IB Docs (2) Team MC test: Entropy & spontaneity
Multiple choice test on 15.2 Entropy & spontaneity
Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).
If you get an answer wrong, read through the explanation carefully to learn from your mistakes.
Which is a correct statement about entropy?
The units of entropy are J K−1 mol−1 and, unlike enthalpy, absolute values can be measured.
Which is a correct statement about the Gibbs free energy change, ΔG?
ΔG = ΔH −TΔS. When ΔS is positive increasing the temperature will make the value of ΔG become more negative.
Which reaction has the greatest increase in entropy?
Only the reaction between zinc and hydrochloric acid has more moles of gas in the products than the reactants.
Which are correct statements about the standard entropy change at 298 K for the following reaction?
2CO(g) + O2(g) → 2CO2(g)
I. ΔS⦵ = 2S⦵(CO2) − 2S⦵(CO) − S⦵(O2)
II. S⦵(O2) = 0
III. ΔS⦵ has a negative value
Unlike ΔHf⦵ for elements, the absolute entropy value of elements is not zero. (In fact, S⦵(O2) = 205.2 J K−1 mol−1.)
Which is a correct statement concerning the following reaction?
CaCO3(s) → CaO(s) + CO2(g) ΔH⦵ = + 178 kJ mol−1
ΔH⦵ is given as being positive and ΔS⦵ must be positive (as one of the products is a gas). From the expression ΔG⦵ = ΔH⦵ − TΔS⦵ it can be seen that ΔG⦵ will become negative above a certain temperature.
Which is a correct statement concerning the following exothermic reaction?
(NH4)2Cr2O7(s) → N2(g) + Cr2O3(s) + 4H2O(g)
Since it is exothermic and much of the solid reactant produces gases ΔH⦵ will be negative and ΔS⦵ will be positive so ΔG⦵ must always have a negative value.
What is the value of ΔG⦵ for the hydrogenation of but-1-ene at 298 K?
H3CCH2CH=CH2(g) + H2(g) → H3CCH2CH2CH3(g) ΔH⦵ = − 124 kJ mol−1
Given:
ΔGf⦵(but-1-ene) = −17.0 kJ mol−1
ΔGf⦵(butane) = − 71.0 kJ mol−1
By Hess's Law ΔG⦵ for the hydrogenation of but-1-ene = ΔGf⦵(butane) − ΔGf⦵(but-1-ene) − ΔGf⦵(H2) = − 71.0 − (−17.0) − 0 = − 54.0 kJ mol−1. (Note that like ΔHf⦵, the Gibbs energy change for the formation of an element in its standard state. ΔGf⦵ is also zero under standard conditions as by definition no change takes place when an element is formed from itself.)
What is the value of ΔS⦵ for the hydrogenation of but-1-ene at 298 K?
H3CCH2CH=CH2(g) + H2(g) → H3CCH2CH2CH3(g) ΔH⦵ = − 124 kJ mol−1
Given:
S⦵ (but-1-ene) = +306 J K−1 mol−1
S⦵ (butane) = +310 J K−1 mol−1
ΔS⦵ = S⦵(butane) − S⦵(but-1-ene) − S⦵(hydrogen). S⦵(hydrogen) is not given. Even though ΔH⦵ is given ΔG⦵ is not given so ΔS⦵ cannot be calculated either using ΔG⦵ = ΔH⦵ − TΔS⦵.
Which expression will give the temperature above which the reaction between hydrogen and ethene will cease to be spontaneous?
C2H4(g) + H2(g) → C2H6(g) ΔH⦵ = − 124 kJ mol−1
ΔG⦵ = ΔH⦵ − TΔS⦵. The reaction will cease to be spontaneous when ΔG⦵ > 0. At ΔG⦵ = 0, T = ΔH⦵ ÷ (ΔS⦵ x 10-3) with the factor of 10-3 required to change the units of ΔS⦵ to kJ K−1 mol−1.
Which statements are correct about spontaneous reactions at 298 K?
I. (ΔH⦵ − 298ΔS⦵) < 0
II. The value of ΔG⦵ does not give any information about the rate of the spontaneous reaction.
III. The reaction will not necessarily also be spontaneous at 398 K.
The value of ΔG⦵ does not give any information about the rate of the spontaneous reaction, for example the reaction of carbon in air is spontaneous yet diamonds are kinetically stable in air as a large amount of energy is required to overcome the activation energy of the reaction. If both ΔH⦵ and ΔS⦵ are negative then there will be temperature above 298 K when the reaction will become non-spontaneous (i.e. ΔG⦵ > 0) and this could be below 398 K.
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