The Alkali Metals
6 marks · NormalRubidium (Rb) and caesium (Cs) are Group 1 alkali metals. Their physical and chemical properties follow group trends seen in other Group 1 metals such as lithium, sodium and potassium.
State two physical properties that rubidium and caesium have in common with lithium, sodium, and potassium.
Rubidium and caesium both react vigorously with water.
Write a balanced chemical equation for the reaction of caesium with water.
Describe what you would observe when a small piece of caesium is added to water. Give two observations.
Explain, in terms of electronic structure, why caesium reacts more violently with water than potassium.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Softness and good electrical conductivity (other suitable physical properties may be accepted) · must show soft texture, good electrical conductivity (or similar e.g. low density, shiny/silvery appearance)
(b) · must show correct reactants, correct products, balanced equation
(c) Caesium bursts into flame/explodes; caesium moves rapidly/fizzes vigorously/disappears very quickly (any two valid observations) · must show vigorous or explosive reaction, rapid movement/fizzing/disappearance
(d) Caesium has more electron shells than potassium, so its outer electron is further from the nucleus and less strongly attracted, making it easier to lose and increasing its reactivity. · must show more shells in Cs, outer electron further away, less nuclear attraction, easier to lose electron
Common mistakes: Incorrect formula or balancing in equation.; Physical properties not unique to Group 1.; Confusing observations of sodium/potassium with caesium.; Reactivity trend not linked to electron distance.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States 1 valid physical property of Group 1 metals common to rubidium and caesium.
B2 (a) · 1 mark · States a second valid physical property of Group 1 metals common to rubidium and caesium.
B3 (b) · 1 mark · Writes a correct balanced equation for caesium with water.
B4 (c) · 1 mark · Gives 1 valid observation for caesium reacting with water (e.g. explosive, bursts into flame).
B5 (c) · 1 mark · Gives a second valid observation (e.g. rapid movement, fizzing, rapid disappearance).
B6 (d) · 1 mark · Explains why caesium is more reactive than potassium using electronic structure (distance of outer electron, weaker attraction, easier to lose).
Worked solution
a) Two physical properties common to rubidium, caesium, lithium, sodium and potassium are:
- They are all soft and can be cut easily with a knife.
- They are good conductors of electricity.
Other acceptable answers: low density (for lighter alkali metals), shiny when freshly cut.
b) Balanced equation for caesium with water:
c) Two observations when caesium is added to water:
- Caesium reacts explosively/bursts into flame.
- Caesium moves around rapidly on the surface, fizzes vigorously, and disappears very quickly.
d) Explanation:
Caesium atoms have more electron shells than potassium atoms, so the outer electron in caesium is further from the nucleus. This means the attraction between the nucleus and the outer electron is weaker in caesium, so the outer electron is lost more easily, making caesium more reactive than potassium.