Investigate the factors affecting the enthalpy change during a displacement reaction.

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The Task

To investigate the factors affecting the enthalpy change during a displacement reaction.

Background Knowledge

In this investigation a displacement reaction will be observed. A displacement reaction occurs when more reactive metal displaces, (or removes) a less reactive metal from a solution of its salt. Also known as chemical bullying because a more reactive metal effectively bullies the original metal out of its compound.

In this investigation zinc and copper sulphate will be reacted together. Zinc is the more reactive metal here, so the copper is displaced from the copper sulphate solution.

Zinc + Copper sulphate ?inc sulphate + Copper

Zn (s) + CuSO4 (aq) ?nSO4 (aq) + Cu (s)

After the reaction it will be observed that the solution will change from blue to clear, colourless. The copper discharged will be observed as a salmon pink deposit in the base of the vessel the reaction occurs in.

Reactions like this can be used to purify metals that are low down in the reactivity series. A more reactive metal can simply be added to purify the less reactive metal, leaving it in its pure state.

Lead Oxide + Tin ?ead + Tin Oxide

In reality thought this process is hardly ever used, as the more reactive metal is the left in an impure form, and thus must be reduced by an even more reactive metal. As this is not economically viable metals are generally reduced using electrolysis, which leaves them in a pure state, and requires no expensive more reactive metals, just electricity and often heat.

The displacement reaction is a redox reaction. This means that on one side of the reaction electrons are being lost and on the other electrons are being gained.

The enthalpy change in a reaction is in effect the energy chance between the reactants and the products. If there is more energy in the bonds of the products than the amount in the bonds of the reactants then it can be seen that the reaction is endothermic. This means that it must take energy in the form of heat from its surroundings.

The opposite is also true. If the bonds between the reactants in a reaction have more energy than the bonds in the product then it can be seen that energy is given out. This takes the form of heat energy, so the reaction is said to be exothermic.

Energy is produced when bonds are formed, and energy is required when bonds are to be broken. This means that in an exothermic reaction more bonds are being formed than are being broken, oppositely in an endothermic reaction more bonds are being broken than are being formed.

To take an accurate reading of the enthalpy within a reaction, (difference in/delta H) can be found. This reading can be calculated off a graph, however it will be more accurate if the formula is used.

/\H = mc/\T

The enthalpy, (delta H) is the difference ion heat energy, in Joules, m is the mass of the solution, c is the specific heat capacity of water, and /\T is the difference in temperature.
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Knowing this formula it is easy to find and compare the enthalpy change when variables are altered.

Secondary Sources

Work covered during the GCSE course. This helped as detailed notes had been taken referring to the nature of displacement reactions and energetics.

Experimental Prediction

I believe that when the concentration of copper sulphate solution is doubled and reacted with a surplus of zinc the enthalpy change will double. This means that the reaction will get twice as hot. I believe this is true, as the ratio of bond energy between the reactants and products ...

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