C2 3.6+ Reversible Reactions and Chemical Analysis

Question 1

What is a reversible reaction?

Answer 1

A reversible reaction is where the products of a reaction can react together to make the original reactants again

Question 2

Reversible reaction symbol:

Answer 2

Question 3

Litmus paper and reversible reactions:

Answer 3

• Litmus is a complex molecule
• We will represent it as HLit where H is hydrogen
• HLit is red
• If alkali is added, HLit turns into the Lit^- ion by losing a H⁺ ion.
• Lit^- is blue
• If more acid is added, blue Lit^- changes back to red HLit and so on

Summed up as:

HLit_{(red litmus)}<=> H⁺+ Lit^-_{(blue litmus)}

Question 4

Ammonium Chloride reversible reaction:

Answer 4

Ammonium chloride <=> ammonia + hydrogen chloride

Ammonium chloride breaks down on heating. It forms ammonia gas and hydrogen chloride gas.

When the gases cool down near the mouth of the tube, they react with each other and re-form the ammonium chloride again

Question 5

What is a food additive?

Answer 5

A food additive is a substance added to food to extend its shelf life or to improve its taste or appearance

Question 6

Why can paper chromatography work to identify food additives?

Answer 6

Paper Chromatography works as some compounds in a mixture dissolve better than others in particular solvents. Their solubility determines how far they travel across the paper

Question 7

What are the advantages of modern instrumental methods?

Answer 7

• They are highly accurate and sensitive
• They are quicker
• They enable very small samples to be analysed

Question 8

What are the disadvantages of modern instrumental methods?

Answer 8

• Usually very expensive
• Needs specialist training
• Gives results that can often only be interpreted by comparison with data from known substances

Question 9

How can compounds in a mixture be separated?

Answer 9

Compounds in a mixture can be separated using gas chromatography

Question 10

What does the mass spectrometer do?

Answer 10

The mass spectrometer identifies the separated compounds. The pattern of peaks it produces identifies the sample

Question 11

Gas chromatography procedure:

Answer 11

• The sample mixture is vaporised
• A ‘carrier’ gas moves the vapour through the coiled column
• The compounds in the sample have different attractions to the material in the column. We say that they have a longer retention time
• The compounds with weaker attractions to the material in the column leave first. They have shorter retention times

Question 12

Gas Chromatography Diagram:

Answer 12

Question 13

What can a mass spectrometer do?

Question 14

Mass Spectrometer Diagram:

Answer 14

The pattern is then quickly matched against a database of known compounds stored on a computer