State the meaning of the term enthalpy change. Enthalpy change refers to the heat (energy) change that occurs in a system at constant pressure, during a chemical reaction or a physical transformation. It reflects the amount of energy absorbed or released by the system when reacting substances form products. 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.

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State the meaning of the term enthalpy change - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 1

Question 1

State the meaning of the term enthalpy change. Enthalpy change refers to the heat (energy) change that occurs in a system at constant pressure, during a chemical re... show full transcript

Worked Solution & Example Answer:State the meaning of the term enthalpy change - AQA - A-Level Chemistry - Question 1 - 2021 - Paper 1

Step 1

State the meaning of the term enthalpy change.

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Answer

Enthalpy change refers to the heat (energy) change that occurs in a system at constant pressure, during a chemical reaction or a physical transformation.

Step 2

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

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Answer

Ca(s) → Ca(g)
Cl2(g) → 2Cl(g)
CaCl2(s) → CaCl2(g)

Step 3

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

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Answer

Using Hess's law, we can rearrange the Born-Haber cycle. The lattice enthalpy (LE) is calculated as:

$LE = \text{Enthalpy of formation} + \text{Enthalpy of atomisation of Ca} + \text{First ionisation energy of Ca} + \text{Second ionisation energy of Ca} + \text{Enthalpy of atomisation of Cl2} - \text{Electron affinity of Cl}$

From Table 1, substituting in the values, we have:

$LE = -795 + 193 + 590 + 1150 + 121 - 364$

Calculating that gives:

$LE = 2242 \, \text{kJ mol}^{-1}$

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