equilibrium and so the nature of the equilibrium is to try and restore this heat somehow, the equilibrium therefore moves to the right to restore the balance.
The equilibrium of the mixed solution containing 25 cm³ each of 0.10 M silver nitrate and iron (II) sulphate solution. There was a precipitate of silver that settled at the bottom of the flask. Without disturbing the precipitate, we transferred 10 cm³ of the solution into another conical flask, and then performed a titration with 0.020 M potassium thiocyanate. The end point was marked by the first permanent red colour.
Variables:
Independent:
Temperature- will be changed by placing the beakers in a water bath and heating them up to the correct temperature using a Bunsen burner and cooling them down with cold distilled water.
Dependant- the concentration of the titrated iron sulphate and silver nitrate, this will be calculated using the formula: concentration = Number of moles
Volume
Control variables:
- Concentration of Silver Nitrate
- Concentration of Iron Sulphate
- Volume of Silver nitrate
- Volume of Iron Sulphate
- Volume of titrated solution from equilibrium
- Concentration of Potassium Thiocyanate
- Volume of titrated Potassium Thiocyanate
- Apparatus
Apparatus:
- 0.1M Silver Nitrate
- 0.1M Iron (II) Sulphate
- 0.02M Potassium Thiocyanate
- 2 25 cm3 measuring cylinder
- 2 10 cm3 titration pipette
- 2 conical flasks
- 1 400 cm3 beaker
- 1 thermometer
- 1 Distilled water
- 2 bungs
- 1 Bunsen Burner
- 1 tripod
- 1 gauze
Method:
- Take the 0.1M Silver Nitrate solution and pour out 25 cm3 using the 25 cm3 measuring cylinder, then pour the solution into a conical flask
- Take the 0.1M Iron (II) sulphate solution and using the 25 cm3 measuring cylinder measure out 25 cm3 and pour the solution into the conical flask containing the 25 cm3 Silver Nitrate solution.
- Place a bung on the conical flask containing both solutions and leave until an equilibrium is established which is when a precipitate of silver forms at the bottom of the conical flask.
- Using one of the 10 cm3 titration pipettes remove the bung and titrate 10 cm3 of solution from the conical flask trying to disturb the precipitate as little as possible and returning the bung when finished.
- Place the titrated solution into the second clean conical flask
- Using the second titration pipette titrate 10 cm3 of Potassium thiocyanate into the second conical flask.
- Place a bung on top of the second conical flask.
- Take the 400 cm3 beaker and fill it half full with water
- Insert the bunsen burner in the gas pipe but do not turn the gas on
- Place the gauze on top of the tripod and place the Bunsen burner underneath
- Place the beaker of water on top of the gauze and tripod and put the thermometer in the beaker.
- Turn on the Bunsen burner and heat the water till the temperature reaches 30oC then remove the Bunsen burner from underneath the tripod.
- Then record the concentration of the solution
- Continue to adjust the temperature by either adding cold water to the 400cm3 beaker of water or by adding heat using the Bunsen burner.
- Then record the concentration of the solution in a table
- Continue this until you have found a concentration at all your temperatures
- Turn off all the apparatus and clear away
