[Shirro]

Hey,

Can someone tell me why does Na⁺ have greater electrical conductivity in water than K⁺. Doesn't conductivity decrease with size in general?

-Shirro

[Borek]

If conductivity decreases with size, it should be smaller for K⁺ - so greater for Na⁺.

[Shirro]

If conductivity decreases with size, it should be smaller for K⁺ - so greater for Na⁺.

Meant: why does K⁺ have greater electrical conductivity in water than Na⁺?

Ionic mobility(correlates with conductivity) in water (u/(10^-8 m²s^-1V^-1))
Na⁺: 5.19
K⁺: 7.62

[Borek]

May have something to do with hydration - smaller ions are strongly hydrated, so they need to pull more water molecules with them. That makes them less mobile.

[cheese (MSW)]

Borek is correctit is a hydration effect.
[M(H2O)n]^+  n includes primary hydration sphere (directly bound to M^+) and secondary hydration sphere 
M^+← δ-OH2δ+...δ-OH2δ+...δ-OH2δ+
The smaller the ion the greater the electrostatic field, the greater n, the slower the ionic mobility:
Li^+ n= ~25.3, Ω = 33.5; Na^+ n= ~16.6, Ω = 43.5; K^+ n= ~10.5, Ω = 64.6; Cs^+ n= ~9.9 Ω = 68.
F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann, Advanced Inorganic Chemistry 6th ed (1999). p 102

[juanrga]

If conductivity decreases with size, it should be smaller for K⁺ - so greater for Na⁺.

Meant: why does K⁺ have greater electrical conductivity in water than Na⁺?

Ionic mobility(correlates with conductivity) in water (u/(10^-8 m²s^-1V^-1))
Na⁺: 5.19
K⁺: 7.62

Hum. My data is different, being greater for Sodium.

Electrical conductivity:

Na⁺: 20.1 x 10⁶ S m^-1
K⁺: 16.4 x 10⁶ S m^-1

I have just checked the electrical conductivity for NaOH and KOH for different concentration in mass percent (range 0.5%--5%) and it is always greater for Sodium.

[cheese (MSW)]

The C&W Ω values are relative and hence no units; they are at ∞ dil., 18°C.
juanra: Two possibilities for the difference: 
Your conductivity values are not molar conductivities. 
You have not corrected for the conductivity of OH^- that like that of H^+
is anomalously high (Grotthuss mechanism).
http://www.lsbu.ac.uk/water/grotthuss.html

[Shirro]

Thanks for the answers everyone. But does this mean that Electrical conductivity increases as size gets bigger in general?

[juanrga]

The C&W Ω values are relative and hence no units; they are at ∞ dil., 18°C.
juanra: Two possibilities for the difference: 
Your conductivity values are not molar conductivities. 
You have not corrected for the conductivity of OH^- that like that of H^+
is anomalously high (Grotthuss mechanism).
http://www.lsbu.ac.uk/water/grotthuss.html

If you check the units they are electrical conductivities, not molar ones.

According to the table "ELECTRICAL CONDUCTIVITY OF AQUEOUS SOLUTIONS" of the CRC Handbook of chemistry and physics (89th) the electrical conductivity of NaOH is larger than KOH for all range of concentrations.

The contribution of OH^- is the same in both solutions (because stoichiometry and concentration are the same), therefore the difference in the measured conductivity must be due to the larger electrical conductivity of the Na⁺ over the K⁺

No?

[Shirro]

The C&W Ω values are relative and hence no units; they are at ∞ dil., 18°C.
juanra: Two possibilities for the difference: 
Your conductivity values are not molar conductivities. 
You have not corrected for the conductivity of OH^- that like that of H^+
is anomalously high (Grotthuss mechanism).
http://www.lsbu.ac.uk/water/grotthuss.html

If you check the units they are electrical conductivities, not molar ones.

According to the table "ELECTRICAL CONDUCTIVITY OF AQUEOUS SOLUTIONS" of the CRC Handbook of chemistry and physics (89th) the electrical conductivity of NaOH is larger than KOH for all range of concentrations.

The contribution of OH^- is the same in both solutions (because stoichiometry and concentration are the same), therefore the difference in the measured conductivity must be due to the larger electrical conductivity of the Na⁺ over the K⁺

No?

hmm interesting... my values are taken from Atkins Physical Chemistry 7th edition.

[cheese (MSW)]

Conductivities (10^-4 m^2 S mol^-1)
   ∞ dil    0.001 M   0.005 M   0.001 M
NaCl   126.4    123.7   120.6           118.5
KCl    149.8  146.9     143.5    141.2
NaI   126.9    124.2   121.2    119.2
KI   150.3    143.3*   144.3*   142.1
NaOH 247.7  244.6   240.7           237.9
KOH   271.5     234           230           228
*Entries back to front?
As can be seen, the conductivity of KOH at infinite dil is higher than
that of NaOH as found, for example, the corresponding chloride and iodide salts. 
This is consistent with the view that the molar conductivity Na^+(aq) is less than K^+(aq)
due to higher hydration of the Na^+ ion.  But at any significant conc the molar conductivity
of aqueous KOH is less than that of NaOH (!).  I suspect this is due to ion pairing: 
K^+(aq)...OH^-(aq) that is stronger for K^+ because K^+(aq) is smaller than Na^+(aq) and
hence δ+ δ-/r  is larger (smaller r).
Data are from 92nd Edition, 2011-2012, of the CRC Handbook of Chemistry and Physics 5-76

[juanrga]

The C&W Ω values are relative and hence no units; they are at ∞ dil., 18°C.
juanra: Two possibilities for the difference: 
Your conductivity values are not molar conductivities. 
You have not corrected for the conductivity of OH^- that like that of H^+
is anomalously high (Grotthuss mechanism).
http://www.lsbu.ac.uk/water/grotthuss.html

If you check the units they are electrical conductivities, not molar ones.

According to the table "ELECTRICAL CONDUCTIVITY OF AQUEOUS SOLUTIONS" of the CRC Handbook of chemistry and physics (89th) the electrical conductivity of NaOH is larger than KOH for all range of concentrations.

The contribution of OH^- is the same in both solutions (because stoichiometry and concentration are the same), therefore the difference in the measured conductivity must be due to the larger electrical conductivity of the Na⁺ over the K⁺

No?

hmm interesting... my values are taken from Atkins Physical Chemistry 7th edition.

Could you say table or page please?. I have 8th edition and I could not find anything about electrical conductivity of electrolytes (only about electrical conductivity of solids).

I attached image of the table of the Handbook

[cheese (MSW)]

Data are from 92nd Edition, 2011-2012, of the CRC Handbook of Chemistry and Physics 5-76
Section 5 page 76
Equivalent Conductivity of Electrolytes in Aqueous Solution
It is three tables down from your table that I note gives conductivities for solns in mass %.

[juanrga]

Conductivities (10^-4 m^2 S mol^-1)
   ∞ dil    0.001 M   0.005 M   0.001 M
NaCl   126.4    123.7   120.6           118.5
KCl    149.8  146.9     143.5    141.2
NaI   126.9    124.2   121.2    119.2
KI   150.3    143.3*   144.3*   142.1
NaOH 247.7  244.6   240.7           237.9
KOH   271.5     234           230           228
*Entries back to front?
As can be seen, the conductivity of KOH at infinite dil is higher than
that of NaOH as found, for example, the corresponding chloride and iodide salts. 
This is consistent with the view that the molar conductivity Na^+(aq) is less than K^+(aq)
due to higher hydration of the Na^+ ion.  But at any significant conc the molar conductivity
of aqueous KOH is less than that of NaOH (!).  I suspect this is due to ion pairing: 
K^+(aq)...OH^-(aq) that is stronger for K^+ because K^+(aq) is smaller than Na^+(aq) and
hence δ+ δ-/r  is larger (smaller r).
Data are from 92nd Edition, 2011-2012, of the CRC Handbook of Chemistry and Physics 5-76

But is not according to usual understanding that ion pairing is not an appreciable effect for 1:1 electrolytes at the concentration ranges reported?

This is all very interesting! 

[cheese (MSW)]

All very interesting indeed!
I would like to propose an explanation for this intriguing observation:   The ion pair in K^+(aq)OH^-(aq) has a reasonably strong H-bonding component:  H-O(-):.(δ+)H-O(H):→K^+
This requires (a) that O be a capable of strong H-bonding which it is, and (b) that H bonding be to a H2O molecule in the primary coordination sphere of K^+ (strong polarization).  The primary hydration sphere of Na^+ and K^+ is, as I recall, 4-6 but closer to 6.  Thus [K(H2O)n]^+ with n = ~10.5 the proposed coordination is possible, but not as likely for [Na(H2O)n]^+  with n = ~16.6.  The only other anion where this H bonding would be possible would be F^- (NH2^- is unstable in H2O) and we therefore would predict that the conductivity of KF soln would also be anomalously low.  (There are no fluorides in the tables of conductivity in the CRC Handbook.)    Note this ion pairing would slow both the conventional conductivity of the K(aq)^+ ion but also the hopping mechanism of conductivity of OH(aq)^+.