Label each of the following reactions as exothermic orendothermic (\"exo\" or \"

Question

Label each of the following reactions as exothermic orendothermic ("exo" or "endo"), and according to whether work isdone on or by the system ("on" or "by")? Note that no "en-on"cases appear here, as these are always thermodynamicallyunfavourable. There is no need to do any calculation, usechemical arguments to predict the sign of the enthalpy change.

Matching pairs

3NaN3(s) --> Na3N(s) + 4N2(g)
(Simplified version of the chemical reaction in air bags.NaN3 is sodium azide.)

2Cl(g) --> Cl2(g)

H2O(l) --> H2O(g)

2Li(s) + H2O(l) --> 2 LiOH(aq) +H2(g)

2H2(g) + O2(g) --> 2H2O(l)

Explanation / Answer

3NaN3(s) --> Na3N(s) + 4N2(g)
Normally, gases have a big volume per mole than a solid and thevolume change in solids is very small and negligible. Here N2 isproduced and so a mass expanse in volume occurs and work is done*by* the system.

2Cl(g) --> Cl2(g)
According to Avagadro's law which states that same number ofmolecules of any gas occupies the same volume at same pressure andtemperature.

H2O(l) -->H2O(g)

2Li(s) + H2O(l) --> 2 LiOH(aq) +H2(g)

Thisis a very common lab exercise and you must be aware that there's achance of Li catching fire so it's highlyexothermic.

Since, a gas is produced, work is done against externalpressure *by* system.

2H2(g) + O2(g) --> 2H2O(l)

Herethe gases condense to form water in liquid and so there's a rapidreduction in volume and so work is done *on* thesystem.

Related Questions

Navigate

• Browse (All)
• Browse L
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.