Chapter 8 - Reactivity Trends

How does group 2 react with oxygen

They react to form a metal oxide (General formula: MO)

How do group 2 elements react with water

They react to form an alkaline hydroxide (general formula: M(OH)2) and hydrogen gas

How do group 2 elements react with dilute acids

They react with dilute acids to form salt and hydrogen gas

Describe trend in reactivity and ionisation energy down group 2

Atoms of group 2 elements react by losing electrons to form +2 ions. Ionisation energies decrease down the group because the attraction between the nucleus and outer electrons decreases. As ionisation energies decrease down the group, the reactivity increases.

How do group 2 oxides react with water

They react to release hydroxide ions and form alkaline solution

How does solubility of hydroxides in water change down the group

Solubility of hydroxides in water increases as you go down the group

How can we show the trend of solubility of hydroxides in water down group 2

1. Add a spatula of each Group 2 oxide to water in a test tube.

2. Shake the mixture. On this scale, there is insufficient water to dissolve all of the metal hydroxide that forms. You will have a saturated solution of each metal hydroxide with some white solid undissolved at the bottom of the test-tube.

3. Measure the pH of each solution. The alkalinity will be seen to increase down the group.

Give 2 uses of group 2 compounds as bases

Agriculture - Calcium hydroxide added to fields to increase pH of acidic soils

Medicine - Group 2 bases used as antacids for treating acid indigestion

What is the trend in boiling point down group 7

Boiling point increases

How can we differentiate iodine and bromine in water

To tell them apart, an organic non-polar solvent such as cyclohexane can be added and the mixture shaken. The non-polar halogens dissolve more readily in cyclohexane than in water. In cyclohexane their colours are much easier to tell apart, with iodine being a deep violet.

How does reactivity change down group 7

As you go down group 7 the reactivity decreases

Define disproportionation

A redox reaction in which the same element is both oxidised and reduced.

Benefits of using chlorine

-Ensure our water is fit to drink

-Ensure bacteria is killed

Risks of using chlorine

-It’s an extremely toxic gas

-Respiratory irritant in small concentrations, in large concentrations it can be fatal

Test for carbonate

1. Add dilute nitric acid

2. If you see bubbles, it could be a carbonate

3. To prove the gas is carbon dioxide, bubble the gas through lime water. If white precipitate is observed, the gas is carbon dioxide hence carbonate ions are present.

Test for sulfate

1. Add dilute hydrochloric acid to remove carbonates

2. Add barium nitrate solution

3. If you observe formation of a white precipitate, sulfate ions are present

Test for halide ions

1. Add dilute nitric acid

2. Add silver nitrate

3. Observe colour of precipitate: Chloride - White

Bromide - Cream

Iodide - Yellow

What can you do to the halide precipitates if you can’t distinguish between the colours

You can add aqueous ammonia to the solutions to test solubility of the precipitates.

Chloride - Completely dissolves

Bromide - Partially dissolves

Iodide - Insoluble (does not dissolve)

What is the order of the sequence of tests (sulfate, halide, carbonate) and why this order

1. Carbonate - In this test you observe for effervescence. Neither sulfates of halides produce bubbles with dilute acid so this can be done first

2. Sulfate - In this test you are looking for the white precipitate of barium sulfate. However, barium carbonate also produces a white precipitate and is insoluble in water so you have to carry out test for carbonate first

3. Halide - In this test you are looking for a coloured precipitate. Silver carbonate and silver sulfate both produce a precipitate, therefore the halide test must be carried out last

What do you do if you’re asked to analyse a mixture of ions

You carry out the tests in the same sequence on the same solution

Test for ammonium ions

1. Add aqueous NaOH

2. Ammonia gas is produced

3. The mixture is warmed and ammonia gas is released.

4. Test the gas with moist pH paper. If the paper turns blue, ammonia is present