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Higher Chemistry Unit 1(c) Ionisation energy

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After today’s lesson you should be able to: explain the meaning of the term ‘first ionisation energy’. write equations to show the first ionisation energy of an atom of any element. explain the trend in first ionisation energy across a period and down a group. write equations to show the second, third and fourth ionisation energies of an element. calculate the energy required to remove two, three and four electrons from an atom of an element.

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First ionisation energy The first ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state. This will leave each of the atoms with a + charge i.e. Mg(g) → Mg + (g) + e -

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Exercise Write ion-electron equations to show the first ionisation energy of: 1. Sulphur 2. Nitrogen 3. Oxygen 4. Potassium 5. Chlorine Using the data booklet write the I.E. next to each equation.

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Trends First ionisation energy increases across a group Why? First ionisation energy decreases down a period Why?

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Second ionisation energy The second ionisation energy is the energy required to remove one mole of electrons from one mole of 1+ charged ions in the gaseous state i.e. Mg + (g) → Mg 2+ (g) + e -

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Third ionisation energy The third ionisation energy is the energy required to remove one mole of electrons from one mole of 2+ charged ions in the gaseous state i.e. Mg 2+ (g) → Mg 3+ (g) + e -

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Exercise Write ion-electron equations to show the second, third and fourth ionisation energies of: 1. Sulphur 2. Nitrogen 3. Oxygen 4. Potassium 5. Chlorine Using the data booklet write the I.E. next to each equation.

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Calculating second, third and fourth ionisation energies Example 1 Calculate the ionisation energy for: Mg(g) → Mg 2+ (g) + 2e - This equation contains 2mol of electrons i.e. it represents the first and second ionisation energies of magnesium. Using the data booklet: 744 + 1460 = 2204kJmol -1

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Example 2 Calculate the ionisation energy for: Ca(g) → Ca 3+ (g) + 3e - This equation contains 3mol of electrons i.e. it represents the first, second and third ionisation energies of calcium. Using the data booklet: = 6686kJmol -1

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Exercise Calculate the ionisation energy of the following: 1. S(g) → S 2+ (g) + 2e - 2. N(g) → N 3+ (g) + 3e - 3. O(g) → O + (g) + e - 4. K(g) → K 4+ (g) + 4e - 5. Cl(g) → Cl 3+ (g) + 3e -

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