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Temperature above which a reaction becomes spontaneous (or is at equilibrium), T = dH/dS

This type of question, asked 5 times: 2021 Paper 21, 2021 Paper 25, 25 Jun 2022, 6 Apr 2024, 24 Jan 2025. All 5 questions below, oldest first.

Try each one first, then open its answer.

  1. Question 1

    2021 Paper 21

    60
    Data given for the following reaction is as follows:
    FeO(s)+C(graphite)⟶Fe(s)+CO(g)FeO_{(s)} + C_{\text{(graphite)}} \longrightarrow Fe_{(s)} + CO_{(g)}
    SubstanceΔH∘ (kJ mol−1)\Delta H^{\circ}\ \left( kJ\ mol^{- 1} \right)ΔS∘ (J mol−1 K−1)\Delta S^{\circ}\ \left( J\ mol^{- 1}\ K^{- 1} \right)
    FeO(s)FeO_{(s)}-266.357.49
    C(graphite)C_{\text{(graphite)}}05.74
    Fe(s)Fe_{(s)}027.28
    CO(g)CO_{(g)}-110.5197.6
    The minimum temperature in KK at which the reaction becomes spontaneous is (Integer answer)

  2. Question 2

    2021 Paper 25

    56
    For a chemical reaction A+B⇌C+DA + B \rightleftharpoons C + D (ΔrH⊖=80 kJ mol−1\left( \Delta_{r}H^{\ominus}= 80\text{ }kJ{\text{ }mol}^{- 1} \right. ) the entropy change ΔrS⊖\Delta_{r}S^{\ominus} depends on the temperature TT (in KK ) as ΔrS⊖=2 T( J K−1 mol−1)\Delta_{r}S^{\ominus}= 2\text{ }T\left( \text{ }J{\text{ }K}^{- 1}{\text{ }mol}^{- 1} \right). Minimum temperature at which it will become spontaneous is ____\_\_\_\_ K.

  3. Question 3

    25 Jun 2022

    54
    The standard entropy change for the reaction 4Fe(s)+3O2( g)→2Fe2O3( s)4Fe(s) + 3O_{2}(\text{ }g) \rightarrow 2Fe_{2}O_{3}(\text{ }s) is −550JK−1- 550JK^{- 1} at 298 K298\text{ }K. [Given: The standard enthalpy change for the reaction is  −165 kJ mol−1]\left. \ - 165\text{ }kJ{\text{ }mol}^{- 1} \right\rbrack. The temperature in KK at which the reaction attains equilibrium is (Nearest Integer)

  4. Question 4

    6 Apr 2024

    146
    For the reaction at 298 K,2 A+B→298\text{ }K,2\text{ }A + B \rightarrow C. ΔH\Delta H =400 kJ mol−1= 400\text{ }kJ{\text{ }mol}^{- 1} and ΔS=0.2 kJ mol−1 K−1\Delta S = 0.2\text{ }kJ{\text{ }mol}^{- 1}{\text{ }K}^{- 1}. The reaction will become spontaneous above ____ KK.

  5. Question 5

    24 Jan 2025

    47
    Standard entropies of X2,Y2X_{2},Y_{2} and XY5XY_{5} are 70,50 and 110 J K−1 mol−1110\text{ }J{\text{ }K}^{- 1}{\text{ }mol}^{- 1} respectively. The temperature in Kelvin at which the reaction
    12X2+52Y2→XY5ΔH⊖=−35 kJ mol−1\frac{1}{2}X_{2}+\frac{5}{2}Y_{2}\rightarrow XY_{5}\quad \Delta H^{\ominus}= - 35\text{ }kJ{\text{ }mol}^{- 1}
    will be at equilibrium is _______\_\_\_\_\_\_\_ (Nearest integer)