Reactivity Trends

Question 1

What is a reducing agent?

Answer 1

A reagent that (adds electron to) another species.

A species that donates it’s electrons to another species, thereby reducing that species.

Question 2

Which group are reducing agents commonly found?

Answer 2

Group 2

Question 3

What is the trend in reactivity down group 2?

Answer 3

Reactivity increases.

Question 4

Why is it that group 2 are referred to as such good reducing agents?

Answer 4

All elements in Group2 (alkali earth metals) have two electrons in their outermost shell, in the s sub-shell (s2 electron configuration).

Redox reactions are the most common reaction of the Group 2 elements.

Each metal atom is oxidised, losing its two valence electrons to form a 2+ion with a noble gas electron configuration.
Ca →Ca2+ + 2e-
The calcium changes from an electron configuration of[Ar]4s to [Ar]

Another species will gain the two electrons lost. This species will therefore be reduced.
The Group 2 element can be referred to as a reducing agent as it has reduced another species. Its also usually the thing that’s been oxidised.

Question 5

Explain the trend of reactivity down group 2

Answer 5

The reactivity of Group 2 elements increases down the group.
As the atomic radii increases there is more shielding

The nuclear attraction therefore decreases and it is easier to remove outer electrons.

Cations can form more easily.

Question 6

What is the product of a group 2 element reacting with oxygen?

Answer 6

Metal oxides, with the general formula MO, made up of M2+ and O2- ions.

Question 7

Name an example of a group 2 REDOX reaction with oxygen, and describe what you would see?

Answer 7

2Mg(s) + 02(g) ———>2MgO(s)

The reaction of magnesium with oxygen in the air burns with a brilliant white and forms a solid white magnesium oxide.

The magnesium has been oxidised, as the oxidisation number has gone from 0 to 2.

The oxygen has been reduced, as the oxidization number has decreased from 0 to -2.

Question 8

Possible sources of error with magnesium experiment?

Answer 8

MgO is a white solid with a high melting point due to its ionic bonding.

Mg will also react slowly with oxygen without a flame.
Mg ribbon will often have a thin layer of magnesium oxide on it
formed by reaction with oxygen.

2Mg + O2  2MgO

This needs to be cleaned off by emery paper before doing
reactions with Mg ribbon.

If testing for reaction rates with Mg and acid, an un-cleaned Mg
ribbon would give a false result because both the Mg and MgO
would react but at different rates.
Mg + 2HCl  MgCl2 + H2
MgO + 2HCl  MgCl2 + H2O

Question 9

What are the products of a group 2 element reacting with water?

Answer 9

Metal hydroxide and hydrogen

Question 10

Give an example of a group 2 REDOX reaction with water, and describe what you would see?

Answer 10

The group 2 elements react with water to form an alkaline hydroxide, with the general formula M(OH)2.

Water and magnesium react very slowly but very fast when the water is steam. However the reaction becomes more vigorous as you go down the group. Remember the reactivity of group 2 metals increases down the group.

The reaction of all group2metals with water follows the following general equation:
M (s)+ 2H O (l)→ M(OH) (s)+ H (g)

We can again see this is a redox reaction by taking a look at changes in the oxidation number of each element in the reaction

M =0 →+2so the metal has been oxidised
H =+1→0 so the hydrogen has been reduced
No change in oxidation number for oxygen
The solubility of the hydroxides increases down the group (well see why later).An example of this reaction would be magnesium with water:

Mg will also react with warm water, giving magnesium
hydroxide as the product. In the photo on the right
magnesium powder is used and there was a slow fizzing. This is a much slower reaction than the reaction with steam and there is no flame

Question 11

When a group 2 metal reacts with water, will the solution become more acidic or alkaline, and what would you observe?

Answer 11

The hydroxides produced make the water alkaline.

One would observe:
fizzing, (more vigorous down group)
the metal dissolving, (faster down group)
the solution heating up (more down group)
and with calcium a white precipitate appearing (less precipitate forms down group).

Question 12

What is the product of a group 2 element reacting with a dilute acid?

Answer 12

Metal salt and hydrogen gas.

Question 13

Give an example of a group 2 REDOX reaction with dilute acids, and describe what you would see?

Both hydrochloric and sulfuric acid.

Answer 13

The Group 2 metals will react with dilute acids to form metal salts
For example, they will form metal chlorides if reacted with hydrochloric acid, HCl

When metals react with an acid, the by-product of this reaction is hydrogen gas
The reaction of the metals with dilute HCl follows the following general equation:
M (s)+ 2HCl(aq)→MCl2 (aq)+ H2(g)

We can again see this is a redox reaction by taking a look at changes in the oxidation number of each element in the reaction
M =0 →+2so the metal has been oxidised
H =+1→0 so the hydrogen has been reduced
No change in oxidation number of chlorine

The reaction ofthe metals with dilute sulfuric acid,H2SO4 ,follows the following general equation:
M (s)+ H2SO4 (aq)→ MSO4 (aq)+ H2 (g)

When some of Group 2 metals react with sulfuric acid rather than hydrochloric, an insoluble sulfate forms.
Going down the group, the Group 2 sulfates become less and less soluble.
Calcium sulfate is sparingly soluble, but strontium sulfate and barium sulfate are insoluble. We ll see whys soon.

Question 14

What is the trend in reactivity and ionisation energy down the group 2 metals?

Answer 14

We can think about the increasing reactivity moving down Group 2i n terms of ionisation energies.
The atoms of Group 2 elements react by losing electrons to form +2ions.
The formation of +2ions from gaseous atoms requires two ionisation energies
M(g)→M+ (g)+ e-
M+ (g)→ M2+ (g)+ e-
Both the first and second ionisation energies decrease down the group as:
The attraction between the nucleus and outer electrons decreases

Atomic radius increases, and there is increased shielding

Despite other energy changes taking place when Group 2 elements react ,the first and second ionisation energies make up most of the energy input.

Therefore, as the total energy input from ionisation energies to form 2+ions decreases down the group, the elements become more reactive and stronger reducing agents .As its easier to lose that electron as we move down the group.

Question 15

What is the trend of solubility down group 2?

Answer 15

Increases

Question 16

Explain the trend of solubility down group 2

Answer 16

As you go down the group, the solution has a greater concentration of OH-. Hence solubility increases

Question 17

Given that group 2 oxides, hydroxides and carbonates neutralise acids, what can they be referred to as?

Answer 17

Bases

Question 18

Given that most group 2 oxides, hydroxides and carbonates are soluble in water, what are they also besides bases?

Answer 18

Alkalis

Question 19

What are the products when group 2 OXIDES react with water?

Answer 19

Group 2 oxides react with water, releasing hydroxide ions, to form alkaline solutions:
CaO (s)+ H O (l)→Ca2+ (aq)+ 2OH- (aq)

These solutions get more alkaline going down the group.

This is due to increasing solubility meaning the resulting solutions formed contain more OH ions. Essentially we get more hydroxide ions down the group, so more soluble.

When the solution becomes saturated any further metal and hydroxide ions will form a precipitate:
Ca2+ (aq)+ 2OH- (aq)→ Ca(OH) (s)

Question 20

Why do the group 2 hydroxides become more soluble down the group?

Answer 20

The solubility of the hydroxides increases down the group, so the resulting solutions contain more OH- ions and the resulting solutions are therefore more alkaline.

Solid magnesium hydroxide, Mg(OH) (s), is only slightly soluble in water
The solution has a low OH- (aq) concentration and a pH ≅10
Barium hydroxide ,Ba(OH) (s), is much more soluble in water
The solution has a greater OH (aq) concentration and a pH ≅13

A suspension of magnesium hydroxide in water will appear
slightly alkaline (pH 9) so some hydroxide ions must
therefore have been produced by a very slight dissolving.

Calcium hydroxide is reasonably soluble in
water. It is used in agriculture to neutralise acidic soils.
An aqueous solution of calcium hydroxide is
called lime water and can be used a test for
carbon dioxide. The limewater turns cloudy
as white calcium carbonate is produce

Barium hydroxide would easily dissolve in
water. The hydroxide ions present would make the solution strongly alkaline.

Question 21

What are the 3 key things that happen to the group 2 metal hydroxides as you go down the group?

Answer 21

1.Solubility increases
2.pH increases
3.Alkalinity increases

Question 22

Describe a practical you could do to show that the solubility of the hydroxides increases down the group?

Answer 22

1.Add a spatula of each group 2 oxide to water in a test tube

2.Shake the mixture. On this scale, there’s insufficient water to dissolve all of the metal hydroxide that forms. You will have a saturated solution of each metal hydroxide with some white solid undissolved at the bottom of the test tube.

3.Measure the pH of each solution. The alkalinity will be seen to increase going down the group.The fact that the pH of the solution increases going down the group shows there’s more OH- ions in each solution, so the hydroxides increase in solubility, going down the group.

Question 23

What happens to the solubility of the sulfates going down the group?

Answer 23

Group II sulphates become less soluble down the group. BaSO4
is the least soluble. All group 2 sulfates when not soluble appear, as white precipitates.

Question 24

Write an equation for the formation of a group 2 precipitate?

Answer 24

An equation for the formation of the precipitate can be written as a full equation or simplest ionic equation:

Full equation : SrCl2(aq) + Na2SO4 (aq)  2NaCl (aq) + SrSO4 (s)
Ionic equation: Sr2+(aq) + SO42-(aq)  SrSO4(s).

Question 25

What are the key uses of group two compounds?

Answer 25

ca(OH)2 - calcium hydroxide- is added to fields as lime to increase the pH of acidic soils. The calcium hydroxide neutralises the acid in the soil, forming a neutral water.

Ca(OH) (s)+ 2H+ (aq)→ Ca2+ (aq)+ 2H2O (l)

Group 2 bases are often used as antacids for treating acid indigestion. Many indigestion tablets use magnesium and calcium carbonates as the main ingredients, whilst milk of magnesia is a suspension of white magnesium hydroxide in water Mg(OH)2.

The acid in your stomach is mostly HCL and below you can see the neutralisation of Mg(OH)2 with HCL.

Magnesium hydroxide, Mg(OH)2 , is partially soluble in water and is
used in suspension (known as ‘milk of magnesia’) to neutralise excess acid in the stomach and treat constipation.
Mg(OH)2 (s)+ 2HCl(aq)→MgCl2 (aq)+ 2H2O (l)
It is safe to use as the magnesium hydroxide is only partially soluble making the solution only
slightly alkaline (pH ≅10) due to the low OH- concentration

Barium sulfate (BaSO4) is used in medicine as a ‘Barium meal’ given to patients who need x-rays of their intestines. The Barium absorbs the x-rays and so the gut shows up on the x-ray image. Even though barium compounds are toxic, barium sulfate is safe to use here because of its low solubility so does not get absorbed into the body.

Question 26

What physical states are the halogens in?

Answer 26

At RTP, all the halogens exist as diatomic molecules, X2.The group contains elements in all three physical states at RTP, changing from gas to liquid to solid , going down the group.

In their solid states, the halogens form lattices with simple molecular structures.

Question 27

What is the trend in boiling point of the halogens going down the group?

Answer 27

Increase down the group

As the molecules become larger they have more electrons and so have larger London forces between the molecules. As the intermolecular forces get larger, more energy has to be put into
overcoming the forces. This increases the melting and boiling points.

Question 28

What is electronegativity of the halogens going down the group?

Answer 28

Electronegativity is a measure of the tendency of an atom to attract the shared pair of electrons, in a covalent bond.

As one goes down the group the electronegativity of the elements decreases.
As one goes down the group the atomic radii increases due to the increasing number of shells. The nucleus is therefore less able to attract the bonding pair of electrons.

Question 29

Appearance of the halogens at RTP?

Answer 29

fLOURINE - pale yellow gas
CHLORINE - pale green gas
BROMINE - brown liquid
IODINE- shiny grey black solid
ASTATINE- never seen before

Question 30

Halogen REDOX reactions?

Answer 30

Each halogen has 7 electrons in their outer shell with two electrons in
the outer s sub-shell and 5 in
the outer sub-shell __s2 __p5
Halogens react with metals by accepting an electron from the metal atom to become an ion with 1-charge For example:
Ca (s)+ Cl2(g)→ CaCl2 (s) consisting of Ca and 2Cl ions
Halogens are therefore oxidising agents:
Halogens oxidise the metal by removing an electron from the metal (the oxidation number of the metal increases).
Halogens become reduced as they gain an extra electron from the metal atom (the oxidation number of the halogen decreases).

Another species loses electrons to halogen atoms - its oxidised. The halogen is called an oxidising agent because it has oxidised another species.

Question 31

What’s a halogen displacement reaction?