Chemistry
Sensors:
Loggers:
Temperature
Any EASYSENSE
Logging time: EasyLog
Teacher’s notes
08 A displacement reaction
Read
Metals can be put into an order of reactivity. A metal higher than another in the reactivity series can
displace a metal below it from a solution of its salt.
The periodic table lists metals in order of reactivity, in general the lower down and further to the left
the metal is positioned in the periodic table the more reactive it is likely to be. As in all lists there are
some exceptions to this rule.
A metal in a reactivity series can displace any metal below it in the series, from the less reactive
metal's oxide, chloride or sulphate or other compound.
E.g. on heating the mixture of a metal (higher in the series) and another metal oxide (lower in the
series), such as magnesium powder and black copper(II) oxide, a very exothermic reaction occurs and
white magnesium oxide is formed with brown bits of copper: The magnesium has displaced the copper
from its oxide.
Magnesium + copper oxide => magnesium oxide + copper
Mg(s) + CuO(s) => MgO(s) + Cu(s)
The more reactive magnesium displaces the less reactive copper.
Or, as in the suggested experiment, adding a metal to a salt solution of another metal e.g. adding
magnesium to blue copper(II) sulphate solution, the blue colour fades as colourless magnesium sulphate
is formed and brown bits of copper metal form a precipitate, heat is also produced which is measured
by the Temperature sensor:
Magnesium + copper sulphate => magnesium sulphate + copper
Mg(s) + CuSO4(aq) => MgSO4(aq) + Cu(s)
If no reaction happens, then it means the added metal is less reactive than the metal in the oxide,
sulphate, chloride etc.
Apparatus
1.
An EASYSENSE logger.
2. A Smart Q Temperature sensor. To prevent the Temperature sensor from becoming
discoloured, wrap the part of the sensor that is in contact with the solution in a single layer of
cling film.
3. 2 beakers for reaction.
4. 2 larger beakers may be needed to form insulating holders for the experiment.
5. Insulating material to surround beaker, plastic foam, expanded polystyrene etc.
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6. 2 retort stands and clamps.
7. Magnesium sulphate solution – MgSO47H2O.
8. Copper sulphate solution – CuSO45H2O.
9. Copper strip – Cu.
10. Magnesium strip – Mg.
Set up of the software and logger
Use EasyLog – experiment should last about 5 minutes.
Notes
For the experiment to work best the metal strips should be free from any surface corrosion or
tarnish, a light rub with a fine emery (carbide) cloth will remove most. Do not use chemical cleaners
(such as might be used for cleaning brass, silver, etc), these often contain oils and lacquers to coat the
newly cleaned surface and slow down future tarnish.
If alternative metals and salts are being provided take care not to cloud the results and analysis by
creating too many combinations. A few well chosen reactions will prove the point more effectively,
The largest surface area for the mass needs to be used to make the reactions fit into the time scale
of an average lesson. The standard ribbon for magnesium is about the ideal form for all the metals.
The temperature rise is being used as a measure of the chemical reaction taking place. If the results
are required to be more qualitative then the solutions should be made up to the same molar strength, 1
mol dm-3 should be adequate for most of the salt solutions. Check with data tables for solubility.
Sulphates are usually a good compromise between availability, solubility and cost.
To get temperature changes that could be measured and looked convincing we found that using a 50 ml
beaker filled to 20 ml mark with 1 mol dm-3 copper sulphate and adding approx 0.2 g of magnesium
ribbon gave a sufficiently vigorous reaction to be completed in 5 minutes.
When taking the temperature measurements it is advised to use some form of insulation around the
reaction vessel, a well cut into a piece of insulating foam is ideal. If there is no objection to using
larger quantities the reaction can be done in a polystyrene drinks cup. A large beaker filled with
insulation and the smaller vessel nested into the insulation works well.
Hazard information
Magnesium ribbon
Highly
flammable
This needs to be carefully monitored. Follow local guidelines about storage, quantities, etc.
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Results and analysis
You should be able to get about a 4 degree temperature change. The temperature change and loss of
colour in the solution are good indicators to students that a chemical reaction has taken place.
Sample results for Magnesium ribbon (0.2 g) added to copper sulphate (50 cm3) solution. Temperature
axis rescaled.
Extension
You have sufficient information to calculate the energy of the reaction (ΔH).
To do the calculations you will need to know,
1. Mass of magnesium used – to calculate moles of magnesium
2. Molarity and volume of copper sulphate – to calculate moles of copper sulphate.
3. Specific heat capacity of the copper sulphate solution and beaker (you can assume all the
apparatus was “water” and use 4.186. For greater accuracy you need to factor in all the
specific heat capacities for all the materials used).
4. The temperature change in the experiment.
From these values you can calculate the energy transfer, energy per gram and energy per mole.
Calculations
Heat released by reaction = heat gained by the solution
1.
2.
3.
4.
Heat gained by the solution = m x c x ΔT
M = Mass of solution (mass of acid + mass of magnesium ribbon)
C = specific heat capacity, assumed to be 4.18j g-1 K-1
ΔT = temperature change
Use the values of Tstart and Tfinal substituted into the calculation to find the energy transferred. Make
sure the volume actually used (in the experiment) of the acid and magnesium are used and not the
suggested volumes and masses.
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E.g. In a similar experiment 100.5 cm3 of acid were used, 4.2 g of magnesium ribbon added and a
temperature increase of 5.2oC was measured from the graph:
100.5 x 4.2 x 5.2 = 2195 J
1. The atomic mass of Magnesium = 24.3
2. Moles of Magnesium in the reaction is 0.500/24.3 = 0.0206 mol
Therefore, the heat released per mole of magnesium = 2195/0.0206 = 106 700 J mol dm-3 or 106.7 kJ
mol dm-3.
Extension to calculations
ΔS (entropy) can be calculated
ΔS = -ΔH/T
Taking value of ΔH from this expt.
T = Kelvin temp of room (298)
ΔS = - - 106.7/298 = 0.355
(Note the double negative above produces a positive number)
Or 355 kJ mol dm-3 (This reaction will take place spontaneously).
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