1. Reducing power of alkali metals in aqueous solution (order).
+
Correct Answer
Li > Cs > Rb > K > Na
Important: Aqueous Solution Trend
The reducing power of alkali metals is determined by their tendency to
lose an electron:
M → M+ + e−
In aqueous solution, the trend is not simply the same as the
ionisation-energy trend. Hydration of the resulting M+
ion is extremely important.
Li+ has exceptionally high hydration enthalpy because
Li+ is very small. This strongly stabilises Li+ in water,
making oxidation of Li especially favourable.
Therefore, lithium is the strongest reducing agent among
the alkali metals in aqueous solution.
Li > Cs > Rb > K > Na
Why is Lithium the Strongest Reducing Agent?
Lithium has the most negative standard reduction potential:
Li+/Li ≈ −3.04 V
This means Li has a very high tendency to undergo oxidation:
Li → Li+ + e−
Although Li has a relatively high ionisation enthalpy, the extremely high
hydration enthalpy of Li+ more than compensates for this.
Very high hydration of Li+ → very stable Li+(aq)
→ strong oxidation tendency of Li
Why is the Order Unusual?
In the gaseous state, reducing tendency broadly follows the ease of electron
loss. However, in water, three major factors contribute:
Ionisation enthalpy + Sublimation enthalpy + Hydration enthalpy
For lithium, the hydration enthalpy is exceptionally large and negative.
This changes the aqueous trend and makes Li the strongest reducing agent.
JEE / NEET Shortcut
Aqueous reducing power:
Li > Cs > Rb > K > Na
Li > Cs > Rb > K > Na
Remember:
Li = Strongest reducing agent in aqueous solution
But do not confuse this with the simple metallic character trend:
Li < Na < K < Rb < Cs
Thus, the aqueous reducing-power order is an important exception.
30 Related MCQs with Solutions
1. The strongest reducing agent among alkali metals in aqueous solution is:
A. Li
B. Na
C. K
D. Cs
Answer: A. Li
2. The correct order of reducing power in aqueous solution is:
A. Li > Na > K > Rb > Cs
B. Cs > Rb > K > Na > Li
C. Li > Cs > Rb > K > Na
D. Na > K > Rb > Cs > Li
Answer: C
3. Why is Li an unusually strong reducing agent in water?
A. Low atomic mass only
B. Very high hydration enthalpy of Li+
C. Large atomic radius
D. High electronegativity
Answer: B
4. Which ion has exceptionally high hydration enthalpy?
A. Cs+
B. Rb+
C. Na+
D. Li+
Answer: D. Li+
5. The standard reduction potential of Li+/Li is approximately:
A. +3.04 V
B. −3.04 V
C. −1.66 V
D. +1.66 V
Answer: B. −3.04 V
6. A more negative standard reduction potential indicates:
A. Greater tendency to be reduced
B. Greater tendency of the metal to be oxidised
C. Lower reducing power
D. No tendency to react
Answer: B
7. In water, Li+ is strongly stabilised because of:
A. Large radius
B. Small radius and strong ion–dipole attraction
C. Low charge density
D. Low hydration
Answer: B
8. Reducing power refers to the tendency of a species to:
A. Gain electrons
B. Donate electrons
C. Gain protons
D. Form precipitates
Answer: B. Donate electrons
9. Which process represents the reducing action of an alkali metal?
A. M + e− → M−
B. M → M+ + e−
C. M+ → M2+ + e−
D. M + H+ → M−
Answer: B
10. Which factor makes Li+ particularly stable in water?
A. Low charge density
B. High hydration enthalpy
C. Large atomic radius
D. Low ionisation enthalpy
Answer: B
11. Which alkali metal has the most negative standard reduction potential?
A. Li
B. Na
C. K
D. Cs
Answer: A. Li
12. In aqueous solution, reducing power of alkali metals is influenced strongly by:
A. Hydration of M+
B. Nuclear mass only
C. Number of neutrons only
D. Melting point only
Answer: A
13. Which statement is correct?
A. Cs is the strongest reducing agent in aqueous solution
B. Li is the strongest reducing agent in aqueous solution
C. Na is always strongest
D. All are identical
Answer: B
14. The high hydration enthalpy of Li+ is mainly due to its:
A. Large size
B. Small size and high charge density
C. Low charge
D. High mass
Answer: B
15. Which order represents increasing atomic size?
A. Cs < Rb < K < Na < Li
B. Li < Na < K < Rb < Cs
C. Na < Li < Cs < K
D. K < Na < Li < Cs
Answer: B
16. Which statement best explains the exception of Li?
A. Li has no valence electron
B. Li+ has exceptionally high hydration enthalpy
C. Li has the largest radius
D. Li is a noble gas
Answer: B
17. Which metal is oxidised most readily in aqueous medium among the following?
A. Li
B. Na
C. K
D. Cs
Answer: A. Li
18. The oxidation half-reaction for sodium is:
A. Na + e− → Na−
B. Na → Na+ + e−
C. Na+ → Na + e−
D. Na + H+ → NaH
Answer: B
19. Which of the following has the greatest reducing character in water?
A. Li
B. Na
C. Rb
D. Cs
Answer: A. Li
20. Why does aqueous medium alter the simple periodic trend?
A. Hydration effects become important
B. Protons disappear
C. Electrons become neutrons
D. Atomic number changes
Answer: A
21. Which species is strongly hydrated?
A. Li+
B. Cs+
C. Rb+
D. K+
Answer: A. Li+
22. A stronger reducing agent has a greater tendency to:
A. Accept electrons
B. Donate electrons
C. Accept protons
D. Remain unchanged
Answer: B
23. The strongest reducing agent among alkali metals in aqueous medium is not Cs because:
A. Cs cannot lose electrons
B. Li+ receives exceptionally strong hydration stabilisation
C. Cs is a non-metal
D. Cs has no valence electron
Answer: B
24. Which property of Li+ is exceptional among Group 1 cations?
A. Very large ionic radius
B. Very high hydration enthalpy
C. Zero charge
D. Low charge density
Answer: B
25. The aqueous reducing power order is an important exception to the:
A. Atomic-size trend
B. Simple metallic-character trend
C. Nuclear-charge trend only
D. Mass-number trend
Answer: B
26. Which metal has the most negative E° value among these?
A. Li
B. Na
C. K
D. Rb
Answer: A. Li
27. In aqueous medium, hydration mainly stabilises:
A. Neutral metal atoms only
B. Metal cations
C. Neutrons
D. Valence electrons in vacuum
Answer: B. Metal cations
28. Which order is correct for reducing power in water?
A. Li > Cs > Rb > K > Na
B. Na > K > Rb > Cs > Li
C. Cs > K > Na > Li > Rb
D. K > Li > Na > Rb > Cs
Answer: A
29. Which statement is true for Li in aqueous solution?
A. It is the weakest reducing agent
B. It is the strongest reducing agent among alkali metals
C. It cannot be oxidised
D. It behaves as a noble gas
Answer: B
30. The key reason for the aqueous reducing-power order is:
A. Only atomic mass
B. Only atomic radius
C. Combined effects of ionisation, sublimation and hydration
D. Only melting point
Answer: C
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