GCSE OCR Gateway C4:1 a-b Alkali metal reactions

C4.1 Predicting chemical reactions of the alkali metals

Summary

Models of how substances react and the different types of chemical reactions that can occur enable us to predict the likelihood and outcome of a chemical reaction. The current periodic table was developed based on observations of the similarities and differences in the properties of elements. The way that the periodic table is arranged into groups and periods reveals the trends and patterns in the behavior of the elements. The model of atomic structure provides an explanation for trends and patterns in the properties of elements. The arrangement of elements in groups and periods reveals the relationship between observable properties and how electrons are arranged in the atoms of each element.

Common misconceptions

Learners consider the properties of particles of elements to be the same as
the bulk properties of that element. They tend to rely on the continuous matter model rather than the particle model. Learners confuse state changes and dissolving with chemical changes. Also, since the atmosphere is invisible to the eye and learners rely on concrete, visible information, this means they therefore often avoid the role of oxygen in their explanations for open system reactions. Even if the role of oxygen is appreciated, learners do not realize that solid products of an oxidation reaction have more mass than the starting solid.

Underlying knowledge and understanding

Tiering
Statements shown in bold type will only be tested in the Higher Tier papers. 
All other statements will be assessed in both Foundation and Higher Tier papers.

Learners should be familiar with the principles underpinning the Mendeleev periodic table; the periodic table: periods and groups; metals and non-metals; the varying physical and chemical properties of different elements; the chemical properties of metals and non-metals; the chemical properties of metal and non-metal oxides with respect to acidity and how patterns in reactions can be predicted with reference to the periodic table.

C4.1a-b

To be able to recall the simple properties of Group 1 both physical and chemical properties.

To be able to explain how observed simple properties of Group 1 depend on the outer shell of electrons of the atoms and predict properties from given trends down the groups including the ease of electron gain or loss.

Group 1 elements are the alkali metals.

Most courses suggest you think about two types of chemical properties when it comes to Group 1.

Reaction with Oxygen:

First the reaction of these metals with oxygen

They burn in oxygen forming different types of oxide and revealing the metal flame colour.

Lithium burns with a crimson flame to form lithium oxide

4Li (s)    +     O₂ (g)    ⟶  2Li₂O  (s)

Sodium burns with a bright yellow/orange flame and forms both the oxide and the peroxide

4Na(s)   +    O₂ (g)     ⟶      2Na₂O (s)

2Na(s)   +    O₂ (g)        ⟶     Na₂O₂ (s)

Potassium burns with a purple/lilac flame to form the oxide and the superoxide

K(s)   +    O₂ (g)        ⟶     KO₂ (s)

4K(s)   +    O₂ (g)     ⟶      2K₂O (s)

These oxides are generally white like most Group 1 compounds unless the anion is coloured as in potassium manganate(VII)  which is purple since the manganite(VII) anion is purple.

The vigor with which these metals react with oxygen increases down the group and is associated with the increasing atomic radius.

Each row of the periodic table adds an extra electron shell.

This puts the outer single electron further from the pull of the nucleus and enhances electron shielding.

Therefore the likelihood of electron loss (oxidation) increases.

The ionization energies of the alkali metals decreases down the group for the same reason.

Reaction with water

The second chemical property usually referred to in courses to illustrate the increasing reactivity of Group 1 metals down the group is the reaction with water.

Reaction with water follows a known pattern:

Metal   +     Water    ⟶       metal hydroxide      +     hydrogen

So the reaction with lithium produces gentle fizzing as the metal flits and floats on the water surface.

Li (s)    +    H₂O (l)     ⟶      LiOH(aq)       +    ½ H₂ (g)

It is from this reaction that the name of these metals derives since the metal hydroxides are alkaline.

There are similar increasingly violent reactions to had with the other alkali metals but each one obeys the same equation.

There are several good videos of these reactions to be found on you tube here

The video cli is accurate but there are several that are more exciting but not accurate like the Brianiac version here.

Enjoy the video clips but remember which is the accurate one!!

Explanation of increasing reactivity to oxygen and water down the group

The vigor with which these metals react with oxygen or water increases down the group and is associated with the increasing atomic radius.

Each row of the periodic table adds an extra electron shell.

This puts the outer single electron further from the pull of the nucleus and enhances electron shielding.

Therefore the likelihood of electron loss (oxidation) increases.

M(s)     ⟶    M⁺ (s)     +    1 outer shell electron

The ionization energies of the alkali metals decreases down the group for the same reason.

The plot shows the energies for the oxidation process:

M(g)    ⟶      M⁺ (g)      +       e^—

IE stands for ionization energy.