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9701/13

Chemistry 9701/13 — October/November 2018

Cambridge AS Level · answer key with instant marking and worked solutions

40
questions
40
marks
60
minutes

Topics Atoms, Molecules and Stoichiometry · Introduction to Organic Chemistry · Equilibria · Atomic Structure · States of Matter · Group 2 · +13 more

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Q11MAtomic StructureFree sample

The first four ionisation energies for element X are shown in the table.

ionisation energy1st2nd3rd4th
value / kJ mol−1\text{kJ mol}^{-1}577198029606190

Which ion of X is produced by removing an electron from a filled shell?

Options

A   X+\text{X}^+
B   X2+\text{X}^{2+}
C   X3+\text{X}^{3+}
D   X4+\text{X}^{4+}

DifficultyMedium-Easy
Worked solution

Working

The large increase between the 3rd and 4th ionisation energies (2960 → 6190 kJ mol⁻¹) shows that the 4th electron is removed from a filled inner shell, which is much more tightly held.

Answer

D (X⁴⁺)

Final answer

D

Detailed explanation

Background Concept

Ionisation energy is the energy required to remove one mole of electrons from one mole of gaseous atoms or ions. Successive ionisation energies always increase because the nuclear charge stays the same while the electron being removed is pulled from an increasingly positive ion and is, on average, closer to the nucleus. A particularly large jump between two successive ionisation energies signals that the next electron is being removed from a new, inner (filled) shell, which is held much more tightly than the valence electrons.

Understanding the Question

The table gives the first four ionisation energies of element X. We are asked which ion is produced when an electron is removed from a filled shell. A filled shell is an inner shell, not the outermost valence shell. The key clue is the size of the jumps between successive ionisation energies.

Approach

Look for the largest jump between successive ionisation energies. The electron removed after that jump comes from a filled inner shell. The ion produced by removing that electron is the one with the corresponding charge.

Step-by-Step Reasoning

  1. The ionisation energies are: 577, 1980, 2960, 6190 kJ mol⁻¹.
  2. Calculate the jumps:
    • 2nd − 1st = 1980 − 577 = 1403 kJ mol⁻¹
    • 3rd − 2nd = 2960 − 1980 = 980 kJ mol⁻¹
    • 4th − 3rd = 6190 − 2960 = 3230 kJ mol⁻¹
  3. The largest jump is between the 3rd and 4th ionisation energies (3230 kJ mol⁻¹).
  4. This means the 4th electron is removed from a filled inner shell.
  5. Removing the 4th electron produces the ion X⁴⁺.

Key Takeaways

A large jump in successive ionisation energies indicates that the next electron is being removed from a filled inner shell. The charge of the ion produced equals the number of electrons removed.

Common Mistakes

Confusing the ion produced with the one before the jump. The electron removed from the filled shell is the 4th electron, so the ion produced is X⁴⁺, not X³⁺.

Things to Be Careful About

The jump is between the 3rd and 4th ionisation energies, so the electron being removed is the 4th one. Make sure the ion charge matches the number of electrons removed.

Techniques used
identify the large jump in successive ionisation energiesdeduce the shell from which an electron is removedrelate ionisation energy jumps to filled shells

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