HardChemistryHigh School (11–12)The Periodic Table › Periodic TrendsNumericalConceptual

The electron gain enthalpy of chlorine is -349 kJ mol⁻¹ and that of fluorine -328 kJ mol⁻¹.

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The electron gain enthalpy of chlorine is $-349\ \text{kJ mol}^{-1}$ and that of fluorine $-328\ \text{kJ mol}^{-1}$ . How much energy is released when $3.55\ \text{g}$ of gaseous chlorine atoms each gain an electron? How much from $1.9\ \text{g}$ of gaseous fluorine atoms? Use $\text{Cl} = 35.5$ and $\text{F} = 19$ . Why is the second smaller?

Need a reminder? The key ideaShow

The first ionisation enthalpy is the energy to remove the outermost electron from an isolated gaseous atom: $\text{X(g)} \rightarrow \text{X}^+\text{(g)} + e^-$ . It rises across a period and falls down a group, with dips at groups 13 and 16. A big jump among the successive ionisation enthalpies shows where an inner shell begins. Electron gain enthalpy is the enthalpy change when a gaseous atom gains an electron, negative when energy is released.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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← The first ionisation enthalpy of nitrogen is 1402 kJ mol⁻¹ and that of oxygen is 1314 kJ mol⁻¹…Use the valences of the groups to write three formulas. →

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