Most of the question came out in the preliminary examinations or the A levels H2 Chemistry examinations usually involve standard definitions that most students are familiar with…

such as enthalpy change of solution, formation, combustion, etc. However, once in a while, there are a few questions that are totally different, and as such, many students will be thrown off their comfort zones and have problems constructing energy cycles.

An example is found in VJC 2012, paper 3:
Below are some thermodynamic equations involving zinc compounds.

ZnO(s) + 3C(s)  ® ZnC2 (s) + CO (g) …… (1)                               ΔHθ = +462 kJmol-1

ZnO(s) + H2O (l)  ® Zn(OH)2 (s)      ………(2)                              ΔHθ = -65kJmol-1

ZnC2(s) + 2H2O(l)  ® Zn(OH)2 (s) + C2H2 (g)  …..(3)               ΔHθ = -126 kJmol-1

Using these data together with the following data,

2C(s)   +   O2 (g)   ®  2CO2 (g) ….  (4)                                              ΔHθ = -221 kJmol-1            
2 H2O (l)  
® 2H2(g)   +  O2(g)  ……(5)                                             ΔHθ = +572 kJmol-1


Determine the standard heat of formation of C2H2 (g) with the aid of an energy cycle.

This equation is a repeat of a much older question that appeared in the early 2000s


Click on the link below for the solution: 

Step by step solution

This is the typical kind of questions we go through in our Chemistry classes : interesting and challenging questions that are not the norm. To receive the full benefits of our classes, ENROL NOW!

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