Examen de Química II URV 2000 · Parcial de Química: Termoquímica y Termodinámica

Problema 1. Combustión de antracita para calentar agua

During the combustion of 1 kg of anthracite coal, an energy of approximately 7300 kcal is released. Assuming that the whole amount of released heat is completely transferred to the system, determine what mass of coal (in grams or kilograms) is needed to heat 4.0 kg of water from room temperature (20 °C) up to its boiling point (100 °C) at an atmospheric pressure of 1 atm.

Data:

Specific heat capacity of liquid water, $c_e(\mathrm{H_2O}) = 4.18\ \mathrm{J\cdot g^{-1}\cdot{^\circ}C^{-1}}$; $1\ \mathrm{cal} = 4.184\ \mathrm{J}$ (take $4.18\ \mathrm{J/cal}$).

Problema 2. Calor de combustión del carbono en calorímetro

A calorimeter constructed from 1500 g of copper contains 2000 g of water. A sample of 3 g of carbon is completely burned inside the device, starting from an initial temperature of 20 °C. At the end of the combustion process, the thermal equilibrium temperature of the calorimeter assembly reaches 31.3 °C. Calculate the specific heat of combustion of carbon expressed in $\mathrm{J\cdot g^{-1}}$.

Data:

$c_e(\mathrm{Cu}) = 0.389\ \mathrm{J\cdot g^{-1}\cdot K^{-1}}$; $c_e(\mathrm{H_2O}) = 4.18\ \mathrm{J\cdot g^{-1}\cdot K^{-1}}$.

Problema 3. Temperatura de equilibrio de mezcla hielo-agua

A mass of 150 g of ice at 0 °C is introduced into a container containing 300 g of liquid water at 50 °C. Assuming an isolated system with negligible heat losses to the surroundings or container walls, calculate the final equilibrium temperature of the mixture.

Data:

Enthalpy of fusion of ice, $Q_{\mathrm{fus}} = 80\ \mathrm{cal\cdot g^{-1}}$; specific heat capacity of liquid water, $c_e = 1\ \mathrm{cal\cdot g^{-1}\cdot{^\circ}C^{-1}}$.

Problema 10. Termodinámica de la vaporización de agua

A sample containing 2.5 moles of liquid water at 100 °C (density $d = 0.96\text{ g}\cdot\text{mL}^{-1}$) is held inside a cylinder fitted with a frictionless movable piston.

The water undergoes complete vaporization against a constant external pressure of 1 atm. Assuming ideal gas behavior for water vapor, calculate:

Data:

Enthalpy of vaporization of water, $\Delta H_{\text{vap}} = Q_{\text{vap}}(\text{H}_2\text{O}) = 40.7\text{ kJ}\cdot\text{mol}^{-1}$; $R = 0.082\text{ atm}\cdot\text{L}\cdot\text{mol}^{-1}\cdot\text{K}^{-1}$; $1\text{ atm} = 101\,325\text{ Pa}$; molar mass of water $M = 18\text{ g/mol}$.

a.

The heat exchanged by the system, $Q$.

b.

The expansion work performed, $W$ (using the IUPAC sign convention $W = -P_{\text{ext}}\Delta V$).

c.

The change in internal energy of the process, $\Delta U$.