Module 3: Chapter 8 (Reactivity Trends)

Question 1

What is another name for Group 2?

Answer 1

Alkaline Earth Metals

Question 2

Describe the physical properties of Group 2

Answer 2

• reactive metals
• never found in their native state
• compounds are common throughout nature, e.g. calcium carbonate

Question 3

Discuss Group 2 in terms of redox reactions

Answer 3

• Called a reducing agent as it reduces another species, gains the two outer electrons from the group 2 element, forms a 2+ group 2 ion

Question 4

Group 2: Describe redox reactions with oxygen

Answer 4

• forms a metal oxide, MO
• For example, 2Mg + O2 -> 2MgO (solid)
• Magnesium is oxidised and Oxygen is reduced

Question 5

Group 2: Describe redox reactions with water

Answer 5

• forms an alkaline hydroxide, M(OH)2, and hydrogen gas
• reaction becomes more vigorous down group (more reactive)
• for example: Sr + 2H2O -> Sr(OH)2

Question 6

Group 2: Describe reactions with dilute acids

Answer 6

metal + acid -> salt + hydrogen (MASH)

• reactivity increases down the group
• e.g. Mg + 2HCl -> MgCl2 + H2

Question 7

Why does reactivity increase down group 2?

Answer 7

• Ionisation energies decrease down the group because of increasing atomic radius and increasing shielding meaning less nuclear attraction for outer electron and lower ionisation energy
• Elements in group 2 react by losing 2 electrons (forming +2 ions), that is two ionisation energies in terms of energy input

Question 8

Group 2 oxides reacting with water

Answer 8

• react with water to release OH- ions forming an alkaline solution of a metal hydroxide
• CaO (solid) + H2O -> Ca2+ + 2OH-
• the group 2 hydroxides are sparingly soluble in water, if the solution is saturated, a solid ppt will form:
  Ca2+ + 2OH- -> Ca(OH)2

Question 9

What is the trend of solubility of group 2 hydroxides?

Answer 9

• increases down the group
• more soluble = more OH- ions in solution = more alkaline
• for example Magnesium hydroxide forms a solution of a low concentration of pH 10 and barium forms a more concentrated solution, pH 13

Question 10

What are the uses of Group 2 compounds?

Answer 10

Agriculture:
Ca(OH)2 added to fields to increase the pH of acidic soils: Ca(OH)2 + 2H+ -> Ca2+ + 2H2O

Medicine:

• Acid in stomach is mainly HCl, can be neutralised
• often used as antacids (treating acid indigestion)
• Mainly Magnesium and Calcium carbonates used

Question 11

What is another name for group 7 elements?

Answer 11

Halogens

Question 12

Describe general properties about halogens

Answer 12

• most reactive non-metal
• only occur as halide ions in nature (never in elemental form)
• in elemental form, exist as diatomic molecules, X2
• structure is simple molecular

Question 13

Describe the elemental states of the halogens at RTP

Answer 13

Fluorine = pale yellow gas
Chlorine = pale green gas
Bromine = an orange-red volatile liquid
Iodine = grey-black crystalline solid

Question 14

What is the trend in boiling points in the halogens?

Answer 14

• increases down the group
• more electrons
• stronger London forces
• more energy required to break intermolecular forces

Question 15

Describe the redox reactions of halogens

Answer 15

• outer shell configuration: s2p5
• gaining one electron achieves noble gas electronic configurations, so redox reactions are the most common reactions for halogens
• e.g Cl2 +2e- -> 2Cl- (half equation), oxidising agents as the chlorine is reduced

Question 16

Describe halide-halide displacement reactions

Answer 16

• reactivity decreases down the group
• if a more reactive halogen is added to less reactive halide a displacement reaction will take place (halogen atom displaces ion in solution)

Question 17

Describe the colour of aqueous halogens

Answer 17

Solution in water:
Cl = pale green
Br = orange
I = brown

top layer colour with cyclohexane:
Cl = pale green
Br = orange
I = violet

Question 18

Explain why as you go down Group 7 the element become weaker oxidising agents?

Answer 18

• increasing atomic radius
• increased shielding from more inner shells means less nuclear attraction to capture an electron from another species
• reactivity decreases down the group/weaker oxidising agent

Question 19

What is a disproportionation reaction?

Answer 19

A redox reaction in which an element is oxidised and reduced

Question 20

What is the redox reaction for chlorine and water?

Answer 20

Cl2 (g) -> H2O (l) -> HClO (aq) + HCl
HClO = chloric (1) acid, kills bacteria and acts a weak bleach
- Chlorine 0 to +1 and -1 (disproportionation)
- Chlorine is used to treat drinking water by killing harmful bacteria, the equation above is the reaction of Cl being added to water

Question 21

What is the reaction between chlorine and dilute sodium hydroxide?

Answer 21

• dilute NaOH will allow more chlorine to dissolve (solubility is lower in water)

Cl2 (g) + 2NaOH (aq) -> NaClO (aq) + NaCl (aq) + H20 (l)

NaClO (sodium chlorate) is a much stronger bleach, large concentration of ClO- ions, chlorate ions
- disproportionation of Cl2, 0 to +1 (in NaClO) and -1

Question 22

What are the benefits and risks of chlorine?

Answer 22

• a toxic gas
• will react with methane(/organic hydrocarbons) in water, methane is formed by decaying vegetation, -> chlorinated hydrocarbons are suspected carcinogens
• without chlorine in our water we would be at risk of disease such as typhoid and cholera

Question 23

What are the properties of Group 1 metal halides?

Answer 23

• white crystalline solids
• very soluble in water forming colourless solutions
• insoluble in cyclohexane

Question 24

Describe the carbonate test

Answer 24

• acids react with carbonates to form a salt, water and carbon dioxide gas (effervescence)
• CO2 produced can be detected by bubbling the gas through lime water, a white precipitate produced (turned limewater cloudy white), calcium carbonate

Question 25

Describe the sulphate test

Answer 25

• barium sulphate is insoluble in water, a white precipitate will form (add Ba2+ ions to a solution so BaSO4 will form, forming a white ppt)
• Application in medicine as a barium meal, drink containing barium sulphate will coat the lining of part of the body (e.g. gut) to detect problems or anomalies

Question 26

Describe the test for ammonium ions

Answer 26

• Add aqueous sodium hydroxide to a solution of ammonium ions and warm.
• Ammonia gas is produced.
• Ammonia is alkaline so hold a damp
  piece of pH indicator paper near the
  mouth of the test tube and observe what happens.

Question 27

Describe the halide test

Answer 27

• Silver ions (in the form of silver nitrate AgNO3) are added to an aqueous halide ions to detect the halide ion present.
• Silver chloride = white ppt, bromide = cream ppt and iodide = yellow
• ppts formed have similar colours (use aqueous ammonia to distinguish halides), addition of dilute ammonia dissolve white chloride ppt, addition of concentrated ammonia dissolves the bromide cream ppt

Question 28

What order should you carry out the tests in?

Answer 28

1. carbonate test only using nitric acid
2. sulfate test, using barium nitrate
3. halide test using silver nitrate and aqueous ammonia

Question 29

Describe the results of the halogen displacement reactions

Answer 29

Chlorine displacing bromine:
orange colour from Br2 formation

Chlorine displacing iodine:
violet colour from I2 formation

Bromine displacing iodine:
violet colour from I2 formation