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AS Chemistry - mod. 2 and 3 and 4 > Chapter 8 Reactivity Trends > Flashcards

Chapter 8 Reactivity Trends Flashcards

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1
Q

What are the group 2 elements?

A

Alkaline earth metals

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2
Q

What is the most common type of reaction of group 2 elements and explain? (Oxygen, water

A

REDOX reactions
- each metal atom is OXIDISED by losing 2 electrons = 2+ ion
- the other species is REDUCED by gaining these 2 electrons
- the group 2 metal = REDUCING AGENT

W/ OXYGEN
- group 2 elements all react with oxygen forming a metal oxide = M2+ O2-

W/ WATER
- group 2 elements react with water forming an ALKALINE (metal) HYDROXIDE and hydrogen gas = M(OH)2 + H (gas)
- not ALL hydrogen atoms are reduced

W/ DILUTE ACIDS
- group 2 metal + (dilute) acid –> salt + H2 (g)
- again, reactivity INCREASES going down the group.

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3
Q

What is the reactivity trend DOWN group 2 and EXPLAIN WHY?

A

Reactivity INCREASES as you go down group 2.

  • the atoms of group react by LOSING 2 electrons to form 2+ ions.
  • the formation of 2+ ions requires the input of 2 ionisation energies.

The IONISATION ENERGIES decrease down the group because the attraction between the nucleus and the outer electrons DECREASE due to…
- increasing atomic radius
- increased shielding

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4
Q

Reaction of group 2 OXIDES with water?

A

W/ WATER
- releases hydroxide ions (OH-) = forming alkaline solutions

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5
Q

Group 2 HYDROXIDES??

A

Only slightly soluble in water. When the solution becomes saturates, any further metal and hydroxide ions = SOLID PRECIPITATE.

The solubility of group 2 metal hydroxides in water INCREASES as you go down the group.
- the resulting solution become more alkaline as they contain more OH- ions.

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6
Q

How can you demonstrate the trend in solubility of group 2 hydroxides?

A

Experiment…

1) add a spatula of each group 2 OXIDE to water in test tube.

2) shake the mixture

3) measure the pH of each test tube solution

  • expected: the alkalinity increases as you go down - pH increases.
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7
Q

Uses of group 2 compounds??

A

As BASES.
- the group 2 oxides, hydroxides, and carbonates have many uses related to their basic properties and ability to neutralise acids.

AGRICULTURE
- Ca(OH)2 - calcium hydroxide is added to fields as lime to INCREASE the pH of acidic soils.
- white lime powder

  • Ca(OH)2 neutralises acid in the soil - forming water.

Ca(OH)2 + 2H+ –> Ca2+ + 2H2O

in MEDICINE
- group 2 bases are often used as antacids for treating acid indigestion.
- tablets of magnesium/ calcium carbonates

Mg(OH)2 + 2HCl –> MgCl2 + 2H2O
CaCO3 +2HCL –> CaCl2 + H2O + CO2

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8
Q

What are the group 7 elements?

A

The HALOGENS

  • most reactive non-metal group
  • occur as stable halide ions on Earth
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9
Q

What is the trend in boiling points going down group 7 and EXPLAIN WHY?

A

Boiling point INCREASES as you go down group 7.

  • more electrons
  • stronger LONDON forces
  • more energy required to break the intermolecular forces
    = increased boiling points
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10
Q

Appearance and state of group 7 elements at RTP?

A

All exists as DIATOMIC MOLECULES
**(x) = boiling points

F = pale yellow gas (-188)

Cl = pale green gas (-34)

Br = red-brown liquid (59)

I = shiny grey-black solid (184)

At = never been seen !!! (230)

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11
Q

REDOX reactions of halogens?? (General)

A
  • each halogen atom is REDUCED by gaining 1 electron = 1- HALIDE ion
  • the other species is OXIDISED by losing 1 electron = 1+ ion
  • the halogen = OXIDISING AGENT
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12
Q

What do the results of halogen-halide displacement reactions show?

A

show DECREASING REACTIVITY of halogens as you go down the group.

Chlorine clearly reacts with Br- and I-
Bromine reacts with I- ONLY
Iodine reacts with neither

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13
Q

Explain halogen-halide displacement reactions.

A

A solution of each halogen is added to aqueous solution of the other halides.
- e.g. solution of Chlorine is added to 2 aqueous solutions of Bromide ions and Iodide ions

IF the halogen added is MORE reactive than the HALIDES present…
- halogen DISPLACES the halide from solution
- solution changes colour

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14
Q

Why would you add cyclohexane to halide-halogen displacement reactions?

A

Non polar halogens dissolve more readily in cyclohexane than water.

Helps tell apart colours easily.

before cyclohexane…
Cl2 - pale green solution
Br2 - orange solution
I2 - brown solution
- bromine and iodine in water can appear a similar orange-brown, depending on the conc.

after cyclohexane is added…
Iodine - BROWN —> VIOLET

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15
Q

Results of halogen-halide displacement reactions??

A

chlorine + chloride ions
x

bromine + chloride ions
- no reaction
- shows that chlorine is more reactive than bromine as bromine doesnt displace the chlorine ions from solution

iodine + chloride ions
- no reaction

chlorine + bromide ions
- orange colour from the formation of Br2
- Cl2 + 2Br- –> 2Cl- + Br2
- chlorine displaces bromide ions from solution
- chlorine is more reactive

bromine + bromide ions
x

iodine + bromide ions
- no reaction

chlorine + iodide ions
- violet colour from iodine formation
- Cl2 + 2I- –> Cl2- + I2
- chlorine is more reactive than iodine as it displaces iodide ions from solution

bromine + iodide ions
- orange colour
- bromine formation
- bromine is more reactive than iodine

iodine + iodide ions
x

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16
Q

Fluorine and Astatine??

A
  • fluorine is a pale yellow gas and reacts with almost any substance that it comes in contact w/
    (most reactive - strongest oxidising agent)
  • astatine is very rare as it is radioactive and decays rapidly
    (predicted to be the least reactive - weakest oxidising agent)
17
Q

What is the trend in reactivity going DOWN group 7?

A

Reactivity DECREASES as you go down the group as the tendency to gain an electron decreases.

  • atomic radius increases
  • more inner shells = more shielding
  • less nuclear attraction (to capture an electron from another species)
  • reactivity decreases
18
Q

What is DISPROPORTIONATION?

A

A redox reaction in which the same element is both OXIDISED and REDUCED.

  • e.g. when chlorine is added to water, a disproportionation reaction takes place…
  • for each chlorine molecule, one chlorine atom is oxidised and the other chlorine atom is reduced.
  • Cl2 + H2O –> HClO + HCl
    (chloric acid - weak bleach)
  • add indicator solution to solution of chlorine in water
  • indicator turns red (acids) - then beaches (chloric acid)
19
Q

How do you test for halide ions?

A

= PRECIPITATION reactions with aqueous silver ions

Aqueous halide ions react with aqueous silver ions to form precipitates of SILVER HALIDES.

  • e.g. Ag + (aq) + X- (aq) —> AgX (s) (silver halide prec.)

(X-) = aqueous solution of ANY halide

20
Q

What is qualitative analysis?

A

Relies on simple observations rather than measurements (quantitive analysis)
- can often be carried out quickly on test-tube scale

= gas bubbles
= precipitates
= colour changes
= identification of gases

21
Q

How do you test for ANIONS?

A

(1) Carbonate test (based on gases)

(2) Sulfate test (based on precipitates)

(3) Halide tests (based on precipitates)

22
Q

(1) Carbonate test method and results??

A

Carbonates react with acids to form CO2 gas.
- this forms the basis for a test for the carbonate ion, CO3 2-

1) add dilute nitric acid to the unknown compound (test tube)

2) if you see bubbles forming, possibly could be a carbonate

3) use lime water test to see if the gas produced (bubbles) is carbon dioxide
- if it is carbon dioxide, a fine white precipitate is formed turning the lime water cloudy

= carbonate ion PRESENT

(test based on gases)

23
Q

(2) Sulfate test method and results??

A

Most sulfates are soluble in water, but barium sulfate (BaSO4) is VERY INSOLUBLE.
- the formation of a white precipitate of barium sulfate is the basis of the sulfate test

Test = adding aqueous barium ions to the solution of an unknown compound
- barium chloride
- barium nitrate (if you are carrying out a halide test afterwards)

If white precipitate is formed
= sulfate ions PRESENT

(test based on precipitates)

24
Q

(3) Halide tests method and results??

A

Most halides are soluble in water, but silver halides are insoluble.
- aqueous silver ions react with aqueous halide ions to form precipitates of silver halides.

Ag+ (aq) + X- (aq) —> AgX (s)

1) add aqueous silver nitrate, AgNO3 to aqueous solution of halide.

2) silver chloride = white
2) silver bromide = cream
2) silver iodide = yellow

3) add aqueous ammonia to test the solubility of the precipitate.
- three precipitate colours can be difficult to distinguish between

silver halide formed = halide ions present

25
Q

What is the sequence/order of tests (for anions)?

A

1) carbonate ions (acid - CO2)
2) sulfate ions (barium sulfate)
3) halide ions (silver halide)

Important to carry out the tests in this order or you could obtain incorrect results and incorrect identifications.

26
Q

How do you test for CATIONS?

A

Test for ammonium ion (NH4 + )

When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas, NH3.

1) add sodium hydroxide to solution of an ammonium ion

2) ammonia gas produced

3) warm the mixture, ammonia gas released

4) use pH indicator paper —> blue (alkaline)

= ammonium ion PRESENT

AS Chemistry - mod. 2 and 3 and 4 (15 decks)

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