Group 2 and the halogens Flashcards
Describe the trends in reactivity of group 2 elements down the group
All elements in group 2 have two electrons in the sub-shell of their outer shell. When group 2 elements react, these two outer electron are lost,. By transferring these two electrons to other species group 2 metals act as reducing agents
The reactivity of metals increases as we move down group 2. When group 2 elements react they lose their outer two electrons. This requires ionisation energy.
For e.g: Both the second and first ionisation energies for calcium are less than for magnesium. This is because calcium has a greater atomic radius than magnesium so the two outer electrons in calcium are further from the nucleus compared to magnesium. Calcium atoms have one more full inner electron shell than magnesium. The full inner electron shells partially shield the outer electrons from the positive charge of the nucleus. Therefore, the outer electrons in calcium are less attracted to the nucleus than the outer electron in magnesium. Calcium takes less energy to remove outer electrons. Therefore, calcium is more reactive than magnesium.
Key idea of elements in group 2
Alkaline earth metals are reactive
Group 2 elements have two electrons in their outer shell subshell. When group 2 elements react they lose they outer two electrons to form a 2+ metal ion. The group 2 metal is oxidised as it loses electrons. These are then used to reduce another chemical as a reducing agent.
The reactivity increases moving down the group as the first and second ionisation energies decrease. This means less energy is required to lose the outer two electrons as we go down the group.
Describe how group 2 elements react with oxygen
2Mg(s) + O2(g) —> 2MgO(s)
Both magnesium and oxygen have an oxidation number of zero as they are elements
During the reaction each magnesium atom is oxidised, losing two electrons. So the oxidation number of each magnesium changes from 0-+2.
Each oxygen atom is reduced, gaining two electrons. So the oxidation number of of each oxygen changes from 0– -2
The changes of oxidation number must balance so the total change of oxidation number is +4 for the two magnesium atoms and -4 for the two oxygen atoms.
Describe how group 2 elements react with water
Ca(s) + 2H2O(l) —> Ca(OH)2 (aq) + H2(g)
Reacting a group 2 metal with water produces the alkaline metal hydroxide and hydrogen gas.
At the start, the calcium has an oxidation number of zero) as it is an element.
The hydrogens in the water each have an oxidation number of +1.
During the reaction, the calcium is oxidised losing two electrons.
The oxidation number of the calcium changes to +2 in the calcium hydroxide.
The two electrons are transferred onto two hydrogen atoms, forming the hydrogen gas. These two hydrogens are reduced and their oxidation number changes to +1 to 0
The oxidation number balance. One calciuma Tom changes to 0 to +2. The two hydrogen Tom changes from +1 to 0.
The two hydrogen atoms in the calcium hydroxide each have an oxidation number of +1. The oxidation number of the two hydrogen atoms had not changed during the reaction.
In water they had an oxidation number of +1 and in calcium hydroxide, they still have an oxidation number of +1
Describe how group 2 elements react with dilute hydrochloric acid
Mg(s) + H2SO4(aq) —> MgSO4(aq) + H2(g)
When group 2 elements react with dilute acids, we make a metal salt and hydrogen gas. (Redox reaction)
At the start the magnesium has an oxidation number of 0 as it is an element.
During the reaction, the magnesium is oxidised,losing two outer electrons. So inthe magnesium sulfate, the magnesium has an oxidation number of +2.
In sulfuric acid the hydrogens each have an oxidation number of +1. During the reaction, these hydrogens are reduced as they each receive one electron from the magnesium. So in hydrogen gas, the oxidation number of each hydrogen is now 0 as they are elements
The changes in oxidation number must balanced. The oxidation number of magnesium has changed from 0 to +2. Each of the two hydrogens have changed from +1 to 0
The sulfate ion SO4^2- does Noto change during the reaction so we don’t need to consider it
Describe the solubility of group 2 hydroxides in water
Reactivity (solubility) of group 2 elements with water increases down the group. This explain the alkalinity of group 2 hydroxides:
When group 2 hydroxides dissolve in water, they release the metal ion and two hydroxide ions. The aqueous hydroxide ions make the solution alkaline. The concentration of the aqueous hydroxide ions determines the alkalinity of the solution. High concentration of hydroxide ions result in a high pH (alkaline solution).
The lower the solubility in water the less alkaline the solution is (lower pH)
Key idea of group 2 hydroxides
A group 2 hydroxide is created by reacting a group 2 oxide with water. (Calcium oxide and water = calcium hydroxide)
As the calcium hydroxide forms, it dissolves in water to form calcium hydroxide solution. However, calcium hydroxide is only slightly soluble in water so this solution becomes saturated. As it is continued to form this no longer dissolves but instead forms a solid calcium hydroxide.
Solutions of group 2 hydroxides become more alkaline as we move down the group
What are the uses of group 2 compounds
They are used for neutralisation reaction in agriculture and in medicine (soldi calcium hydroxide, (lime) is spread on fields. This neutralizes the acids on the soil making the soil more favourable for crops.
It is also used to treat indigestion caused by excess hydrochloric acid in the stomach. A suspension of magnesium hydroxide in water is called milk of magnesia. This neutralizes hydrochloric acid to produce magnesium chloride and water. (Tablets of calcium carbonate can be eaten). This is used to neutralise hydrochloric acid producing calcium chloride, carbon dioxide and water. (We don’t use calcium hydroxide because the alkalinity of calcium hydroxide would be harmful to body issues e.g tissue lining in the throat. /
State the colors of the elements in group 7 ( at room temperature and pressure
Fluorine = pale yellow gas
Chlorine = pale green gas
Bromine = red-brown liquid
Iodine = grey-black solid
Astatine = highly radioactive and has never been observed
How does the m.p/b.p of halogens change as it moves down group 7
When the halogens are cooled, they form simple molecular lattices. The covalent bond between the two halogen atoms is strong. However, between the halogen molecules there are induced dipole-dipole interactions. (London forces) . These intermolecular forces are relatively weak and do not take much energy to break
However, the strength of London forces increases with the increased number of electrons.Therefore, London forces will be stronger on molecules which are lower down on group 7 of the periodic table.
Key idea of group 7 halogens
When halogens react they remove an electron from another species
Halogens are diatomic molecules (therefore when a redox reaction is written it is shown as one e.g 2Cl)
More reactive halogens are stronger oxidising agents (top of group 7) than less reactive halogens which are less powerful oxidising agents (bottom of group 7)
The reactivity of the halogens decreases down group 7 as they become less powerful oxidising agents
The solubility of halogens decreases as you go down the group. This is is because the number of electron increases therefore, there are more electron shells thus the atomic radius increases.
Explain why does the reactivity of the halogens decreases down group 7
When a homages reacts as an oxidising agent the halogens are stronger atom removes an electron from another species. This electron adds into the outer shell of the halogen atom, to form a full outer shell.
Why is bromine less reactive than fluorine
Bromine has a greater atomic radius than fluorine. This means that the outer electrons in bromine are further from the nucleus,
Bromine has more inner electron shells so there is greater shielding between the nucleus and the outer electrons. Therefore, there is less attraction between the nucleus and the outer electrons.
This makes it harder for a bromine atom to gain an electron from another species.Thsi makes it less reactive than fluorine and a less powerful oxidising agent.
What are the uses of chlorine
Chlorine is a widely used halogen and one of its uses is in drinking water. Small amounts of chlorine are added to drinking water to kill harmful bacteria.
By adding small amounts of of chlorine to drinking water chloric (I) acid which kills bacteria. This prevents water-borne bacteria diseases such as cholera.
Explain the reaction between chlorine and water:
Cl2(g) + H2O(l) —-> HClO(aq) + HCl(aq)
If we look at oxidation numbers we can see that this is a redox reaction:
On the left, chlorine has an oxidation number of zero as it is an element.
In hydrochloric acid, the chlorine atom has an oxidation number of zero as -1 showing that it has been reduced. However, in chloric (I) acid, the chlorine atom as an oxidation number of +1 (oxidised)
A redox reaction in which atoms of the same element are oxidised and reduced is called a disproportionation reaction
