Investigation into the Factors that Affect the Enthalpy Change During a Displacement Reaction
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 à Zinc sulphate + Copper
Zn (s) + CuSO4 (aq) à ZnSO4 (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 à Lead + 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
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... middle of paper ...
...pacity of
the solution (assume this equals 4.2 J/g/degrees Celsius), /T =
temperature change (estimated from extrapolation).
The results here show the heat energy in each reaction. To calculate
the enthalpy change one must account for the respective moles involved
in each reaction.
To equal out the number of moles simply multiply the results by the
respective amounts required to make them all amounts of Joules/1 mole.
When this calculation is performed on all the heat energy results
enthalpy change can be compared. In an accurate experiment the
enthalpy change when copper sulphate solution is reacted with zinc
powder should remain constant. By taking into account the number of
moles in each equation when multiplying the results will be brought
into proportion, so the enthalpy changes should be roughly equal.
== = Hess’s law of heat summation states that the value of DH for a reaction is the same whether it occurs directly or as a series of steps. This principle was used to determine the change in enthalpy for a highly exothermic reaction, the combustion of magnesium metal. Enthalpy changes for the reactions of Mg in HCl (aq) and MgO (s) in HCl (aq) were determined experimentally, then added to that for the combustion of hydrogen gas to arrive at a value of –587 kJ/mol Mg.
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= 3 ´ E(C-H) + 1 ´ E(C-O) + 1 ´ E(O-H) + 1.5 ´ E(O=O)
-2152.7 x (56.1 / 1.37) = -88150.7 J.mol. 1. H = -88.15 kJ.mol. Hess' law states that: 1"The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided initial and final conditions are the same.
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