18.1 Lewis Acids & Bases

Revision of Topic 8.

Bronsted-Lowry theory stated that an acid was a proton -__________________

And a base was a proton -__________________________

Remember also that a proton is just another name for a _________ ion.

And that protons don’t really exist uncombined in water but (almost) all react with water as below:

H⁺_(aq) + H₂O_(l) --> H₃O⁺_(aq)

The H₃O⁺ ion is called the __________________ ion.

Ammonia solution absorbs Hydrogen Chloride gas, & is neutralised in the process

NH_3(aq) + HCl_(g) --> NH₄⁺_(aq) + Cl^-_(aq)

______is the base because __________________________________________

______is the acid because __________________________________________

NH₃ + BF₃--> NH₃BF₃

The Ammonia is neutralized because the Nitrogen atom can no longer accept protons.

This makes BF₃ an acid.

What sort of bond forms between the Ammonia and Boron Trifluoride?

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Why doesn’t Bronsted-Lowry theory allow BF₃ to be acidic?

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Lewis knew that electrons are important in Chemistry so his theory was based on those.

In the first case:

NH_3(aq) + HCl_(g) --> NH₄⁺_(aq) + Cl^-_(aq)

Lewis said that the Ammonia is being a base because it donated its lone pair to the coordinate bond (rather than because it accepted the H⁺ ion).

This means that the HCl is an acid because it accepted the lone pair (rather than because it donated an H⁺ ion)

How does this solve the problem of NH₃/BF₃?

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It may help to draw the dot-cross diagram for the outer shells of each reactant and product.

Q2 Identify the Lewis Acids and Lewis bases.

We studied Transition Elements and their complexes in Topic 13.

Each of the water ligands in the complex ion above has acted as a nucleophile.

Q. What is the textbook definition of a nucleophile?

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Q. This means that every nucleophile is a Lewis _______

The central metal ion in all complex ions acted as an electrophile.

Q. What is the textbook definition of a electrophile?

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Q. This means that every electrophile is a Lewis _______