This chemistry grades 11-12 lesson on group 2 and halogens notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on group 2 and halogens and check yourself with the quiz.
Quick answer: Group 2 metals are reactive metals that lose two outer electrons to form 2+ ions, and their reactivity rises down the group. Also, the halogens are reactive non-metals that gain one electron to form 1− ions, and their oxidising power falls down the group. In addition, both groups show clear trends that you can test in the lab.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.2.2-3.2.3
Group 2 and halogens notes: the key ideas
How do the group 2 metals react?
Moreover, the group 2 elements (beryllium, magnesium, calcium, strontium and barium) each have two electrons in their outer s subshell. Also, they react by losing both: M → M²⁺ + 2e⁻. Reactivity increases down the group, because the atoms get larger, the outer electrons sit further from the nucleus and are shielded by more inner shells, so less energy is needed to remove them.
The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms: M(g) → M⁺(g) + e⁻. In addition, it is measured in kJ mol⁻¹.
With oxygen the metals burn to form oxides (2Mg + O₂ → 2MgO). Then with dilute acids they give a salt and hydrogen (Mg + 2HCl → MgCl₂ + H₂). With water, magnesium reacts very slowly when cold and rapidly with steam to give magnesium oxide and hydrogen, while calcium, strontium and barium react with cold water to form the hydroxide and hydrogen: Ca + 2H₂O → Ca(OH)₂ + H₂.
Aarav in Pune drops 0.80 g of calcium into water. Calculate the volume of hydrogen at room temperature and pressure (24.0 dm³ mol⁻¹, Ar of Ca = 40.1).
n(Ca) = 0.80 ÷ 40.1 = 0.01995 mol. The equation shows 1 mol Ca gives 1 mol H₂, so n(H₂) = 0.01995 mol. V = 0.01995 × 24.0 = 0.479 dm³, which is 479 cm³.
How do the hydroxides and sulfates change down group 2?
Hydroxides become more soluble down the group, so the solutions are more strongly alkaline. Sulfates show the opposite trend and become less soluble. This gives a standard test for sulfate ions: add dilute hydrochloric acid, then barium chloride solution. Moreover, a white precipitate of barium sulfate forms: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
| Compound | Mg | Ca | Sr | Ba |
|---|---|---|---|---|
| Hydroxide M(OH)₂ | almost insoluble | slightly soluble | soluble | fairly soluble |
| Sulfate MSO₄ | soluble | slightly soluble | almost insoluble | insoluble |
Down group 2: reactivity with water increases, hydroxide solubility increases, sulfate solubility decreases, and the hydroxide solutions become more alkaline.
What are the halogens like?
Also, the halogens (fluorine, chlorine, bromine, iodine) exist as diatomic molecules, X₂, held together by London forces. These forces grow with the number of electrons, so melting and boiling points rise down the group and the colour darkens.
How do halogens displace each other?
A halogen atom gains one electron to become a halide ion, so the halogens are oxidising agents. Oxidising power falls down the group, because the incoming electron is added further from the nucleus and is more shielded. A more reactive halogen therefore displaces a less reactive halide from solution: Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). In this reaction chlorine is reduced (0 → −1) and bromide is oxidised (−1 → 0).
| Halogen added | Chloride | Bromide | Iodide |
|---|---|---|---|
| Chlorine water | no reaction | orange solution | brown solution |
| Bromine water | no reaction | no reaction | brown solution |
| Iodine solution | no reaction | no reaction | no reaction |
Chlorine also reacts with water and with alkali. In water, Cl₂ + H₂O ⇌ HCl + HClO, and the chloric(I) acid kills bacteria. With cold dilute sodium hydroxide, Cl₂ + 2NaOH → NaCl + NaClO + H₂O, making bleach. In addition, here chlorine is both oxidised (0 → +1) and reduced (0 → −1), which is called disproportionation.
How do you test for halide ions?
Acidify the sample with dilute nitric acid, which removes carbonate and hydroxide ions that would also give precipitates, then add silver nitrate solution.
| Halide | Precipitate | Dilute NH₃ | Concentrated NH₃ |
|---|---|---|---|
| Chloride | white | dissolves | dissolves |
| Bromide | cream | stays | dissolves |
| Iodide | yellow | stays | stays |
Wanjiru adds excess silver nitrate to 30.0 cm³ of 0.100 mol dm⁻³ sodium chloride. Calculate the mass of silver chloride (Mr = 143.4).
n(Cl⁻) = 0.0300 dm³ × 0.100 mol dm⁻³ = 0.00300 mol. Ag⁺ + Cl⁻ → AgCl is 1 : 1, so n(AgCl) = 0.00300 mol. Mass = 0.00300 × 143.4 = 0.430 g.
Never acidify with hydrochloric acid or sulfuric acid before the silver nitrate test. Hydrochloric acid adds chloride ions, giving a false white precipitate, and sulfuric acid gives slightly soluble silver sulfate.
Also, remember the halide precipitates as white, cream, yellow: the colour deepens as the halide ion gets bigger. Down the group the oxidising power of the halogen falls but the reducing power of the halide rises.
- Group 2: M → M²⁺ + 2e⁻; first ionisation energy falls from Be (900) to Ba (503 kJ mol⁻¹)
- Down group 2: hydroxide solubility rises, sulfate solubility falls
- Halogens X₂ get darker and boil at higher temperatures down group 7 (London forces)
- In addition, a more reactive halogen displaces a less reactive halide: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
- Silver nitrate test: AgCl white, AgBr cream, AgI yellow
Reactivity trends in group 2 and the halogens: frequently asked questions
Why is barium more reactive than magnesium?
Barium atoms are larger and their outer electrons are more shielded, so the first and second ionisation energies are lower. Less energy is needed to form Ba²⁺, so it reacts faster.
Why does chlorine displace bromine from bromide solution?
Chlorine is a stronger oxidising agent than bromine because its smaller atom attracts an incoming electron more strongly. Moreover, it takes electrons from bromide ions, forming chloride ions and bromine.
Why is nitric acid added before silver nitrate in the halide test?
Nitric acid removes carbonate and hydroxide ions, which would give a false precipitate with silver ions. Nitrate ions do not interfere because all nitrates are soluble.
Why is barium sulfate safe in a barium meal when barium ions are toxic?
Barium sulfate is so insoluble that almost no Ba²⁺ ions enter the body. Also, it passes through the gut and shows up on X-rays.
What is disproportionation?
It is a reaction in which the same element is both oxidised and reduced. Chlorine in cold sodium hydroxide goes from 0 to +1 in NaClO and from 0 to −1 in NaCl.
