Lawrie Ryan and Roger Norris Solutions for Chapter: Enthalpy Changes, Exercise 5: Questions

Calculate the energy transferred when the temperature of $75 {\mathrm{cm}}^3$ of water rises from $23 °\mathrm{C}$ to $54 °\mathrm{C}$.

When $8 \mathrm{g}$ of sodium chloride is dissolved in $40 {\mathrm{cm}}^3$ of water, the temperature falls from $22 °\mathrm{C}$ to $20.5 °\mathrm{C}$. Calculate the energy absorbed by the solution when sodium chloride dissolves.

A student added $50 {\mathrm{cm}}^3$ of sodium hydroxide to $50 {\mathrm{cm}}^3$ of hydrochloric acid. Both solutions were at $18 °\mathrm{C}$ to start with. When the solutions were mixed a reaction occurred. The temperature rose to $33 °\mathrm{C}$. How much energy is released in this reaction?

Explain why the enthalpy change of neutralisation of one mole of sulphuric acid, ${\mathrm{H}}_2{\mathrm{SO}}_4$ is not the standard enthalpy change of neutralisation in $\mathrm{kJ} {\mathrm{mol}}^{-1}$.

A student added $10 \mathrm{g} (0.25 \mathrm{mol})$ of sodium hydroxide to $40 {\mathrm{cm}}^3$ of water to make a concentrated solution. All the sodium hydroxide dissolved. The student measured the maximum temperature rise. The student suggested that these results would give an accurate value for the standard enthalpy change of solution. Give two reasons why the student is incorrect.

A student calculated the standard enthalpy change of combustion of ethanol $∆{\mathrm{H}}_{\mathrm{c}}^{°}\left[{\mathrm{C}}_2{\mathrm{H}}_5\mathrm{OH}\right]$ by calorimetry as $- 870 \mathrm{kJ} {\mathrm{mol}}^{-1}$. The data book value is $-1367 \mathrm{kJ} {\mathrm{mol}}^{-1}$. Explain why there is a difference between these values.