Enthalpy changes

Nitrogen dioxide gas is formed when magnesium nitrate is heated.

Table 4 shows some thermodynamic data.

Give the standard conditions indicated by the symbol in Δ_f and

Use the equation and the data in Table 4 to calculate the temperature at which this reaction becomes feasible.

Nitrogen dioxide can also be produced in a reaction between ammonia and oxygen.

Table 5 shows some bond enthalpy values.

Assume that the bonds between N and O in NO₂ are the same.

Calculate the mean bond enthalpy, in kJ mol^–1, for a bond between N and O in NO₂

An alloy of platinum and rhodium is used as a catalyst in the reaction.

What type of catalyst is this?

Some ammonia and oxygen are reacted and the mixture is left to reach equilibrium at a fixed temperature.

Explain why increasing the temperature of this equilibrium mixture decreases the mole fraction of NO₂ in the equilibrium mixture.

This question is about metal chlorides.

Table 2 shows some enthalpy change data.

Use the data in Table 2 to calculate the molar enthalpy change when calcium chloride dissolves in water.

Molar enthalpy change
kJ mol^–1

Use your answer to Question 05.1 to deduce how the temperature changes when calcium chloride dissolves in water.

Explain why the enthalpy of hydration of fluoride ions is more negative than the enthalpy of hydration of chloride ions.

Figure 2 shows an incomplete Born–Haber cycle for calcium chloride.

Complete the Born–Haber cycle by writing the formulas of the missing species on each of the two blank lines.

Table 3 shows some enthalpy change data.

Use Figure 2 and data from Table 3 to calculate the second ionisation energy of calcium.

Second ionisation energy
kJ mol^–1

Explain why the second ionisation energy of calcium is greater than the first ionisation energy of calcium.

Table 4 shows lattice enthalpies based on a perfect ionic model and lattice enthalpies from Born–Haber cycles for three metal chlorides.

Discuss the values in Table 4.

In your answer you should

compare the three values based on a perfect ionic model

compare the values based on a perfect ionic model to the values from a Born–Haber cycle for each compound.

Which reaction has a standard enthalpy change equal to the standard enthalpy of formation for barium chloride?

This question is about enthalpy changes for calcium chloride and magnesium chloride.

State the meaning of the term enthalpy change.

Figure 1 shows an incomplete Born–Haber cycle for the formation of calcium chloride.

Complete Figure 1 by writing the formulas, including state symbols, of the appropriate species on each of the three blank lines.

Table 1 shows some enthalpy data.

Use Figure 1 and the data in Table 1 to calculate a value for the enthalpy of lattice dissociation of calcium chloride.

Magnesium chloride dissolves in water.

Give an equation, including state symbols, to represent the process that occurs when the enthalpy of solution of magnesium chloride is measured.

Table 2 shows some enthalpy data.

Use your answer to Question 01.4 and the data in Table 2 to calculate a value for the enthalpy of solution of magnesium chloride.

The enthalpy of hydration of Ca²⁺(g) is –1650 kJ mol^–1

Suggest why this value is less exothermic than that of Mg²⁺(g)

A bomb calorimeter can be used for accurate determination of the heat change during combustion of a fuel.

A bomb calorimeter is a container of fixed volume that withstands the change in pressure during the reaction.

The fuel is mixed with pure oxygen in the calorimeter, ignited and the temperature change is recorded.

The total heat capacity (C_cal) of the calorimeter is calculated using a fuel for which the heat change is known.

In an experiment to calculate C_cal, 2.00 g of hexane (M_r = 86.0) is ignited. A temperature change (T) of 12.4 °C is recorded.

Under the conditions of the experiment, 1.00 mol of hexane releases 4154 kJ of energy when combusted.

The heat energy released in the calorimeter, q = C_calT

Calculate the heat capacity (C_cal) in kJ K¹

When the experiment is repeated with 2.00 g of octane (M_r = 114.0) the temperature change recorded is 12.2 °C

Calculate the heat change, in kJ mol¹, for octane in this combustion reaction.

If you were unable to calculate a value for C_cal in Question 05.1, use 6.52 kJ K¹ (this is not the correct value).

State why the heat change calculated from the bomb calorimeter experiment is not an enthalpy change.

The thermometer used to measure the temperature change of 12.2 °C in Question 05.2 has an uncertainty of 0.1 °C in each reading.

Calculate the percentage uncertainty in this use of the thermometer.

Suggest one change to this experiment that decreases the percentage uncertainty while using the same thermometer.

Which reaction has an enthalpy change equal to the standard enthalpy of formation of silver iodide?