AQA AS Physical Chemistry - Ionisation Energies PDF

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE 1st MOLAR IONISATION ENERGIES The energy required to remove 1 mole of electrons from one mole of gaseous atoms X(g) → X+(g) + e- FACTORS...

AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE 1st MOLAR IONISATION ENERGIES The energy required to remove 1 mole of electrons from one mole of gaseous atoms X(g) → X+(g) + e- FACTORS THAT INFLUENCE 1st I.E. The greater the attraction between the positive nucleus and outer electron, the greater the value for the 1st I.E. The following factors influence this attraction: Nuclear Charge : The stronger the nuclear charge, the greater the attraction (and vice versa). Shielding: AKA, the full orbitals / energy levels that are between the nucleus and outer electron. The greater the shielding, the weaker the attraction (and vice versa). Distance from the Nucleus: Same deal as shielding. The greater the distance between the nucleus and outer electron (i.e. the bigger the atom), the weaker the attraction (and vice versa). Orbitals: Orbitals can also play a part in explaining trends in 1st Ionisation energy. However, we ONLY REFER TO THESE WHEN EXPLAINING THE EXCEPTIONS! IN CONTEXT You need to be able to use these factors to answer questions about the trends in 1st Ionisation Energy for the first 20 elements. You must be able to: Explain the difference / trend in ionisation energies down a group Explain the difference / trend in ionisation energies across a period Explain why there are exceptions in the trend in ionisation energy across a period AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE 1st I.E. GRAPH 3000 1st Ionisation Energy (kJ.mol-1) 2250 1500 750 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number TRENDS IN THE GRAPH At first glance there doesn’t seem to be much we can get from this. However…. Elements 3 to 10 are the elements in Period 2. Elements 11 to 18 are the elements in Period 3. You can see that there is a general increase in 1st I.E. across a period (with exceptions) The “peaks” in this graph are the Group 8 elements. These always have the highest 1st I.E. in their period. The “troughs” in this graph are the Group 1 elements. These always have the lowest 1st I.E. in their period. If you compare elements that are in the same Group, you can see that 1st I.E. decreases down a group. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE TREND DOWN A GROUP 3000 1st Ionisation Energy (kJ.mol-1) 2250 1500 750 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number THE EXPLANATION 1st I.E. DECREASES down any group. e.g. Down Group 8 (see graph) FACTS - Down a Group: Shielding increases and Distance between nucleus & outer He electron increases. Why does 1st I.E. decrease down a Group? Ne “As you move down the group, both shielding and distance of the outer electron from the nucleus increases. This means there is a weaker attraction between the outer electron and the nucleus, so less energy is required to remove it”. Ar Why does Ar have a lower 1st I.E. than Ne? “Ar has greater shielding and a greater distance between the outer electron and the nucleus compared to Ne. This means there is a weaker attraction between the outer electron and the nucleus, so less energy is required to remove it” AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE TREND ACROSS A PERIOD 3000 1st Ionisation Energy (kJ.mol-1) 2250 1500 750 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number THE EXPLANATION 1st I.E. generally INCREASES across Periods 2 & 3. e.g. Period 2 Li Be B C N O F Ne Nuclear 3+ 4+ 5+ 6+ 7+ 8+ 9+ 10+ Charge FACTS - Across a Period: Nuclear Charge increases and Shielding remains constant (the same) Why does 1st I.E. generally increase across a Period? “As you move across a period, nuclear charge increases, but the amount of shielding remains the same. This means there is a stronger attraction between the outer electron and the nucleus, so more energy is required to remove it” There are exceptions though! See next page AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE EXCEPTIONS TO THE TREND ACROSS A PERIOD 3000 1st Ionisation Energy (kJ.mol-1) 2250 1500 750 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number THE EXPLANATION 1st I.E. generally INCREASES across Periods 2 & 3, but there are exceptions where it DECREASES. These are found when moving from: Group 2 to 3: Be to B & Mg to Al Group 5 to 6: N to O & P to S Why it decreases between Group 2 to 3 Be: 1s2 2s2 B: 1s2 2s2 2p1 “In B, the 2s orbital shields the 2p orbital, weakening the nuclear attraction to the outer electron, lowering the energy needed to remove it” The same explanation can be applied to Mg & Al, but it’s the 3s and 3p orbitals. Why it decreases between Group 5 to 6 N: 1s2 2s2 2p3 O: 1s2 2s2 2p4 ↼ ↼ ↼ ↼ ↼ ↼ ↼ “In O, the 2p orbitals contain a paired set of electrons. Repulsion between them means less energy is required to remove one of those electrons compared to N which does not have a paired set of electrons.” The same explanation can be applied to P & S, but it’s the 3p orbitals. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE THE 2nd I.E. GRAPH 8000 1st Ionisation Energy (kJ.mol-1) 6000 4000 2000 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Atomic Number THE EXPLANATION At first glance this doesn’t seem to be any different from the 1st I.E’s graph, however….. The “peaks” in this graph are now the Group 1 elements. These always have the highest 2nd I.E. in their period. This is because they only had 1 electron in their outer energy level. The 2nd electron that is removed is being taken from an energy level closer to the nucleus and has less shielding! This means a stringer nuclear attraction to that electron and therefore a high 2nd I.E. The “troughs” in this graph are the Group 2 elements. This is because the 2nd electron is being taken from the same energy level as the first electron that was removed. This is also true for the rest of the period (Groups 3-8), but we do see a general increase as nuclear charge increases. If you compare elements that are in the same Group, you can see that 2nd I.E. decreases down a group just like 1st I.E. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE SUCCESSIVE IONISATION ENERGIES The 1st I.E. is the energy required to remove the 1st (outermost) electron from an atom. The 2nd I.E. is the energy required to remove the next electron so on. i.e. 1st I.E. X(g) → X+(g) + e- Notice how the ionisation number is 2nd I.E. X+(g) → X2+(g) + e- the same as the 3rd I.E. X2+(g) → X3+(g) + e- etc charge on the ion we make! e.g. 1st I.E. of magnesium = Mg(g) → Mg+(g) + e- 3rd I.E. of oxygen = O2+(g) → O3+(g) + e- 7th I.E. of phosphorous = P6+(g) → P7+(g) + e- There is a general increase in successive ionisation energies for any element. The number of protons remains the same as each electron is removed. This means that the relative charge increases causing a stronger attraction between the nucleus and each successive electron that is removed. Therefore, more energy is required to remove each electron compared to the last. AQA www.chemistrycoach.co.uk © scidekick ltd 2024 AS CHEMISTRY 3.1.1 ATOMIC STRUCTURE e.g. SUCCESSIVE I.E’s of SODIUM 1000000 Ionisation Energy (kJ.mol-1) 100000 10000 1000 1 2 3 4 5 6 7 8 9 10 11 Ionisation Number Here we see all 11 I.E’s for sodium. The 1st electron removed is the outermost and requires the LEAST amount of energy to remove. The 11th (last) electron removed is the innermost and requires the MOST amount of energy to remove. There’s a general increase, but there are also large jumps in the data between the 1st & 2nd and the 9th & 10th. How To Answer Ionisation Energy Questions AQA www.chemistrycoach.co.uk © scidekick ltd 2024