Download History of definition of acids and bases

History of the definitions of
acids/bases
Test your knowledge
Note: discuss polyprotic, amphoteric
acids/bases – dissociation equations
For the following examples create a table writing the name of the acid
or base, whether it is monoprotic, diprotic, or triprotic, and the
dissociation equation(s).
• HBr
• KOH
• Ba(OH)2
• H2CO3
• H3PO3
Definitions of Acids/Bases
• 17th century – Boyle “sour taste”
• Observed the action of acids in solvents, change colour with certain vegetable
extracts (litmus).
• Alkalis reverse these observed effects (e.g. alkalis made acids less acidic).
• Limitation:
• Definition is based on observations rather than explicitly ‘what’ an acid is.
• Late 17th century – Lavoisier “acids contain oxygen”
• Thought that the presence of oxygen made compounds acidic.
• Explains H2SO4, HNO3, H3PO4.
• Limitation:
• Doesn’t explain acids like HCl.
Definitions of Acids/Bases
• 1810 – Davy “acids contain hydrogen”
• Discovered a lack of oxygen in HCl whilst still behaving like an acid.
• Proposed that acids contain hydrogen which improves Lavoisier’s definition.
• Two limitations:
• Not all substance that contain hydrogen are acids
• Definition wasn’t inclusive of substances like aluminium chloride which are acidic.
• 1838 – von Liebig “acid + metal  hydrogen gas”
• Defined an acid based on its reactivity with a metal rather than on the constituent
atoms within the acid.
• Improves the definition of an acid by being more inclusive of more acidic compounds
• Limitation:
• Definition based on reactivity of the compound rather than directly addressing ‘what’ an acid is.
Definitions of Acids/Bases
• 1887 – Arrhenius Theory “acid in water  hydrogen ions”
• Proposed that some compounds dissociate (break apart) to form ions when dissolved in
water.
• Acids, being hydrogen containing compounds (Davy & von Liebig), produce hydrogen ions
when dissolved in water
• E.g. HCl(g) + H2O(l)  H+ (aq) + Cl-(aq)
• Whilst bases produce hydroxide ions when dissolved in water
• E.g. NaOH(s) + H2O (l)  Na+ (aq) + OH- (aq)
• Arrhenius’ definitions are very useful and greatly improve the prior definitions which
were mainly based on reactivity of acids.
• Limitation:
• Still doesn’t encompass all substances that are acidic and basic. Examples include NaHCO3 and
NH3 which act as bases but don’t produce hydroxide ions.
• This definition requires water to be the solvent whereas some acids and bases are water insoluble
Definitions of Acids/Bases
• 1923 – Bronsted & Lowry Theory “acids are proton donors, bases are
proton acceptors”
• Both scientists, working independently, refined Arrhenius’ theory:
• An acid is a proton donor
• A base is a proton acceptor
• Improved definition of acids and
bases as it applies to all solvents,
not just water.
• E.g. HCl(aq) + NH3(aq)  NH4+ (aq) + Cl-(aq)
• Limitaion:
• Not all acids are substances easily
defined by proton donation
Definitions of Acids/Bases
• 1923 – Lewis Theory “acids are electron acceptors, bases are electron
donors”
• Defines an acid as being a substance which can accept a lone pair of electrons
from another substance.
• Whilst a base is a substance which can donate a lone pair of electrons to
another substance.
• Lewis bases = Bronsted-Lowry bases as a lone pair of electrons need to be
donated in order to accept a proton.
Definitions of Acids/Bases
• 1923 – Lewis Theory “acids are electron acceptors, bases are electron
donors”
• Defines an acid as being a substance which can accept a lone pair of electrons
from another substance.
• Whilst a base is a substance which can donate a lone pair of electrons to
another substance.
• Lewis bases = Bronsted-Lowry bases
as a lone pair of electrons need to be
donated in order to accept a proton.
Summary
Amphiprotic? Amphoteric?
Check your understanding
HSC Exam Question 2019
1. What is the verb? (It will tell you the information you need to know)
Assess = make a judgement of value, quality, outcomes, results, or size.
2. What is the content about? (Identify key words – content area)
What is the definition of a Bronsted-Lowry acid and base?
3. Look at the limiting word given for the response (e.g. impact, influence, limitations, contribution)
Usefulness – how is this definition helpful? What are the positives of this model of acids and bases? Considering
the verb used is assess it would be ideal to also include the limitations of this model in making your judgement.
4. Look for plurals, and/or, verbs, adverbs
Try and think of two equations that add to your answer, this could be to aid in your explanation of what an acid
or base is based on the Bronsted-Lowry model. You could also provide an equation to demonstrate a limitation
or limitations of this model.
HSC Exam Question 2019