8- Reactivity Trends

Question 1

What is the outer shell electron configuration for Beryllium ions?

Answer 1

(Be2+) - [He]

Question 2

What is the outer shell electron shell of Calcium ions?

Answer 2

(Ca2+) - [Ar]

Question 3

How many electrons are in the outer shell of group 2 elements?

Answer 3

2

Question 4

What happens when a Group 2 electron undergoes a redox reaction?

Answer 4

-metal atom is oxidised
-losses two electrons
-forms a 2+ ions (with electron configuration of a noble gas)
-group 2 metals are reducing agents (reduced another species)

Ca → Ca2+ 2e-

Question 5

How do group 2 elements react with oxygen?

Answer 5

-form at metal oxide

2Mg + O2 → 2MgO

Question 6

What are the changes in oxidation numbers when magnesium reacts with oxygen?

Answer 6

2Mg + O2 → 2MgO

Mg - starts with oxidation state of 0 and ends in +2
O- starts with oxidation state of 0 and ends in -2

Mg is oxidised
O2 is reduced

Question 7

What is the reaction of the group 2 elements with water?

Answer 7

-form an alkaline hydroxide (general formula M(OH2) )
-form hydrogen gas

Sr + 2H2O ➝ Sr(OH)2 + H2

Question 8

How does reactivity change down Group 2?

Answer 8

least reactive at the top (Beryllium)
most reactive at the bottom (Radium)

Question 9

What is the reaction of group 2 elements with dilute acids?

Answer 9

-form a salt + hydrogen gas

metal+acid ➝ salt + hydrogen

Question 10

What is the reaction between magnesium and dilute hydrochloric acid?

Answer 10

Mg + 2HCl ➝ MgCl2 + H2

Mg - goes through oxidation (goes from oxidation state of 0 to +2)
H- goes through reduction (oxidation state of +1 to 0)

Question 11

Why does the first ionisation energies decrease down group 2?

Answer 11

The first ionisation energies decrease down the group because the attraction between the nucleus and the outer electrons decrease due to increased atomic radius and increased shielding

Question 12

Why does reactivity increase down group 2?

Answer 12

-↑atomic radius ∴ increase in shielding
-decreased attraction between the outer electrons and the positive nucleus
-less energy is required to remove outer electrons

Question 13

What happens when you react a carbonate with an acid?

Answer 13

carbonate + acid = salt + water + C02

Question 14

What is the reaction like between Group 2 oxides and water?

Answer 14

-release hydroxide ions (OH-)
-form alkaline solutions of the metal hydroxides

Question 15

Write an equation for the reaction between calcium oxide and water

Answer 15

CaO + H2O ➝ Ca2+ + 2OH-

Question 16

Are group 2 hydroxides soluble in water?

Answer 16

-slightly
-when the solution becomes saturated, any further metal and hydroxide ions will for a solid precipitate
eg.

Ca2+ + 2OH- ➝ Ca(OH)2

Question 17

How does the solubility of hydroxides change down group 2? And why?

Answer 17

-increases as you go down the group
-results in solutions containing more OH- ions are more alkaline
-solubility increases along with pH and alkalinity

Question 18

How are Group 2 compounds used in agriculture?

Answer 18

-Calcium Hydroxide (Ca(OH)2)
-Added to fields as lime
-Increase the pH of acidic soils
-Neutralises the soil forming neutral water

Ca(OH)2 + 2H+ ➝ Ca2+ + 2H2O

Question 19

How are Group 2 compounds used in medicine?

Answer 19

-used as antacids for treating acid indigestion
-magnesium and calcium carbonates as main ingredients
-milk of magnesia is a suspension of magnesium hydroxide (Mg(OH)2) in water
-because magnesium hydroxide is only slightly soluble in water
-acid in stomach is mainly HCl and a neutralisation reaction takes place

Mg(OH)2 + 2HCl ➝ MgCl2 + 2H20
CaCO3 + 2HCl ➝ CaCl2 + H20 + CO2

Question 20

How do halogens exist?

Answer 20

-diatomic molecules at room temperature and pressure (Cl2, Br2)

Question 21

What is the trend of boiling points of halogens?

Answer 21

-boiling point increases as you go down the group
-more electrons
-stronger london forces
-more energy required to break the intermolecular forces

Question 22

How many electrons do halogens have in there outer shell?

Answer 22

7

Question 23

What is the charge of a halide ion?

Answer 23

1-

Question 24

How do halogens react in a redox reaction?

Answer 24

-halogen atom is reduced
-gain one electron to form a 1- halide ion (electron configuration of nearest noble gas)
-oxidising agent (oxidised another species)

Cl2 + 2e- ➝ 2Cl-

Question 25

What happens in a reaction if a halogen is more reactive then the halide?

Answer 25

-halogen displaces halide -colour change occurs

Question 26

What do displacement reactions between halogens and halides show?

Answer 26

-reactivity of halogens decrease down the group

Question 27

Due to I2 and Br2 being hard to tell apart in water what can happen to be able to tell them apart?

Answer 27

-organic non-polar solvent (cyclohexane) is added -mixture shaken -non-polar halogens dissolve more readily in cyclohexane -colours much easier to tell apart

Question 28

What are the colours of Cl2, Br2 and I2 in water?

Answer 28

Chlorine - pale green Bromine - orange Iodine - brown

Question 29

What are the colours of Cl2, Br2 and I2 in water?

Answer 29

Chlorine - pale green Bromine - orange Iodine- violet

Question 30

What is the reaction between chlorine and Br-?

Answer 30

Cl2 + 2Br- ➝ 2Cl- + Br2

Question 31

What is the reaction between bromine and I- ions?

Answer 31

Br2 + 2I- ➝ 2Br- + I2

Question 32

What are the trends in reactivity of the halogens?

Answer 32

-reactivity decreases down the group -atomic radius increases -more inner shells so shielding increases -less nucleus attraction to capture an electron from another species

Question 33

Which halogen is the strongest oxidising agent?

Answer 33

-fluorine -gaining electrons from other species more readily than the other halogens

Question 34

What is a disproportionation reaction?

Answer 34

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

Question 35

What is the reaction of chlorine with water like?

Answer 35

-Cl2 + H20 ➝ HClO + HCl -disproportionation reaction

Question 36

How is the reaction of chlorine with water used?

Answer 36

-drinking water treatment

Question 37

What is the reaction of chlorine with cold, dilute aqueous sodium hydroxide? What is it used for?

Answer 37

-limited by the low solubility of chlorine in water -water contains dissolved NaOH, much more chlorine dissolves -disproportionation reaction Cl2 + 2NaOH ➝ NaClO + NaCl + H2O -results in a large concentration of ClO- ions from sodium chlorate -use as household bleach

Question 38

What are the benefits and risks of chlorine use?

Answer 38

-ensures our water is fit to drink and bacteria are killed, no risk of diseases like typhoid,cholera -extremely toxic gas -respiratory irritant in small concentrations, large concentrations are fatal -chlorine can react with organic hydrocarbons which are suspected of causing cancer

Question 39

What is the reaction between aqueous halide ions and aqueous silver ions?

Answer 39

Ag+ + X- ➝ AgX (aq) X = aqueous soloution of any halide

Question 40

How do you preform the carbonate test?

Answer 40

1. In a test tube, add dilute nitric acid to the solid or solution 2. If you see bubbles the unknown gas could be a carbonate 3. To prove gas is carbon dioxide, bubble the gas through limewater (aqueous calcium hydroxide) . If CO2 if present will make a white precipitate

Question 41

What is the reaction between sodium carbonate and nitric acid?

Answer 41

Na2CO3 + 2HNO3 ➝ 2NaNO3 + CO2 + H2O

Question 42

How do you perform the sulfate test?

Answer 42

-add a few drops of dilute HCL -add a few drops of Barium Chloride -positive result = white precipitate formed

Question 43

Write an equation for the sulphate ion test

Answer 43

Ba2+ + SO4 2- ➝ BaSO4

Question 44

How do you test for halide ions?

Answer 44

1. Add aqueous silver nitrate , AgNO3 2. Record the colour change 3. If difficult to tell colours apart, add aqueous ammonia 4. Note the solubility of the precipitate

Question 45

What is the equation and result for the Cl- test?

Answer 45

-white precipitate, soluble in dilute aqueous ammonia Ag+(aq) + Cl-(aq) → AgCl(s)

Question 46

What is the equation and result for the Br- test?

Answer 46

-cream precipitate -soluble in concentrated ammonia only Ag+(aq) + Br-(aq) → AgBr(s)

Question 47

What is the equation and result for the I- test?

Answer 47

-yellow precipitate -insoluble in concentrated and dilute aqueous ammonia Ag+(aq) + I-(aq) → AgI(s)

Question 48

When testing for carbonate, sulfate and halide ions, in which order should you carry out the tests and why?

Answer 48

1. Carbonate 2. Sulfate 3. Halide Because barium ions forms insoluble precipitate of BaCO3 and silver ions form insoluble precipitate Ag2SO4

Question 49

How do you test fir ammonium ions, NH4+?

Answer 49

1. add sodium hydroxide (NaOH) to the sample 2. Warm the mixture 3. Test the gas produced with red litmus paper

Question 50

What are the observations for positive ammonia test?

Answer 50

-litmus paper turns blue -ammonia has a pungent smell

Question 51

Write an equation for the ammonia test?

Answer 51

NH4+ (aq) + OH- (aq) ➝ NH3 (aq) + H2O (aq)