18.1 Lewis Acids & Bases
Syllabus
What does this mean?
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.
Applying B-L Theory
Ammonia solution absorbs Hydrogen Chloride gas, & is neutralised in the process
NH3(aq) + HCl(g) --> NH4+(aq) + Cl-(aq)
______is the base because __________________________________________
______is the acid because __________________________________________
The Problem with Bronsted-Lowry Theory
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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So a new theory is needed and was provided by Gilbert Lewis, the man who first came up with an explanation of Covalent Bonding.
Lewis Theory.
Lewis knew that electrons are important in Chemistry so his theory was based on those.
In the first case:
NH3(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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Video
QUESTIONS
Q1.
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.
A confusion of nomenclature
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 _______