Chapter 8 - Reactivity Trends

Question 1

What happens when a group 2 element reacts in terms of electron configuration?

Answer 1

Act as reducing agents
They form 2+ ions
They are oxidised

Question 2

What happens when the group 2 elements react with oxygen?

Answer 2

A redox reaction occurs
Form a metal oxide
General formula: MO

Question 3

What are the half equations of group 2 elements reacting with oxygen? (Use Mg)

Answer 3

Mg -> Mg2+ +2e- (Loss of electrons and increase in oxidation number = oxidation)
1/2O2 + 2e- -> O2- (Gain of electrons and decrease in oxidation number = reduction)

Question 4

What happens when the group 2 elements react with water?

Answer 4

Redox reaction occurs
Form an alkaline hydroxide
General formula: M(OH)2, and hydrogen gas

Question 5

What happens when the group 2 elements react with dilute acids?

Answer 5

Redox reaction occurs
Form a salt and hydrogen gas

Question 6

Why does the reactivity increase down group 2?

Answer 6

Loses 2 outer-shell electrons to form 2+ ions
The formation of +2 ions involves two ionisation energies
These ionisation energies decrease down the group as atomic radius increases and inner-shell shielding increases
So nuclear attraction decreases

Question 7

What happens when group 2 oxides react with water?

Answer 7

Form alkaline solutions
Release hydroxide ions

Question 8

What are the properties of group 2 hydroxides?

Answer 8

Only slightly soluble in water
When the solution becomes saturated, any further metal and hydroxide ions will form a white precipitate

Question 9

What is one of the uses of group 2 compounds as bases?

Answer 9

In agriculture calcium hydroxide, Ca(OH)2, is added to fields as lime by farmers to increase the pH of acidic soils in a neutralisation reaction

Question 10

What is the other use of group 2 compounds as bases?

Answer 10

In medicine, group 2 bases are often used as antacids for treating acid indigestion

Question 11

What are group 7 elements also known as?

Answer 11

Halogens

Question 12

What do group 7 elements exist as?

Answer 12

Diatomic molecules, X2, at RTP

Question 13

What is the general trend in the boiling point as you go down group 7?

Answer 13

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

Question 14

What happens when a group 7 element reacts in terms of electron configuration?

Answer 14

Each atom in the molecule gains 1 electron into its outer shell to form 1- ions
During the reaction, another species loses electrons which will be gained by the halogen atoms in each molecule
The group 7 element is called an oxidising agent because it has oxidised another species

Question 15

What colours appear when chlorine, bromine and iodine are dissolved in water?

Answer 15

Cl2 - Pale green
Br2 - Orange
I2 - Brown

Question 16

What happens when a non-polar solvent is added to the mixture containing the halogen dissolved in water?

Answer 16

Layers form
The non-polar halogens dissolve more readily in the non-poplar solvent

Question 17

What happens when a solution of each halogen is added to aq solution of other halides? (if the halogen added is MORE reactive than the halide present)

Answer 17

A reaction takes place, the halogen displacing the halide from solution
The solution changes colour

Question 18

What is the halogen-halide displacement reaction between Cl2 (aq) and 2Br- (aq)?

Answer 18

2Cl- (aq) + BR2 (aq) form

Question 19

What is the halogen-halide displacement reaction between Cl2 (aq) and 2I- (aq)?

Answer 19

2Cl- (aq) and I2 (aq) form

Question 20

What is the halogen-halide displacement reaction between Br2 (aq) and 2I- (aq)?

Answer 20

2Br- (aq) and I2 (aq) form

Question 21

What happens to the reactivity as you go down group 7?

Answer 21

Decreases

Question 22

What is a redox reaction?

Answer 22

A redox reaction in which the same element is both oxidised and reduced

Question 23

What is the reaction between chlorine and water? (Include oxidation numbers)

Answer 23

HClO and HCl formed
Bacteria are killed by chloric(I) acid and chlorate(I) ions

Question 24

What is the reaction between chlorine with cold, dilute aq sodium hydroxide?

Answer 24

Question 25

What is the benefit of chlorine usage?

Answer 25

Killing all bacteria present

Question 26

What are the risks of chlorine usage?

Answer 26

Extremely TOXIC gas: -In small concentrations it is a respiratory irritant -In large concentrations it can be fatal Chlorine is added to drinking water can react with organic hydrocarbons, such as methane, to form chlorinated hydrocarbons - a suspected carcinogen

Question 27

How do we test for halide ions?

Answer 27

They all react with aq silver ions to form a precipitate of silver halide

Question 28

How do we test for CO₃²⁻?

Answer 28

Add dilute nitric acid to a test tube containing the unknown compound Carbonates react with acids to form carbon dioxide gas Observation - bubbles of gas

Question 29

How do we test for SO₄²⁻?

Answer 29

Add barium nitrate to a test tube containing a solution of the unknown compound Observation - a white precipitate (of barium sulfate) forms

Question 30

What is the order in which we test for anions?

Answer 30

1. Carbonate 2. Sulfate 3. Halides

Question 31

What is the test for carbonate in a mixture of ions?

Answer 31

If bubbles are visible, continue adding nitric acid until the bubbling stops All carbonate ions will have been removed and there will be none left to react in the next tests

Question 32

What is the test for sulfate in a mixture of ions?

Answer 32

To the solution left from the carbonate test, add an excess of Ba(NO3)2 Any sulfate ions initially present have already been removed. Therefore, any precipitate formed must involve halide ions Add NH3 to confirm which halide you have

Question 33

What is the test for NH4⁺?

Answer 33

Add aq sodium hydroxide to a solution containing ammonium ions and heat gently Test gas released with moist pH paper