Calculating the Heat of Phase Transition from Standard Enthalpies of Formation Use values of ΔH°f to calculate the heat of vaporization, ΔH°vap, of carbon disulfide at 25°C. The vaporization process is CS2(l) → CS2(g) PROBLEM STRATEGY The vaporization process can be treated like a chemical reaction

Question

Calculating the Heat of Phase Transition from Standard Enthalpies of Formation

Use values of [latex]\Delta H_f^{\circ}[/latex] to calculate the heat of vaporization, [latex]\Delta H_{ ext {vap }}^{\circ}[/latex], of carbon disulfide at 25°C. The vaporization process is

[latex]CS_2(l) → CS_2(g)[/latex]

PROBLEM STRATEGY

The vaporization process can be treated like a chemical reaction, with [latex]CS_2(l )[/latex] the “reactant” and [latex]CS_2(g)[/latex] the “product.” It is convenient to record the values of [latex]\Delta H_f^{\circ}[/latex] under the formulas in the equation, multiplying them by the coefficients in the equation. You calculate [latex]\Delta H_{ ext {vap }}^{\circ}[/latex] by subtracting values for the “reactant” from values for the “product.”

Accepted Answer

Here is the equation for the vaporization, with values of [latex]\Delta H_f^{\circ}[/latex] multiplied by coefficients (here, all 1’s).

[latex]\underset{1(89.7)}{CS_2(l)} \longrightarrow \underset{1(116.9) (kJ)}{CS_2(g)} [/latex]

The calculation is

[latex]\begin{aligned}\Delta H_{ ext {vap }}^{\circ} &=\Sigma n \Delta H_f^{\circ} ext { (products) }-\Sigma m \Delta H_f^{\circ} ext { (reactants) } \&=\Delta H_f^{\circ}\left[ CS _2(g) ight]-\Delta H_f^{\circ}\left[ CS _2(l) ight] \&=(116.9-89.7) kJ = 2 7 . 2 k J \end{aligned}[/latex]

Question 6.8: Calculating the Heat of Phase Transition from Standard Entha...

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