C4
Predicting and identifying reactions and products
C4.1
Predicting chemical reactions
C4.1.1
Group 1 - the alkali metals
Learning outcomes
After studying this lesson you should be able to: • recall some physical and chemical properties of Group 1 elements • explain the reactions of Group 1 elements • predict properties from given trends.
Specification reference
C4.1a, C4.1b
density (g/dm?)
0,97
Na
Group 1 element
Density
A substance will float on water if its density is less than 1.00 g/cm?, the density of water.
What are the Group 1 elements like?
Group 1 elements are placed in the vertical (1) column on the far left of the Periodic Table. They are called the alkali metals because they react with water to form alkaline solutions. Group 1 elements have some typical properties of metals. They are shiny when freshly cut and are good conductors of electricity, but they also have some unusual properties.
The alkali metals are in the solid state at room temperature, but they are all soft enough to cut with a knife.
Lithium is the hardest and each alkali metal is softer as you go down the group.
A general pattern or direction of change is a trend.
The alkali metals show other trends in their physical properties. Going down the group: • their density increases, although sodium is denser than expected • their melting point decreases.
From Figure 3 predict and explain which of the elements in Group 1 will float on water, and which will sink.
Use Figure 4 to predict the melting point of rubidium, Rb.
What are the reactions of Group 1 elements?
The only Group 1 metals you are likely to see in school are lithium, sodium, and potassium. They react rapidly with oxygen in the air and with water, so they are stored in oil.
The alkali metals react with water to produce the metal hydroxide and hydrogen.
For example, sodium reacts with water to produce sodium hydroxide and hydrogen: 2Na(s) + 2H,O(l) → 2NaOH(aq) + H,(g)
The reactivity of alkali metals with water increases down the group:
• lithium fizzes steadily and slowly disappears • sodium melts to form a silvery ball, fizzes vigorously, and quickly disappears • potassium immediately ignites, burns with a lilac flame, and very quickly disappears.
How do you explain the trend in reactivity?
The Group 1 elements have similar chemical properties because all their atoms have one electron in their outer shell. They lose these outer electrons in reactions, leaving a complete outer shell and forming ions with a single positive charge.
This general ionic equation models the change (M stands for the symbol of the metal):
M→M* + e
The easier it is for an atom of a Group 1 element to lose its outer electron, the more reactive the elements is.
For example, potassium is more reactive than lithium because its atoms lose their outer electron more easily than lithium atoms do.
Explain how the general equation above shows why Group 1 metals are oxidised in their reactions with water and non-metals such as oxygen.
SODIUN
1 Explain the colour you expect universal indicator to be in potassium hydroxide solution.
(2 marks)
2 Caesium reacts explosively on contact with water.
a Write a balanced equation to model the reaction, including state symbols.
b Explain why rubidium is less reactive than caesium, but more reactive than potassium.
(3 marks)
Predict the physical and chemical properties of francium, and explain your answers.
(3 marks)
