- We live in a world full of chemicals. Some of these chemicals are compounds called acids and bases. We will talk about the properties and various ways of identifying acids and bases. In 1663, Robert Boyle started to classify compounds as acids and bases by their physical properties, but it would be another 200 years before scientists began to explain their chemical properties.
Properties of Acids
- Sour taste:
- citric acid makes fruit juice tart
- acetic acid makes pickles sour
- acetylsalicylic acid makes aspirin sour
- lactic acid gives the smell and flavor to sour milk
- When some acids dissolve, they are able to conduct and electrical current. For example, HCl ionizes into hydrogen ions and chlorine ions. Any substance that ionizes to conduct electricity in a solution is called an electrolyte.
- An acid turns blue litmus paper red.
- Acids react with active metals to produce hydrogen gas and a salt.
- Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Properties of Bases
- Bitter taste
- Slippery
- Red litmus paper turns blue.
- Neutralization reaction between an acid and a base in an aqueous solution produces a salt and water.
- HCl (aq) + NaOH (aq) → NaCl (aq) + H2O
The Arrhenius Model
- Acids:
- Give a H+ ion (a proton) when in water.
- This hydrogen ion is not stable so it bonds with the water to form the hydronium ion (H3O+)
- Examples:
- HCl + H2O → H3O+ + Cl-
- CH3COOH + H2O → H3O+ + CH3COO-
- Bases:
- Gives an OH- ion (a hydroxide) when in water
- Examples:
- NaOH + H2O → OH- + Na+ + H2O
- NH3 + H2O → OH- + NH4+
- Limitations:
- Not all substances with Hs or OHs are an acid or a base.
- Examples: CH4 and CH3OH
- Only works for acids and bases in water.
The Bronsted Lowry Model
- Acids: donate proton(s)
- This is the same definition as in the Arrhenius model.
- Donating or losing a proton is called deprotonation.
- Bases: accept proton(s)
- Much wider than Arrhenius because it includes substances that might not have an OH group.
- Gaining or accepting a proton is called protonation.
- Conjugate pairs:
- This refers to acids and bases with common features. These common features are the equal loss/gain of protons between pairs. Conjugate acids and conjugate bases are characterized as the acids and bases that lose or gain protons.
- Acid + base → conjugate base + conjugate acid
- Example:
- HC2H3O2 + H2O <—> H3O+ + C2H3O2-
- Acid: HC2H3O2 because it donates a proton
- Base: H2O because it accepts the proton
- Conjugate acid: H3O+ because it would donate a proton
- Conjugate base: C2H3O2- because it would accept the proton
- HClO2 + H2O → ClO2- + H3O+
- Acid: HClO2
- Base: H2O
- Conjugate acid: H3O+
- Conjugate base: ClO2-
- OCl- + H2O → HOCl + OH-
- Acid: H2O
- Base: OCl-
- Conjugate acid: HOCl
- Conjugate base: OH-
- HCl- + H2PO4- → Cl- + H3PO4
- Acid: HCl-
- Base: H2PO4-
- Conjugate acid: H3PO4
- Conjugate base: Cl-
Lewis Model
- Acids: accept a pair of e- (one empty orbital)
- Bases: donate a pair of e- (one unbonded pair of electrons)
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