Use of Enthalpy Changes of Metal Reactions


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Use of Enthalpy Changes of Metal Reactions

The problem: The problem that I will investigate is how to use
enthalpy changes of metal reactions to experimentally test the
reactivity series of common metals. Therefore the area that I am
looking at is reactions involving metals and the heat energy that is
evolved when these take place. In brief the reactivity series of
metals is a list in which metals are ranked according to their
reactivity, from the most reactive metals (such as Pottassium) at the
top to Gold the least reactive metal at the bottom. Therefore my
research question is “Can enthalpy changes of reaction be used to
predict the place of a metal in the metals reactivity series?”

Hypothesis:

“In reactions between various metals and the same other single
reactant, the more reactive the reacting metal the stronger will be
the bonds in the metal product formed . This will be observable
through more exothermic enthalpy changes of reaction for more reactive
metals.”

The hypothesis is the prediction which should help me to answer the
research question. If the hypothesis is proved correct through
experimentation then I can apply it to predict the place of the metals
I have investigated in the reactivity series. This will be as simple
as arranging the metals investigated in order of decreasing enthalpy
change of reaction and then comparing this list with published and
accepted data. If they match then the hypothesis will be proved
correct. If this is true then the answer to the research question will
be yes as I will then be able to use enthalpy change of reaction to
work out the place of a metal in the reactivity series without the
need to refer to a book.

The hypothesis is based on the following explanation. The enthalpy
change of a reaction is the heat evolved when a reaction takes place.
All chemical bonds hold heat energy within them. Enthalpy change of
reaction is the difference between the heat held in the bonds of

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reactants, and the heat held in the bonds of the products, of a
reaction. If the bonds in the products are stronger than the bonds in
the reactants then heat will be given off by the reaction as a strong
bond is more stable and holds less energy within it. This is called an
exothermic reaction and would be considered to have a negative
enthalpy change of reaction because heat energy has been given up into
the surroundings. Likewise a reaction in which the product bonds are
weaker than the reactant bonds will take up heat from the surroundings
and will be considered to have a positive enthalpy change as heat
energy has been gained. This is called an endothermic reaction. My
hypothesis is based on the understanding that the more reactive metals
will create more stable compounds on reaction, meaning that less
energy will be held in their strong and stable bonds.

I have chosen to test my hypothesis using the reaction between copper
sulphate solution and metals of various reactivity. I have chosen this
reaction because it is simple and safe, and because based on the
standard electrode potential for the reduction of copper ions to
copper metal it should be spontaneous with a good number of common
metals.

For example: The reaction between Lithium metal and copper sulphate.
The half cell equations are:

Þ [IMAGE] Cu2+ + 2e- Cu = + 0.34 v

Þ [IMAGE] 2 Li 2 Li+ + 2e- = + 6.06 v

Þ Thus the reaction will be spontaneous with a positive S.E.P of 6.4 v

I can then measure the heat given off by this reaction to calculate
the enthalpy change of the reaction. I calculate the enthalpy change
using:

Δ H = m c ( Tinitial – Tfinal) in joules, where c = specific heat
capacity water, m = mass of copper sulphate, and T = temperature.

The controlled variables in the investigation are:

* The quantity of copper sulphate used. (25 cubic centimetres)

* The quantity of metal used. (In excess in all cases)

The independent variable is the choice of metal in the experiment.
This changes from experiment to experiment. The dependant variable is
the aspect which will change because of the changing metal: It is ΔH
in all cases in this investigation.

The uncontrolled variables are aspects over which I have no effective
control. These are things like the ambient temperature in the cup
which may change with external temperatures. I am minimising this by
taking a new initial temperature reading for every experiment. The
surface area of the metal is an uncontrolled variable in that it
affects the rate of reaction. Rate of reaction has no impact on the
enthalpy change but the temperature change will be harder to measure
the slower the reaction, as there will be a greater period in which
heat can escape from the reaction vessel where temperature is
measured. This can be mitigated by using metal filings wherever
possible.

[IMAGE]Apparatus:
Thermometer

[IMAGE]

[IMAGE]


* Insulated Styrofoam cup

* Plastic lid with hole for above

* 0 – 100 oC
thermometer
Cup

* [IMAGE][IMAGE]samples of common metals

* Copper sulphate solution

* [IMAGE]Stopwatch

Method:

The reaction will be conducted in the insulated Styrofoam cup to
minimise the heat loss to the surroundings. The lid should also
minimise the heat lost to air circulation. The thermometer will be
introduced through a small hole in the lid and the initial and final
temperatures of the reaction measured in situ thus. 25cm3 of copper
sulphate will be measure into the cup and then 0.5 (g) ( an excess) of
the metal will be added and the stopwatch started. A temperature
reading will be taken every fifteen seconds until the temperature of
the reaction has dropped for three readings in succession. This means
that the reaction has completed, passing its peak temperature. Heat
leaving the cup is responsible for the fall in temperature after this
point. It is important to take further readings after the peak
temperature has passed as these allow the true peak temperature to be
extrapolated accounting for the heat lost to surroundings during the
experiment. A graph of time on the x axis and temperature on the y
axis will be plotted for each reaction. I will draw a best fit line
through the points with a positive gradient and another through the
points with negative gradients. Where these two line cross will be the
true peak temperature and thus enthalpy change of the reaction. This
is because heat loss error will mean that the temperature measured
will not take account of all the heat that has been generated by the
reaction.

[IMAGE]Temp (oC)

[IMAGE]


[IMAGE] Time (s)

To provide sufficient data for analyses I have chosen five metals with
which to conduct the reaction. I have chosen them on the basis that I
expect them to react with copper ions on the basis of their standard
electrode potentials and that I expect them to cover a reasonable
range of the reactivity series, so that their should be significant
differences in the enthalpy change values for each of them. Therefore
the metals for which I shall measure the enthalpy change of reaction
with copper sulphate are:

* Lithium

* Magnesium

* Zinc

* Nickel

* Lead


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