Energy changes
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What two experiments can we do to indirectly determine the enthalpy change of the reaction of copper with hydrochloric acid bearing in mind that one of them must be a displacement of copper. How would you calculate the value? Do you suggest looking at data values? Thank you.
Igloo writes ..
Carry out a risk assessment before starting any practical work, and check with your teacher.
Copper does not react with hydrochloric acid under any conditions (the total entropy change is negative), so, to determine the hypothetical value for the enthalpy change we need to construct a suitable Hess cycle.
The clue you have been given is to use a displacement reaction involving copper, so, how about using zinc and aqueous copper(II) chloride to form copper and aqueous zinc chloride?
Reaction 1: Zn + CuCl2 -> Cu + ZnCl2
Next, we need to consider the reaction involving zinc and hydrochloric acid:
Reaction 2: Zn + 2HCl -> ZnCl2 + H2
Both of these reactions can be carried out in the laboratory, using, for example, an expanded polystyrene cup, and then, having calculated the standard enthalpy changes for these two processes, we can set up the Hess cycle below:
From this, DHreaction = DH2 - DH1
The result should be a positive one (endothermic), but I emphasise that this reaction cannot take place in practice.
Alternatively, using standard enthalpy changes of formation from the appropriate tables in the Nuffield Book of data, it would be possible to determine the value by calculation.
Why exactly is the reaction between copper and hydrochloric acid endothermic? I thought that a reaction between an acid and a metal was exothermic. Also, are the enthalpy changes for the other two reactions (zinc +HCl) and zinc and CuCl2 displacement both exothermic? Furthermore, I cannot find the data values for the enthalpy of formation of the compounds on the internet or any books.
Could I also just ask why we decided to use zinc and not magnesium or iron with hydrochloric acid?
Also what is the colour of the zinc chloride salt formed, because I cannot find this information?
Igloo replies
The reactions of metals with hydrochloric acid are exothermic, e.g. Mg, Al, Zn, Fe etc., but, in the case of copper, we are dealing with a metal which does not react with hydrochloric acid in practice. The endothermic value for this enthalpy change is an indication of the fact that this value is totally hypothetical.
If you consider entropy changes - at A2 this is what you should be doing - and take into account the endothermic value for this hypothetical process, you will establish a negative value for the total entropy change and therefore confirm that we are dealing here with a process which simply doesn’t occur in the laboratory!
Displacement reactions (e.g. Zn with CuCl2) are exothermic, as you rightly point out.
Values for the relevant enthalpy changes of formation are found in Table 5.6 (pages 112-113) of the Nuffield Book of Data: “Thermochemical data for ions in aqueous solution”.
I’ll show you the relevant calculation for
Cu(s) +2HCl(aq) -> CuCl2) (aq) + H2 (g)
Written in ionic format this is:
| Cu(s) + | 2H+(aq) -> | Cu2+ (aq) + | H2 (g) |
| DHformation (from Book of Data): | |||
| 0 | 0 | +64.8 | 0 kJ mol-1 |
DHformation (from Book of Data) : 0 0 +64.8 0 kJ mol-1
DHreaction = DHformation[products] - DHformation[reactants] = +64.8 - 0
= +64.8 kJ mol-1 (endothermic, as already mentioned)
It would be a useful exercise now for you to convert the other two equations (Zn with HCl and Zn with CuCl2) into ionic equations and then to apply the relevant data from the same table. If similar calculations are carried out you will arrive at answers of -153.9 kJ mol-1 and -218.7 kJ mol-1, respectively (both exothermic processes, as already mentioned).
I chose zinc because it was the first appropriate metal to come to mind! I could equally well have chosen metals such as magnesium or iron instead, though with iron the reaction with HCl is rather slow in practice, so this is a problem which would need to be overcome in the laboratory.
Finally, the colour of solid zinc chloride (and other zinc compounds too) is white, so the solutions formed will be colourless.
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updated: 28 August 2004
