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A Level Chemistry > Chapter 8- reactivity trends > Flashcards

Chapter 8- reactivity trends Flashcards

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1
Q

Describe a halogen-halide displacement reaction.

A
  • A solution of a halogen is added to the aqueous solution of a halide.
  • Cyclohexane is then also added.
  • A change in colour denotes that the halogen is more reactive than the halide so a reaction has occurred where the halide is displaced.
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2
Q

Why is cyclohexane used in halogen-halide displacement reactions?

A

It forms the top layer of the solution and shows the colour change more clearly as the colours are easier to distinguish from each other.

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3
Q

What colours does cyclohexane go for the following molecules?

  • Cl2
  • Br2
  • I2
A
  • Cl2- pale green.
  • Br2- orange
  • I2- violet.
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4
Q

Explain what you would expect to see occur for the following and state any ionic equations relevant…

  • If chlorine was added to a solution of bromide ions.
  • If bromine was added to a solution of iodide ions.
  • If Iodide was added to a solution of chloride ions.
  • If Chlorine was added to a solution of chloride ions.
A
  • The chlorine would displace the bromide ions forming bromine and chloride ions. The formation of bromine would result in the cyclohexane turning orange. The ionic equation for this displacement is…
    Cl2 +2Br^- → 2Cl^- + Br2
  • The bromine would displace the iodide ions forming iodine and bromide ions. The formation of iodine would result in the cyclohexane turning violet. The ionic equation for this displacement is…
    Br2 +2I^- → 2Br^- + I2
  • The iodine would not displace the chloride ions soon reaction would occur.
  • No reaction/ change would occur.
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5
Q

Describe the anion test you would perform to test for the presence of carbonate (CO3^2-).

A
  • Add nitric acid (HNO3) to a sample in a test tube connected to another test the containing lime water (Ca(OH)2).
  • If the lime water goes cloudy white, the CO3^2- ions are present.
  • It goes cloudy due to CO3^2- forming calcium carbonate.
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6
Q

Describe the anion test you would perform to test for the presence of sulphate (SO4^2-).

A
  • Most compounds of sulphate are soluble in water, but barium sulphate is not (BASO4).
  • Therefore if you added barium ions to the sample and sulphate is present a white precipitate forms.
  • Barium nitrate should be used if you want to test for halides after.
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7
Q

Describe the anion test you would perform to test for the presence of halides and what you’d expect to see.

A
  • Most halides are soluble in water, but silver halides are not, instead forming precipitates.
  • Hence silver nitrate is added to a solution of the halide ion.
  • The ionic equation for this reaction is…
    Ag^+(aq) + X^-(aq) → AgX (s)
  • The compounds go the following colours…
    AgCl is white
    AgBr is cream
    AgI is pale yellow
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8
Q

How can you confirm what sliver halide precipitate is produced when testing for the presence of halides?

A

Ammonia can be used to determine what halide is present due to their different solubilities.

  • Dilute aqueous ammonia dissolves AgCl.
  • Concentrated aqueous ammonia dissolves AgBr.
  • No form of ammonia dissolves AgI.
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9
Q

Name the correct order to perform the anion tests and why.

A
  • Do the carbonate test first, sulphate test second and halide test third.
  • Carbonate- If the test produces no bubbles, then no carbonate is present and you can move on.
  • Sulphate- BaCO3^- is insoluble so the carbonate test must occur first to remove all carbonate from being present.
  • Halide- Ag2CO3 and AgSO4 are precipitates that can form so this test must be done last to remove those ions.
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10
Q

Describe things to remember when carrying out an anion tests.

A
  • Complete the tests in the correct sequence.
  • Carbonate- keep adding dilute nitric acid until no bubbles are seen to insure all carbonate ions are removed and do not use H2SO4 to insure the results are not contaminated for the next test.
  • Sulphate- Add an excess of Ba(NO3)2 to insure all SO4^2- is removed and then filter it to remove BaSO4. Also don’t use BaCl2 if testing halides after.
  • Halide- if all carbonate and sulphate ions are removed then the precipitate formed must be of halide ions.
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11
Q

How can you test for ammonium (NH4^+)?

A

When heated hydroxide and ammonium ions form ammonia (NH3). The ionic equation for this reaction is as follows…
NH4^+ (aq) + OH^- → NH3(g) +H2O(l)
- First add NaOH(aq) to the sample solution.
- If ammonium is present ammonia ions are produced, but bubbles are unlikely to be seen as it is soluble in water.
- The solution is then warmed to release the gas.
- When the gas is released you can smell it and also test it with moist pH paper (indicator paper) which will go blue to show the alkaline nature.

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12
Q

What is the name of the type of reaction where the same element is oxidised and reduced?

A

A disproportionation reaction.

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13
Q

Describe the reaction of chlorine with water and its significance in water purification.

A

This is a disproportionation reaction. The reaction is as follows…
Cl2 +H2O → HClO +HCl
As both chloric (I) acid and ClO^- ions kill bacteria chlorine is used in water purification. Chloric (I) acid is bleach.

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14
Q

How can you prove that chloric (I) acid produced when chlorine is used for water purification is a bleach?

A

If an indicator is added the solution goes red due to being acidic, however the presence of a bleach removes this colour so it can no longer be seen.

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15
Q

Explain why sodium hydroxide is used in water purification.

A

If water contains dissolved NaOH then more chlorine dissolves and a disproportionation reaction occurs producing NaCl, NaClO and H2O. NaClO is used in household bleach.

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16
Q

Describe the potential negative impacts of using chlorine in water.

A
  • Chlorine is a respiratory inhabitant in small concentrations.
  • Chlorine can react with methane from decaying vegetation to form chlorinated hydrocarbons that are suspected of causing cancer.
  • Despite these risks, there is a greater risk if you don’t use chlorine such as diseases like cholera and typhoid.
17
Q

Describe and explain the trend in reactivity as you go down group 2.

A
  • The reactivity increases as you progress down the group.
  • This is due to the ionisation energy required decreasing.
  • This is because the attraction between the nucleus and outer electrons decreases as a result of increasing atomic radius and increasing shielding.
18
Q

If magnesium is added to oxygen what element is oxidised and what is reduced?

A

Magnesium is oxidised as it loses electrons.

Oxygen is reduced as it gains electrons.

19
Q

Describe the solubility of group 2 hydroxides and how you can test this.

A
  • Group 2 elements react with water to produce OH^_ ions forming an alkaline solution of the metal hydroxide…
    CaO(s) + H2O(l) → Ca^2+(aq) + 2OH^-(aq)
  • Group 2 hydroxides are only slightly soluble in water, so once saturated further metal and hydroxide ions form a solid precipitate…
    Ca^2+ + OH^- → Ca(OH)2(s)
  • The solubility increases as you go down the group as the resulting solution contains more OH^- ions making them more alkaline. This can be tested by fully saturating a solution until a white solid forms, then testing the pH.
20
Q

Describe the use of a group 2 compound in agriculture.

A

Ca(OH)2 is added to fields as lime by farmers to increase the pH of acidic soils. It neutralises the acid in the soil forming water.
Ca(OH)2 + 2H^+ → Ca^2+ + 2H2O

21
Q

Describe the use of a group 2 compound in medicine.

A

Group 2 bases are used as antiacids for treating indigestion. Magnesium and calcium carbonated are often used. The acid in the stomach is HCl, which is then neutralised.

22
Q

Describe the trend in reactivity for the halogens as you go down the group.

A
  • The atomic radius increases.
  • More inner shells so the shielding increases.
  • Less nuclear attraction to capture an electron from other species.
  • Reactivity decreases.
23
Q

Describe the trend in boiling point of the halogen group.

A

As you go down the group…

  • The e^- present increases.
  • The strength of the London forces increase.
  • More energy is required to break the intermolecular forces.
  • The boiling point increases.

A Level Chemistry (12 decks)

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