NDA (I) 2017 General Ability Test · Question 59 of 149

Correct — A, Lithium. All four options are alkali metals from Group 1, and every one of them reacts with water to give a hydroxide and hydrogen gas. What separates them is vigour, and that increases steadily as you go down the group. Lithium is at the top and is the gentlest — it reacts with cold water at a steady, visible pace, fizzing quietly and staying solid throughout. Sodium reacts faster, and the heat melts it into a moving ball. Potassium goes further and the hydrogen it releases catches fire with a lilac flame. Caesium, at the bottom of the group, reacts explosively even with cold water. Going down the group the atoms get larger and the outermost electron is held less tightly, so it is lost more easily and the reaction gets fiercer, leaving lithium as the least reactive of the four.

• (b)Sodium — Sodium is more reactive than lithium, not less. It reacts briskly enough with cold water for the heat of the reaction to melt the metal into a ball that skims over the surface — which is why sodium is kept under kerosene in the laboratory.
• (c)Potassium — Potassium lies below sodium and reacts still more violently. The hydrogen it liberates usually ignites at once, burning with the lilac flame typical of potassium, so it is far from being the least reactive.
• (d)Cesium — Caesium is the lowest of the four in Group 1 and the most reactive of them, reacting explosively with water. Its outermost electron is the most loosely held, which is exactly the opposite of what the question asks for.

The alkali metals each carry a single electron in their outermost shell and react by giving it away. Down the group the atoms grow larger, the outer electron is further from the nucleus and more shielded from it, and the ionisation enthalpy falls, so the electron is surrendered more readily and the reactions with water and with air become more violent. Lithium is additionally the hardest and highest-melting of the group, so it does not melt during the reaction and never exposes a large fresh surface the way molten sodium does.

There is a genuine subtlety here that rewards a careful student. Lithium has the most negative standard electrode potential of all the alkali metals, about minus three volts, which suggests it should be the most eager to react — and in the thermodynamic sense it is, because the very small lithium ion releases a great deal of energy on being surrounded by water molecules. What the exam asks about is the observed speed of the reaction, and that is governed by kinetics, where lithium's high melting point and its slow-forming surface layer keep it the gentlest of the four. Remember the trend as reactivity increasing down the group, and treat the electrode-potential fact as the footnote it is. One small printer's point about this paper — option (d) is set as 'Cesium', the American spelling, in a paper that otherwise uses British spellings; the element is the caesium of the periodic table and the spelling makes no difference to the answer.

• Reactivity of the alkali metals with water increases down Group 1, from lithium to caesium.
• Lithium reacts steadily with cold water without melting or catching fire; sodium melts into a ball; potassium ignites; caesium reacts explosively.
• Every alkali metal reacting with water gives the metal hydroxide together with hydrogen gas.
• Down the group the atomic size increases and the ionisation enthalpy falls, so the single outer electron is lost more easily.
• Sodium and potassium are stored under kerosene to keep air and moisture away from them.

• Reversing the trend and assuming reactivity falls down the group as it does for the halogens.
• Quoting lithium's very negative electrode potential as proof that it reacts fastest — the observed order with water runs the other way.
• Forgetting that all four give hydrogen gas, so the difference is one of vigour, not of product.

NDA asks for the most or least reactive metal in a set, or for a reactivity series to be put in order, so fix the Group 1 trend and the common series in memory.

• practice — not a real PYQ

  Which one of the following alkali metals reacts most violently with cold water?

  • (a)Lithium
  • (b)Sodium
  • (c)Potassium
  • (d)Caesium

  Answer(d) Caesium — reactivity increases down Group 1, so the lowest member of the four reacts most violently.

• practice — not a real PYQ

  When an alkali metal reacts with water, the gas evolved is

  • (a)oxygen
  • (b)hydrogen
  • (c)carbon dioxide
  • (d)nitrogen

  Answer(b) hydrogen — the metal displaces hydrogen from water, leaving the metal hydroxide in solution.