GROUPS 1&2: REACTIONS WITH OXYGEN AND CHLORINE

Lithium burns in air to form a mixture of two products. One of them is lithium oxide. What is another product?

Write an equation for the reaction of lithium with oxygen.

Answer explanation

Write an equation for the reaction of sodium with chlorine.

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Write an equation for the reaction of calcium with oxygen.

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Write an equation for the reaction of strontium with chlorine.

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What do you think you would see if you burned lithium in chlorine?

• Lithium would burn with a red flame to produce a white solid product – lithium chloride.

• Lithium would burn with a white flame to produce a white solid product – lithium chloride.

• Lithium would burn with a yellow flame to produce a white solid product – lithium chloride.

• Lithium would burn with no flame to produce a white solid product – lithium chloride.

The table shows the main product(s) formed when the Group 1 elements burn in a plentiful supply of oxygen. Write the formulae for sodium oxide, potassium peroxide and caesium superoxide.

Oxide ion = O^2-

Peroxide ion = O₂^2-

Superoxide = O₂^-

Answer explanation

Draw a simple diagram to show the structure of a peroxide ion.

Answer explanation

Explain why sodium forms a peroxide, but lithium doesn't. The O-O single bond in the peroxide ion is quite weak, and a nearby positive charge will ​ (a)   , possibly to the point that both negative charges will end up on the oxygen nearest the positive charge to ​ (b)   . The remaining oxygen atom breaks away and will end up in an oxygen molecule as oxygen gas. This is most effective if the positive ion has a​ high charge density – in other words, ​ (c)   In the lithium case, the charge density is high because of ​ (d)   . Any peroxide ion nearby would be instantly converted into an oxide ion and oxygen. Sodium is bigger, and so the charge density is lower, and the peroxide ion remains intact.

• polarise it strongly
• a lot of charge packed into a small volume.
• form a simple oxide ion
• the small size of the atom

When Group 2 metals burn in oxygen, you may get either the simple oxide or a peroxide formed. Name the metals which only form a simple oxide. (3 answers)

• Beryllium

• Calcium

• Magnesium

• Strontium