3.3.1 Discuss the similarities and differences in the chemical properties of elements in the same group.
Group 1 metals, often referred to as Alkali Metals, are elements which share similarities and differences in their chemical properties. These elements are all quite reactive, especially with air and water – to ensure that the metal doesn’t react (and in some cases, cause serious damage), alkali metals are often stored in paraffin to avoid contact with air.
The alkali metals are as below:
- Lithium (Li)
- Sodium (Na)
- Potassium (K)
- Rubidium (Rb)
- Caesium (Cs)
- Francium (Fr)
Alkali Metals with water
Alkali metals react quite vigorously with water, and this becomes even more apparent when react francium (literally impossible, as of today). Reacting lithium with water will most likely lead to a slow reaction. However, reacting caesium with water is a different story. Chances are, you’ll hear an explosion.
In the exam, you have to be able to write equations describing the reactions between alkali metals and water.
The reaction is exothermic (releases energy), a metal hydroxide and hydrogen.
A = generic alkali
Alkali reaction with water
2A (s) + 2H2O (l) → 2AOH (aq) + H2(g)
Or to put this in words:
Alkali metal + Water →Metal hydroxide + hydrogen
We can also write this reaction in its ionic form, as alkali metals form ionic compounds:
2A (s) + 2H2O (l) →2A+ (aq) + 2OH– (aq) + H2 (g)
Obviously in the exam, you will substitute “A” with the alkali specified by the question.
Group 7: Halogens
Group 7 elements are often referred to as Halogens. Halogens are elements with 7 outer electrons, so require 1 more electron in its outer-shell to achieve the electron configuration of a noble gas. Reactivity decreases down the group, and this is because the distance between the nucleus and outer-electron increases as you go down the group.
The halogens in the periodic table are:
- Fluorine (F)
- Chlorine (Cl)
- Bromine (Br)
- Iodine (I)
- Astatine (As)
Similar to that of Group 1 metals, you will have to learn how to describe the reactions between halogens with Alkalis AND halogens with halide ions.
Halogens reaction with alkalis
Again, let’s take
A = generic alkali
Ha = generic halogen
Halogen reaction with alkalis
2A (s) + Ha2→2AH2 (s)
Reason I didn’t add state symbols to the generic halogen is because we don’t know what the halogen is! If it’s chlorine, it’ll be gaseous, if it’s iodine, it’ll be solid.
The product formed here is an ionic halide. Essentially, the halogen is gaining one electron from the alkaline metal.
Reaction with halide
We often call this “Displacement reactions”
3.3.2 Discuss the changes in nature, from ionic to covalent and from basic to acidic, of the oxides across period 3.
As you go across period 3, nature of oxides changes from basic to acidic.
Here is a table to describe this change in nature:
|Oxide||Acid or base|
So as you can see, as you progress through period 3, the nature of the oxides gradually changes from basic to acidic – this is a critical learning point you should remember.
Amphoteric is likely to be a new definition as well – if a substance shows amphoteric properties, the substance posses properties of both acids and bases. More about this later!
In the exam, you have to be able to write equations describing reactions between Na2O,MgO, P4O10 and SO3 and water.
Here are the equations:
Na2O (s) + H2O (l) →2NaOH (aq)
Sodium oxide + water → Sodium hydroxide
MgO (s) + H2O (l)→ Mg(OH)2 (aq)
Magnesium oxide + water → Magnesium oxide
*It’s (OH)2 for the second reaction mainly to balance valence charges (Unit 1), and since Mg’s valency is +2, and OH is 1-, we need two OH to balance this charge.
P4O10 (s) + 6H2O (l) → 4H3PO4
Phosphorous (v) oxide + water → Phosphoric (V) acid
SO3(l) + H2O (l) → H2SO4(aq)
Sulfur trioxide + water → Sulfuric (VI) acid.
Amphoteric oxides are species which show properties of both both acids and bases – in that when it reacts with an acid, it behaves as a base, and vice-versa. The most common example, and the one that will most likely be tested is aluminum oxide.
When aluminum oxide reacts with an acid such as hydrochloric, aluminum oxide acts as a base.
Al2O3 (s) + 6HCl (aq) → 2AlCl3 (aq) + 3H2O (l)
Now, when aluminum oxide reacts with a base such as Sodium hydroxide, it acts as an acid.
Al2O3 (s) + 2NaOH (aq) + 3H2O (l) → 2NaAl(OH)4
Additionally, as you move across a period 3, there is a change from metallic to non-metallic. E.g. Sodium, Magnesium and Aluminium all have the typical properties of metallic substances (E.g. good conductors of electricity), and where elements on the right of Period 3 such as Chlorine and Sulfur are non-metals.