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Chemistry > M3 Reactity Trends > Flashcards

M3 Reactity Trends Flashcards

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1
Q

Define a reducing agent

A

Something that causes another species to be reduced.
eg. Ca —> Ca2+ + 2e- Ca is oxidised
Another species will gain these two electrons and be reduced.

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2
Q

Describe group 2 redox reactions with oxygen

A
  • Group 2 elements all react with oxygen to form a metal oxide with the general formula MO, made up of M2+ and O2- ions.
  • The group 2 element is oxidised (increases by +4) and the oxygen is reduced (decreases by -4)
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3
Q

Describe group 2 redox reactions with water

A
  • The group 2 elements react with water to form an alkaline hydroxide, with the general formula M(OH)2 and hydrogen gas.
  • Water and magnesium react very slowly but the reaction becomes more vigorous with metals further down the group.
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4
Q

Describe group 2 redox reactions with dilute acids

A
  • Many metals take part in redox reactions with dilute acids to form a salt and hydrogen gas.
    metal + acid —> salt + hydrogen
  • All group 2 elements react in this way and reactivity increases down the group.
  • The metal is oxidised and the hydrogen is reduced.
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5
Q

Describe the trend in reactivity and ionisation energy

A
  • Ionisation energies decrease down the group because the attraction between the nucleus and the outer electrons decreases as a result of atomic radius increasing.
  • Reactivity increases as the metallic bonds become weaker as the ion gets bigger and outer electrons are more weakly held by the nucleus. The bigger the atom the further away the delocalised electrons are from the positive nucleus, so the smaller the energy required to break the electrostatic forces of attraction.
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6
Q

Describe the reactions of group 2 compounds with water

A
  • The oxides of group 2 elements react with water, releasing hydroxide ions and forming alkaline solutions of the metal hydroxide.
    CaO (s) + H2O (l) —> Ca2+ (aq) + 2OH- (aq)
  • Group 2 hydroxides are only slightly soluble in water. When the solution becomes saturated, any further metal and hydroxide ions will form a solid precipitate:
    Ca2+ (aq) + 2OH- (aq) —> Ca(OH)2 (s)
  • The solubility of the hydroxides in water increases as you go down the group, so the resulting solutions contain more OH- (aq) ions and are more alkaline as you go down the group.
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7
Q

Describe the physical properties of halogens

A
  • The most reactive non-metallic group, these elements do not occur as elements in nature.
  • Halogens occur as stable halide ions (Cl-, Br-, I-) dissolved in sea water or combined with sodium or potassium solid deposits.
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8
Q

What is the appearance and state of fluorine at RTP?

A

Pale yellow gas

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9
Q

What is the appearance and state of chlorine at RTP?

A

Pale green gas

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10
Q

What is the appearance and state of bromine at RTP?

A

Red-brown liquid

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11
Q

What is the appearance and state of iodine at RTP?

A

Shiny grey-black solid

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12
Q

Describe the trend in boiling points as you go down the halogens

A
  • more electrons
  • stronger London forces
  • more energy to break the intermolecular forces
  • boiling point increases
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13
Q

Describe redox reactions of halogens

A
  • Each halogen has 7 outer-shell electrons. Two electrons are in the outer s sub-shell and 5 are in the outer p sub-shell - s2p5.
  • In redox reactions each halogen atom is reduced, gaining one electron to form a 1- halide ion.
    Cl2 + 2e- —> 2Cl- chlorine is reduced
  • Halogens are oxidising agents as they oxidise another species.
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14
Q

Define oxidising agents

A

Something that can cause another species to be oxidised

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15
Q

Describe halide-halide displacement reactions

A
  • Displacement reactions of halogens with halide ions can be carried out in test-tubes. The results of the displacement reactions show that the reactivity of the halogens decrease down the group.
  • Eg. A solution of chlorine is added to two aqueous solutions containing bromine and iodine, if the halogen added is more reactive than the halide present a reaction takes place, displacing the halide from solution and the solution changes colour.
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16
Q

What colour are halogens in water?

A

Chlorine: pale green
Bromine: orange
Iodine: brown

17
Q

What colour are the top layers of halogens in cyclohexane?

A

Chlorine: pale green
Bromine: orange
Iodine: violet

18
Q

Describe trends in reactivity as you go down the halogens

A
  • atomic radius increases
  • more inner shells so shielding increases
  • less nuclear attraction to capture an electron from another species
  • reactivity decreases

In halogens fluorine is the strongest oxidising agent, gaining electrons more readily than the other halogens, they become weaker oxidising agents down the group.

19
Q

What is disproportionation?

A

Disproportionation is a redox reaction in which the same element is both oxidised and reduced. eg. the reaction with chlorine and water

20
Q

Describe the reaction of chlorine with water

A
  • Chlorine is used in water purification, when small amounts of chlorine are added to water, a disproportionation reaction takes place, for each chlorine molecule one chlorine atom is oxidised and the other reduced.
    Cl2 (aq) + H2O (l) —> HClO (aq) + HCl (aq)
  • Bacteria are killed by chloric(I) acid and chlorate(I) ions, ClO- rather than chlorine.
21
Q

Benefits and risks of chlorine use

A
  • Chlorine in drinking water can react with organic hydrocarbons such as methane, formed from decaying vegetation. Chlorinated hydrocarbons are formed, which are suspected of causing cancer.
  • However, without chlorine, the quality of drinking water would be compromised and diseases such as typhoid and cholera might break out.
22
Q

What occurs in the test for halide ions

A
  • Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides.
    Ag+ (aq) + X-(aq) —> AgX (s)
23
Q

Describe the carbonate test

A
  1. Add dilute nitric acid to the solid or solution to be tested.
  2. If you see bubbles, the unknown compound is a carbonate. Bubble the gas though limewater to provide it is carbon dioxide.

Na2CO3 (aq) + 2HNO3 (aq) —> 2NaNO3 (aq) + CO2 (g) + H2O (l)

24
Q

Describe the sulfate test

A
  • Barium sulfate is very insoluble. The formation of a white precipitate of barium sulfate is the basis for the test.
  • Usually barium ions are added as barium chloride or barium nitrate.
    Ba2+ (aq) + SO4 2- (aq) —> BaSO4 (s)
25
Q

Describe the halide test

A

Silver halides are insoluble. Aqueous silver ions react with aqueous halide ions to from precipitates of silver halides.
- nitric acid
1. Add aqueous silver nitrate to an aqueous solution of a halide.
2. The silver halides precipitates are different colours.
Chloride = white
Bromide = cream
Iodide = yellow
3. Add aqueous ammonia to test the solubility of the precipitate. (Silver chloride and silver bromide are soluble, silver iodide is insoluble)

26
Q

What is the correct sequence of anion tests?

A
  1. Carbonate CO3 2-
  2. Sulfate SO4 2-
  3. Halides, Cl-, Br-, I-
27
Q

What are you looking for in each anion test?

A

Carbonate test - add dilute acid and looking for effervesce. Neither sulfate or halide ions produce bubbles, therefore there is no possibility of the incorrect conclusion.
Sulfate test - add a solution with Ba 2+ ions and looking for a white precipitate of BaSO4. Barium carbonate (BaCO3) is white and insoluble, if you carry out a sulfate test in a carbonate you will get a white precipitate too.
Halide test - you add a solution containing Ag+ ions and looking for a precipitate. Silver carbonate and silver sulfate are both insoluble in water and will form precipitates so the halide test needs to come last.

28
Q

What are the tests for a mixture of ions?

A
  1. Carbonate test
    - If you see bubbles, continue adding dilute nitric acid until bubbling stops.
    - All carbonate ions will have been removed and there will be none left to react.
  2. Sulfate test
    - To the solution test from the carbonate test, add an excess of Ba(NO3)2 and any sulfate ions will precipitate out as barium sulfate.
    - Filter the solution to remove the barium sulfate.
  3. Halide test
    - To the solution left from the sulfate test, add AgNO3.
    - Any carbonate or sulfate ions initially present will have been removed. Any precipite now must involve halide ions.
    - Add NH3 (aq) to confirm which halide you have.
29
Q

Describe the test for ammonium ions (NH4 +)

A

When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas.
NH4+ (aq) + OH- (aq) —> NH3 (g) + H2O (l)
1. Add aqueous sodium hydroxide to the solution of an ammonium ion
2. Ammonia has is produced, you are unlikely to see bubbles as ammonia is very soluble in water
3. The mixture is warmed and ammonia gas is released
4. Ammonia gas will turn damp red litmus paper blue

Chemistry (15 decks)

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