Similarities and trends in the properties of the Group 2 metals, magnesium to barium

AS Level Chemistry · Group 2. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

Test this topic freeAll AS Level Chemistry topics

The idea

The Group 2 metals have two outer-shell electrons. They lose both to form M2+ ions, so their oxidation number in compounds is always +2. Down the group the atoms get larger and the outer electrons are more shielded, so they are lost more easily and reactivity increases from magnesium to barium.

The oxides and hydroxides are bases, and the carbonates react with acids to give carbon dioxide. Carbonates and nitrates decompose on heating. Thermal stability increases down the group, because a larger M2+ ion has a lower charge density and polarises the large anion less.

Two solubility trends must be learned: hydroxides become more soluble down the group, sulfates become less soluble.

Rules to remember

Common mistake

Students often swap the two solubility trends. Remember that Mg(OH)2 is the sparingly soluble hydroxide and BaSO4 is the insoluble sulfate, so hydroxides become more soluble and sulfates less soluble down the group.

Worked example

0.0200 mol of strontium nitrate, Sr(NO3)2, is heated until it has decomposed completely. What is the total volume of gas produced, measured at room conditions (24.0 dm3 mol−1)?

  1. Equation: 2Sr(NO3)2 → 2SrO + 4NO2 + O2.
  2. 2 mol of nitrate give 4 + 1 = 5 mol of gas, so 1 mol gives 2.5 mol of gas.
  3. Moles of gas = 0.0200 × 2.5 = 0.0500 mol.
  4. Volume = 0.0500 × 24.0 = 1.20 dm3.

Answer: 1.20 dm3 of gas (NO2 and O2 together).

Practice questions

Try each one, then open the answer.

1. Which gases are produced when magnesium nitrate is heated strongly?

  1. A
    NO2 and O2
  2. B
    NO and O2
  3. C
    N2 and O2
  4. D
    N2O only
Show answer

Answer: A. Group 2 nitrates decompose to the metal oxide, nitrogen dioxide and oxygen: 2Mg(NO3)2 → 2MgO + 4NO2 + O2. The brown fumes seen are NO2.

2. Barium reacts steadily with dilute hydrochloric acid, but its reaction with dilute sulfuric acid soon stops. Why?

  1. A
    an insoluble layer of barium sulfate forms on the metal and prevents further reaction
  2. B
    barium is not reactive enough to release hydrogen from sulfuric acid
  3. C
    sulfuric acid is a weaker acid than hydrochloric acid
  4. D
    barium hydroxide precipitates and neutralises the acid
Show answer

Answer: A. BaSO4 is insoluble, so as it forms it coats the barium and stops the acid reaching the metal. BaCl2 is soluble, so the reaction with HCl continues.

3. Which equation shows the thermal decomposition of calcium nitrate?

  1. A
    Ca(NO3)2 → Ca(NO2)2 + O2
  2. B
    2Ca(NO3)2 → 2CaO + 4NO + 3O2
  3. C
    Ca(NO3)2 → CaO + 2NO2 + O2
  4. D
    2Ca(NO3)2 → 2CaO + 4NO2 + O2
Show answer

Answer: D. Group 2 nitrates decompose to the oxide, brown nitrogen dioxide and oxygen. O atoms: 12 on the left and 2 + 8 + 2 = 12 on the right. Ca(NO3)2 → CaO + 2NO2 + O2 has 7 O on the right, and forming a nitrite is not Group 2 behaviour.

More questions on this topic

Read the full notes

Keep going

← Chemical periodicity of other elementsPhysical properties of the Group 17 elements →All rules on one pageStuck? Ask a question