C2 - Investigate the reactions of acids

• Acids can react with metals, metal oxides, metal hydroxides and metal carbonates.

• A number of characteristic observations can be made for these reactions.

• In these experiments you will measure temperatures increase and test hydrogen and carbon dioxide gases produced in acid reactions.

Acids can react in a number of ways. For example, acids can react with:

• metals

• metal oxides

• metal hydroxides

• metal carbonates

You will carry out four different experiments to investigate these reactions of acids.

An acid will release hydrogen gas when reacted with a metal:

acid + metal → salt + hydrogen

In this experiment, you will react magnesium with hydrochloric acid to produce magnesium chloride (a salt) and hydrogen gas:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

The hydrogen gas produced can be trapped and tested with a lit splint.

• Boiling tubes

• Boiling tube rack

• Measuring cylinder (25 cm³)

• Safety glasses

• Wooden splint

• 1 mol/dm³ hydrochloric acid (15 cm³)

• 2 cm strip of magnesium

1. Measure 15 cm³ of hydrochloric acid using the measuring cylinder and add to the boiling tube.

2. Add the magnesium strip to the boiling tube – ensuring that the magnesium is fully immersed in the acid by swirling and allow the reaction to proceed for 10 seconds.

3. After 10 seconds, place a second boiling tube upside-down over the top of the first boiling tube to trap the gas produced.

4. Light a splint. Insert the splint into the upside-down boiling tube.

Reaction of magnesium and hydrochloric acid:

• gas produced (hydrogen).

• heat produced.

• solid metal disappears.

• colourless solution forms.

Lit splint inserted into trapped gas:

• squeaky pop produced.

An acid will react with metal oxides:

acid + metal oxide → salt + water

In this reaction, you are going to use sulfuric acid and copper(II) oxide:

sulfuric acid + copper(II) oxide → copper(II) sulfate + water

In this reaction no gas is produced.

However, there will be other evidence that a chemical reaction is taking place.

• Kettle

• 250 cm³ beaker

• 100 cm³ beaker

• Measuring cylinder

• Spatula, watch glass, glass rod

• Heatproof mat

• Safety goggles

• Sulfuric acid 0.5 mol/dm³

• Copper(II) oxide (2 g)

• pH paper

1. Using pH paper determine the pHof the acid at the start, record this in your results.

2. Using a measuring cylinder, measure out 25 cm³ of sulfuric acid into the small beaker.

3. Collect approximately 2 g of copper(II) oxide on a watch glass.

4. Using hot water from a kettle, fill 1/3 of the larger beaker with hot water.

5. Warm the sulfuric acid beaker by letting it rest (carefully) in the hot water bath (leave for 2 min).

6. Carefully remove the small beaker and add copper(II) oxide to the acid slowly, stir with a glass rod.

7. Keep adding the copper(II) oxide, until there is some left over at the bottom of the beaker. Let the beaker sit for 2 min to allow the black powder to settle.

• At the start of the reaction the pH of the acid is 1-2

• At the end of the reaction the pH of the solution is close to pH 7

• Blue solution produced

• Heat produced

• Most of the black solid disappears

An acid will react with a metal hydroxide in a similar manner to the reaction with a metal oxide:

acid + metal hydroxide → salt + water

In this experiment we will react hydrochloric acid with sodium hydroxide and measure the temperature increase during the reaction:

hydrochloric acid + sodium hydroxide → sodium chloride + water

No gas is produced.

Also, in this reaction there is no colour change.

However, the temperature will increase and the pH will change – you could measure this as described in the previous reaction.

• Polystyrene cup and lid

• 250 cm³ beaker

• Thermometer

• Measuring cylinder (25cm³)

• 1 mol/dm³ hydrochloric acid (25cm³)

• 1 mol/dm³ sodium hydroxide (25cm³)

1. Place the polystyrene cup into the 250 cm³ beaker to stabilise it.

2. Carefully measure 25 cm³ of hydrochloric and place it in the polystyrene cup.

3. Take the initial temperature of the hydrochloric acid and record it in the results table.

4. Wash out the measuring cylinder and rinse it with a little sodium hydroxide solution.

5. Then measure 25 cm³ of sodium hydroxide solution into the measuring cylinder.

6. Add the sodium hydroxide solution to the acid, stir with the thermometer, measure and record the highest temperature reached during the reaction.

During this reaction the H⁺ ions from the acid and the OH^- ions from the alkali react together to form water.

As this happens, energy is released which is detected as a temperature increase on the thermometer.

The polystyrene cup is a good insulator which prevents heat from escaping from the reaction.

The final characteristic reaction of an acid is that it will produce carbon dioxide gas when reacted with a metal carbonate.

acid + metal carbonate → salt + water + carbon dioxide

In this experiment you will react calcium carbonate with hydrochloric acid:

hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide

The carbon dioxide gas which is released during the reaction will cause limewater to change from a colourless solution to milky.

• Measuring cylinder

• Boiling tube, test-tube

• Disposable pipette/dropper

• Test-tube rack

• Hydrochloric acid 0.5 mol/dm³ (15 cm³)

• Calcium carbonate (3 g)

• Limewater (5 cm³)

1. Using a measuring cylinder, measuring 15 cm³ of hydrochloric acid and place into the boiling tube.

2. Using a clean measuring cylinder, measure 5 cm³ of limewater and place into a test-tube. Place the test-tube and boiling tube side by side in a test-tube rack.

3. Carefully add the calcium carbonate to the acid – record your observations.

4. Using the disposable pipette/dropper, collect the gas produced by opening and closing the dropper above the reaction in the boiling tube.

5. Once the gas has been collected in the disposable pipette/dropper, bubble the gas through the limewater and record your observations.

Reaction of calcium carbonate and hydrochloric acid:

• Gas produced.

• White solid disappears.

• Colourless solution formed.

Gas bubbled through limewater from pipette

• Limewater turns from colourless to milky/cloudy – this indicates the gas produced was carbon dioxide.