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Chemistry > Chapter 8: reactivity trends > Flashcards

Chapter 8: reactivity trends Flashcards

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1
Q

What are the key properties of group 2 elements?

A

The elements in group 2 of the periodic brake are normally metals called the alkaline earth metals (from the alkaline properties of their metal hydroxides). Do not occur naturally on earth but instead as stable compounds e.g. CaCO3.

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2
Q

How do group 2 elements act as reducing agents?

A

Each group 2 elements have to outer electrons in redox reaction each group 2 element is oxidised loosing their outer 2 electrons and forming 2+ ions. The other species gaining these electrons become reduced and group 2 becomes the reducing agent.

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3
Q

What is the reaction of group 2 elements and oxygen?

A

Group 2 metals react with oxygen in a redox reaction to form metal ions (MO).
E.g. 2Mg (s) + O2 (g) —> 2MgO (s)
Mg oxidised
O reduced

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4
Q

What is the reaction between group 2 elements and water?

A

Group 2 react with water forming
- alkaline hydroxide M(OH)2
- hydrogen gas (H2)
E.g. Sr (s) + 2H2O (l) —> Sr(OH)2 (aq) + H2 (g)
Sr oxidised
H reduced

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5
Q

What is the reaction between group 2 and dilute acids?

A

Groups 2 react with dilute acid ( MASH) forming
- salt
- hydrogen
E.g. Mg(s) + 2HCl (aq) —> MgCl2 (aq) + H2 (g)
Mg is oxidised
H reduced

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6
Q

What is the trend in reactivity of group 2 elements and why?

A

Reactivity increases down group 2. This is because ionisation energy decreases down the group as
- increasing atomic radius
- increasing electron shielding
- decrease in nuclear attraction
This all mean that the ease of removing the outer 2 electron is easier travelling down the group.

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7
Q

Equations for first and second ionisation energy of a group 2 element?

A

M (g) —> M+ (g) + e-
M+ (g) —> M2+ (g) + e-

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8
Q

What reactions happens when group 2 oxides react with water?

A

When group 2 oxides react with water the form
- alkaline solutions of hydroxide ions
CaO (s) + H2O (l) —> Ca+ (aq) + 2OH- (aq)
Group 2 hydroxides are only slightly soluble in water so when the solution becomes saturated a solid precipitation will form
Ca+ (aq) + 2OH-(aq) —> Ca(OH)2 (s)

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9
Q

What is the tend of the solubility of group 2 hydroxides down the group?

A

The solubility of the metal hydroxides increase down the group, realising more OH- ion and increasing the alkalinity of the solution.

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10
Q

How can you use group 2 compounds in agriculture?

A

Calcium hydroxide Ca(OH)2, is added to fields as lime by farmers, increasing the pH of acidic soils.
Ca(OH)2 (s) + 2H+ (aq) —> Ca2+ (aq) +2H2O (l)

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11
Q

How can group 2 compounds be used in medicine?

A

They can often be used as antiacids fro treating acid indigestion.
Mg(OH)2 (s) + 2HCl (aq) —> MgCl2 (aq) + 2H2O (l)
CaCO3 (s) + 2HCl (aq) —> CaCl2 (aq) + H20 (l) + CO2 (g)

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12
Q

What are the key properties of group 7?

A

Group 7 are known has the halogens and are the most reactive non-metal group. They do don’t occur in their elements but as stable halide ions (Cl-, Br-, I-) dissolved in sea water of as sold deposits (NaCl)

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13
Q

What are the trend in boiling lint down group 7 and why?

A

At RTP all halogens exist as diatomic molecules. Down group 7 the boiling point decreases as
- more electrons present
- stronger London forces
- more energy require to break the London forces

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14
Q

Why is halogens oxidising agents?

A

Each halogen has 7 electrons on its outer shell, they take place in redox reactions gaining a singular electrons and reduced to form a 1- ion.
Cl2 + 2e- —> 2Cl-
Another specie has to loose an electron and there’re is oxidised making the halogen an oxidising agent.

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15
Q

What is the halogen - halide displacement reaction?

A

A solution is each halogen (Cl, Br, I) is added to aqueous solution of the other two halides (Cl-, Br-, I-). A reaction may take place where the halogen displaces the halide changing the colour of the solutions)

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16
Q

Results of the halogen - halide displacement reaction?

A

Cl- halide is not displaces by either the Br2 or I2 meaning there is no change of colour (stays pale green)
Br- halide is displaced by only the Cl2 changing the solution colour to orange.
Cl2 (aq) + 2Br- (aq) —> 2Cl- (aq) + Br2 (aq)
I- is displaced by both Cl2 and Br2 changing the colour of the solution to a violet colour form the formations of I2
Cl2 (aq) + 2I- (aq) —> 2Cl- (aq) + I2 (aq)
Br2 (aq) + 2I- (aq) —> 2Br-(aq) + I2 (aq)

17
Q

How to tell the colours of halogen part in a solution?

A

An organised non-polar solvent such as cyclohexane can be added and shaken. The non-polar halogen will dissolve more readily in cyclohexane than water making colours more visible.

18
Q

What is the trends in reactivity down group 7?

A

Down group 7 reactivity decreases as
- atomic radius increases
- more electron shielding
- less nuclear attraction to capture an electron from another species.

19
Q

What is meant by the term disproptionation?

A

A redox reaction in which the same element is both oxidised and reduced.
E.g reaction with chlorine and water and chlorine and dilute sodium hydroxide.

20
Q

What reaction takes place when chlorine reacts wiht water?

A

When chlorine is added to water a disproportionation reaction take place forming
- Chloric acid (HClO)
- hydrochloride acid (HCl)

Cl2 (aq) + H2O (l) —> HClO (aq) + HCl (aq)
Cl is both reduced and oxidised

21
Q

What reaction takes place when chlorine reacts with cold, dilute aqueous sodium hydroxide?

A

In aqueous sodium hydroxide chlorine dissolves and another disproportionation reaction takes place forming
- sodium chlorate NaClO
- sodium chloride (NaCl)
- water (H2O)

Cl2 (aq) + 2NaOH (aq) —> NaClO (aq) + NaCl (aq) + H2O (l)
Cl is both reduced and oxidised

22
Q

What are the benefits of chlorine use?

A

Chlorine is extremely beneficial in killing bacteria in water and creating portable water. This reduces the spread of water borne diseases such as typhoid and cholera.

23
Q

What are the risks of chlorine use?

A
  • chlorine is an extremely toxic gas and can become a irritant in small concentration and fatal in large concentrations
  • in water it can react with organic hydrocarbons (form decaying vegetation) forming chlorinated hydrocarbons which are suspected of causing cancer.
24
Q

How do you test for halide ions?

A

Aqueous halide ion react with aqueous silver ions forming a solid precipitate
Ag+ (aq) + X- (aq) —> AgX (s)

25
Q

How do you test for carbonates?

A

Carbonates react with acids to form carbon dioxide gas.
1. Add dilute nitric acid to solid/solution
2. If bubble are produced the unknown could be a carbonate
To prove the gas is CO2 bubble it thought lime water, it will go cloudy.
Na2CO3 (aq) —> 2HNO3 (aq) —> 2NaNO3 (aq) + CO2 (g) + H2O (l)

26
Q

How do you test for sulfate?

A

Aqueous barium ions are added to the solution of unison compounds. BaSO4 is insoluble and so if sulphate ions are present a white precipitate will form
Ba2+ (aq) + SO4(2-) (aq) —> BaSO4 (s)

27
Q

How to test for halides method?

A
  1. Add aqueous silver nitrate AgNO3 to aqueous solution of halide
  2. A solid predicate will form with each halide giving different colours
    Chlorine - white
    Bromine - cream
    Iodine - yellow
  3. Aqueous ammonia can be used to test the solubility of predicates
    Chlorine - soluble in dilute NH3
    Bromine - soluble in concentrated NH3
    Iodide - insoluble
28
Q

What the create order to test an unknown anions and why?

A

Yes there it
1. Carbonate test (it can be carried out with out the possibility of an incorrect conclusion as neither sulfate nor halide ions produce bubble skirt dilute acids)
2. Sulfate test (a carbonate will also rash with Ba2+ forming a white predicate so carbonate test needs to be be first)
3. Halide test (both sulfate and carbonate ion will form solid precipitate so halide rest need to be carried out last)

29
Q

How do you test for ammonium ions?

A

When heated together, aqueous ammonium ions and aqueous hydroxide ions react to for ammonia gas NH3
NH4(+) (aq) + OH- (aq) —> NH3 (g) + H2O (l)

30
Q

What’s the method to test for ammonium ions?

A
  1. Aqueous sodium hydroxide (NaOH) is added to a solution of an ammonium ion
  2. Ammonia gas (unlikely to see gas bubble as ammonia is very soluble in water)
  3. Warm the mixture which releases the ammonia gas
  4. You may be able to smell the ammonia. Additionally you can get with moist pH indicator paper which will turn blue.

Chemistry (11 decks)

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