8- Reactivity Trends

Question 1

Redox reaction definition.

Answer 1

An oxidation-reduction (redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species.

Question 2

Write a half equation showing how group two metals react. State if its oxidation or reduction.

Answer 2

M —> M(2+) +2e-
[No] 4s2 [No]
Group two metals lose electrons, so they are oxidised. But they are reducing agents.

Question 3

Show how group two metals react with oxygen. Show oxidation numbers.

Answer 3

2Mg + O2 —> MgO (s)

0 0 +2 -2

Question 4

Show how group 2 metals react with water.

Answer 4

Sr(s) + 2H2O (l) —> Sr(OH)2 (aq) + H2 (g)
0 +1 +2 0
Hydrogen is reduced

Question 5

Explain the trend in boiling point of the halogens.

Answer 5

The boiling point inreases as you go down the group.

• More electrons.
• Stronger London forces.
• More energy required to break intermolecular forces.
• Boiling point increases.

Question 6

Show how halides react using half equations. Explain the oxidation and reduction.

Answer 6

Cl2 + 2e- –> 2Cl-
Chlorine gains electrons so it is reduced.
It is an oxidising agent.

Question 7

Describe how displacement reactions can be used to show the trend in reactivity of group 7

Answer 7

A solution of each halogen is added to an aqueous solution of each other halide. If the halide added is more reactive: the halide is displaced, the solution changes colour.

Question 8

State the colour made by each halide in water.

Answer 8

Chlorine: pale green. Bromine: Orange Iodine: Brown

Question 9

State the equation of halide displacement in water.

Answer 9

Cl2(aq) + 2Br-(aq) –> 2Cl-(aq) +Br2 (aq)

Question 10

Iodine and bromine can look the same in water. What could you do to tell the difference between them?

Answer 10

To tell them apart, an organic non polar solvent such as cyclohexane must be used to dissolve the non polar halides.

Question 11

What colours do the halides make in cyclohexane.

Answer 11

Chlorine: pale green. Bromine: Orange Iodine: Violet.

Question 12

State the equation of the redox reaction between aqueous chlorine and sodium bromine.

Answer 12

Cl2 (aq) + 2NaBr (aq) –> 2NaCl (aq) + Br2(aq)

0 -1 -1 0 oxidation n.

Question 13

State the ionic equation of the redox reaction between aqueous chlorine and sodium bromine.

Answer 13

Cl2 + 2Br- –> 2Cl- + Br2

Question 14

Explain the trend in reactivity down group 7.

Answer 14

Reactivity decreases down group 7.

• Atomic radius increases.
• More shells so more shell sheilding.
• Less nuclear attraction to capture an electron from another species.

Question 15

What is a disproportionation reaction?

Answer 15

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

Question 16

Give an example of a disproportionation reaction.

Answer 16

Chlorine + water.
Cl2(aq) + H2O(l) –> HCl(aq) + HClO(aq)
0 -1 +1

Question 17

The reaction of chlorine and water is limited by chlorines low solubility in water. How can you make more chlorine dissolve?

Answer 17

If the water contains dissolved sodium hydroxide, much more of the chlorine will dissolve and a disproportionation reaction takes place.

Question 18

State the equation for the reaction of chlorine with aqueous sodium hydroxide.

Answer 18

Cl2(g) + 2NaOH(aq) –> NaClO(aq) + NaCl(aq) +H2O(l)

0 +1 -1

Question 19

How is bleach made. Give a risk of a chemical in bleach.

Answer 19

Bleach is made from reacting chlorine gas with cold dilute aqueous sodium hydroxide. The chlorate ions from the NaClO are toxic. Chlorine can bond with hydrocarbons in water. Chlorinated hydrocarbons cause cancer.

Question 20

How would you test for carbonate ions.

Answer 20

Carbonates react with acids to form carbon dioxide. So add dilute nitric acid to the solid or solution. If there is effervesence, bubble it through lime water. If the solution goes cloudy there is CO2.

Question 21

How would you test for sulphate ions?

Answer 21

Barium sulphate is insoluable in water. So add aqueous Barium to the solution and watch for a white precipitate. Ba 2+(aq) + SO4 2-(aq) –> BaSO4 (s)

Question 22

How would you test for halide ions?

Answer 22

Silver halides are insoluabe in water. So add dilute silver nitrate. Precipitate: silver chloride = white. Silver bromide = cream. Silver iodide = yellow. Add aqueous ammonia to test the soluability of the precipitate. Iodine is insuluable and bromine is only soluable in conc. ammonia.

Question 23

Explain the order of the anion tests.

Answer 23

Neither halide or sulphate ions produce effervesense when reacting with dilute acid. So adding acid for the carbonate test produces no error in results. BaCO3 is a white precipitate formed if there is a carbonate in the sulphate test. So carbonate must be done first. Both Ag2SO4 and Ag2CO3 are precipitates. SO silver nitrate must be added last for the halide test.