18.9 - Reactions of Carboxylic Acids
The experiment
Aim
To compare dilute ethanoic acid with dilute hydrochloric acid of the same concentration, using pH and two reactions, to find out what the comparison shows about a weak acid and a strong acid.
Background
- Ethanoic acid, CH3COOH, is a carboxylic acid. It's a weak acid, which means it only partly ionises.
- Hydrochloric acid is a strong acid, which ionises completely. At the same concentration, it gives a higher concentration of hydrogen ions than ethanoic acid.
- Both acids react with a metal in the same way, giving a salt and hydrogen, so they can be compared reaction for reaction.
- You can't see how much an acid has ionised, but you can measure its pH and see how fast it reacts with a metal.
- So in this practical you compare the pH of the two acids, and how fast each one reacts with magnesium and with calcium. The acid with the higher pH and the slower reactions is the weaker acid.
Variables
- Independent variable - Which acid is used, ethanoic or hydrochloric.
- Dependent variable - The pH of each acid, how fast it fizzes with magnesium, and the time for a piece of calcium to disappear.
- Control variables - The concentration and volume of each acid, and the size and shape of each piece of metal.
Method
- Compare the pH - Put two pieces of universal indicator paper on a white tile. Use a glass rod to add a drop of each acid to its own piece, rinsing the rod in between, and match each colour to the pH chart.
- React each acid with magnesium - Add a 1 cm strip of magnesium to the same volume of each acid in two test tubes, and compare how fast each one fizzes. Test the gas from the hydrochloric acid with a lit splint.
- React each acid with calcium - Use tweezers to add a piece of calcium to the hydrochloric acid in a small beaker, and time how long it takes to disappear. Repeat with the same volume of ethanoic acid and an identical piece of calcium.
- Draw a conclusion - Decide which acid is weaker from its higher pH, slower fizzing, and longer time for the calcium to disappear. Three results that agree are much stronger evidence than any one on its own.
Safety
- Eye protection - Both acids are irritants, so wash off any splashes at once.
- Hydrogen is flammable - Only bring a lit splint to a tube once it's fizzing, and never point a fizzing tube at anyone.
- Calcium - Handle it with tweezers, because it reacts with moisture on your skin, and keep your face away from the beaker, because it can spit.
Common exam questions
Question 1
Using your results, state and explain which acid reacted faster with calcium.
Hydrochloric acid, because the calcium disappeared in a shorter time. It's fully ionised, so it has a higher concentration of hydrogen ions than ethanoic acid at the same concentration.
Question 2
Write a word equation for the reaction between ethanoic acid and magnesium.
Ethanoic acid + magnesium → magnesium ethanoate + hydrogen
Question 3
Write a balanced symbol equation for the reaction between ethanoic acid and calcium.
2CH3COOH + Ca → (CH3COO)2Ca + H2
Question 4
Describe a test that would show a substance is a carboxylic acid rather than an alcohol.
Add magnesium or a metal carbonate. A carboxylic acid fizzes, giving hydrogen that pops with a lit splint, or carbon dioxide that turns limewater cloudy. An alcohol doesn't react with either.
Question 5
Suggest how you could improve the accuracy of the pH comparison.
Use a pH probe instead of universal indicator paper. It gives a number, rather than a colour matched to a chart by eye.