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Which is more basic, Cl or F?

Published in Chemical Properties 2 mins read

F is more basic than Cl.

Understanding Basicity of Halides

When comparing the basicity of halide ions like F⁻ (fluoride) and Cl⁻ (chloride), we look at the strength of their corresponding conjugate acids. The general rule in chemistry is that the stronger an acid is, the weaker its conjugate base is. Conversely, a weaker acid has a stronger conjugate base.

Conjugate Acid-Base Pairs

  • HF is the conjugate acid of F⁻.
  • HCl is the conjugate acid of Cl⁻.

Acid Strength Comparison

The strengths of the hydrohalic acids (HX) in water generally increase down the group:
HCl > HBr > HI >> HF

This means that HF is the weakest acid in the halogen series.

Basicity Comparison

Following the rule:

  • Since HF is the weakest acid, its conjugate base, F⁻, will be the strongest base among the halide ions (F⁻, Cl⁻, Br⁻, I⁻).
  • Since HCl is a stronger acid than HF, its conjugate base, Cl⁻, will be a weaker base than F⁻.

Therefore, based on the relative strengths of their conjugate acids, F⁻ is a stronger base than Cl⁻. This aligns with the information provided in the reference: "Because HF is the weakest acid in the halogen series, its conjugate base F− will be the stronger base than Cl−."

Let's summarize the relationship:

Acid Acid Strength (Relative) Conjugate Base Base Strength (Relative)
HF Weaker F⁻ Stronger
HCl Stronger Cl⁻ Weaker

This comparison highlights why fluoride is a more basic ion in aqueous solution compared to chloride. The relatively weak acidity of HF means that F⁻ has a greater tendency to accept a proton (H⁺) and revert back to HF than Cl⁻ does to accept a proton and form HCl.