18.2 - Reactions of Dilute Acids
The experiment
Aim
To find out what happens when a reactive metal, an unreactive metal, a metal oxide, an alkali, and a carbonate are added to dilute hydrochloric acid or dilute sulfuric acid, and to measure the temperature change where one occurs.
Background
- Acids react to make a salt. What else forms depends on what the acid reacts with.
- A metal gives a salt and hydrogen. A metal oxide or an alkali gives a salt and water, which is neutralisation. A carbonate gives a salt, water, and carbon dioxide.
- You can't see the products forming, but you can see a gas given off, a solid dissolving, or a colour change, and you can measure a temperature rise.
- So in this practical you add each substance to an acid, test any gas given off, and use what you see to tell which reaction has happened.
Variables
- Independent variable - The substance added to the acid.
- Dependent variable - What you see, and for the alkali, the highest temperature reached.
- Control variables - The volume and concentration of the acid, the amount of substance added, and starting each test at room temperature.
Method
- Metals - Measure 15 cm3 of dilute hydrochloric acid into a boiling tube and add a 2 cm strip of magnesium. After about ten seconds, hold a lit splint at the mouth of the tube and record what you see. Repeat with zinc, then with copper.
- Metal oxide - Warm 25 cm3 of dilute sulfuric acid in a beaker of hot water for two minutes, and test its pH with universal indicator paper. Add copper(II) oxide a spatula at a time, stirring, until some black powder stays undissolved, then test the pH of the blue solution.
- Alkali - Put 25 cm3 of dilute hydrochloric acid in a polystyrene cup stood in a beaker, and record its temperature and the temperature of 25 cm3 of sodium hydroxide solution. Add the alkali to the acid, stir with the thermometer, and record the highest temperature. The temperature change is the highest temperature minus the mean of the two starting temperatures.
- Carbonate - Add a spatula of calcium carbonate to 15 cm3 of dilute hydrochloric acid in a boiling tube. Collect the gas in a plastic pipette, bubble it through limewater, and record what happens.
Safety
- Eye protection - The acids are irritants, and copper(II) oxide is harmful.
- Hydrogen is flammable - Only bring the lit splint to the tube once the reaction has started, and point the tube away from people.
- Hot water - Take care handling the beaker of hot water.
Common exam questions
Question 1
How do you test a gas to show it is hydrogen, and what is the result?
Hold a lit splint at the mouth of the tube. Hydrogen burns with a squeaky pop.
Question 2
How do you test a gas to show it is carbon dioxide, and what is the result?
Bubble the gas through limewater. Carbon dioxide turns the limewater cloudy.
Question 3
Why doesn't copper react with dilute hydrochloric acid or dilute sulfuric acid?
Copper is below hydrogen in the reactivity series, so it can't displace hydrogen from the acid.
Question 4
Write a word equation for the reaction between magnesium and hydrochloric acid.
Magnesium + hydrochloric acid → magnesium chloride + hydrogen
Question 5
Write a word equation for the reaction between calcium carbonate and hydrochloric acid.
Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide