Edexcel International A Level Chemistry The Elements of Groups 1 and 2 Exam Questions

About Edexcel International A Level Chemistry The Elements of Groups 1 and 2

This Unit 2 topic in Edexcel International A Level Chemistry looks at the s-block metals and their compounds. You explain why reactivity increases down Groups 1 and 2 using ionisation energies, describe reactions with water, oxygen and chlorine, and compare the oxides and hydroxides. A key idea is the thermal stability of carbonates and nitrates, explained by the polarising power of the cation, linking back to Ionic bonding. You also learn solubility trends of hydroxides and sulfates and flame test colours. Examiners like equations for thermal decomposition, explanations of stability trends and descriptions of how to carry out a flame test.

What you need to know

• Reactivity trends Down Group 2 first and second ionisation energies decrease because atomic radius and shielding increase, so the metals lose electrons more easily and become more reactive.
• Reactions of Group 2 metals Mg reacts slowly with cold water but burns in steam to form MgO and H2. Ca, Sr and Ba react with cold water to form hydroxides and hydrogen. They burn in oxygen to form oxides and react with chlorine to form chlorides.
• Oxides and hydroxides Group 2 oxides react with water to form hydroxides and with acids to form salts. Solubility of hydroxides increases down the group, while solubility of sulfates decreases (BaSO4 is insoluble).
• Thermal stability Group 2 carbonates decompose to oxide and CO2; Group 2 nitrates give oxide, NO2 and O2. Stability increases down a group because larger cations with lower charge density polarise the anion less.
• Group 1 compounds Group 1 carbonates (except Li2CO3) are stable to heat. Group 1 nitrates (except LiNO3) decompose to the nitrite and oxygen, for example 2NaNO3 gives 2NaNO2 + O2.
• Flame tests Use a nichrome wire cleaned in concentrated HCl. Colours: Li red, Na yellow, K lilac, Ca orange-red, Sr red, Ba pale green. The colour arises when excited electrons fall back to lower energy levels, emitting visible light.

Common mistakes in The Elements of Groups 1 and 2

• Saying reactivity increases down the group because atoms gain more electrons. Explain with larger radius, more shielding and lower ionisation energy.
• Explaining thermal stability with the size of the anion. It depends on the polarising power of the cation.
• Writing that Group 2 nitrates give the nitrite. Group 2 nitrates give the metal oxide, nitrogen dioxide and oxygen.
• Mixing up the solubility trends: hydroxides become more soluble down the group, sulfates less soluble.
• Describing flame colour as electrons gaining energy. Light is emitted when electrons return to lower energy levels.

Try three The Elements of Groups 1 and 2 exam-style questions