GRADE 12 PHYSICAL SCIENCES

CHEMISTRY FOCUS: ACIDS & BASES

Arrhenius Definition of Acids and Bases

AcidSubstance that produces Hydrogen ions (H+) or Hydronium ions (H3O+) when dissolved in water

BaseSubstance that produces Hydroxide ions (OH) when dissolved in water

Lowry-Brnsted Definition of Acids and Bases

AcidProton (H+) donor

BaseProton (H+) acceptor

Nitric Acid donates a single proton; Sodium Hydroxide accepts it

Acid-Base reactions are called protolytic reactions or protolysis

Common Household Acids & Bases

Acetic AcidCH3COOHAlso ethanoic acid. In vinegarLactic AcidC3H6O3Found in dairy productsAscorbic AcidC6H8O6Vitamin CSulfuric AcidH2SO4Car BatteriesHydrochloric AcidHClStomach AcidCarbonic AcidHCO3Soft Drinks

Caustic SodaNaOHOven cleanerAmmoniaNH3Cleaning; fertilizersMilk of MagnesiaMg(OH)2Neutralises stomach acidBaking SodaNaHCO3Bicarbonate of SodaWashing SodaCNa2O3Soap / detergentBleachConjugate Acid-Base Pairs

Pairs differ by only one proton always Square brackets are used to indicate each pair

Ampholytes & Amphiprotic Substances

Substances that can act both as an acid and a base

Water: or

Hydrogen Sulfate Ion:or

Strength and Weakness of Acids & Bases

Extent to which the acid or base is ionized or dissociated

Strong acids are fully ionized in the solutionHydrochloric Acid, Sulfuric Acid, Nitric Acid

Strong bases are fully dissociated in the solutionSodium Hydroxide, Potassium Hydroxide

Weak acids are partially ionized in the solutionEthanoic Acid, Oxalic Acid

Weak bases are partially dissociated in the solutionAmmonia, Calcium Carbonate, Potassium Carbonate, Sodium Hydrogen Carbonate

Concentration of Acids & Bases

The number of moles of acid or base dissolved per volume of water Equations for Reactions of Aqueous Acids and Bases

Sulfuric acid is diprotic

Neutralisation Reactions of Acids and Bases

The pH scale

Indicates the concentration of the H3O+ ion in a solution Scale of numbers from 0 to 14 When the H3O+ concentration is high, the pH is low

0714

AcidicNeutralBasic

pH = log10[H3O+]

The Ionisation Constant of Water

KW = [H3O+][OH] = 1 x 1014 at 298K = 25C

The Auto-Ionization of Water

Water is weakly ionized as follows:

Acting as an acid and a base

Hydrolysis Reactions

Reaction of a salt with water

Salt of Weak Acid and Strong Base Alkaline SolutionSodium Ethanoate, Sodium Oxalate, Sodium Carbonate

Salt of Strong Acid and Weak Base Acidic SolutionAmmonium Chloride

Salt of a strong acid and strong base does not undergo hydrolysis(pH will be 7)

Indicators

A chemical substance that is used to detect the presence of other chemicals in a solution

Equivalence PointThe point at which the acid or base has completely reacted with the base or acid in the titration

EndpointThe point in a titration where the indicator changes colour

IndicatorpH rangeBelowAboveEndpoint

Bromothymol Blue6.0 7.6YellowBlueGreen

Methyl Orange3.1 4.4RedYellowOrange

Phenolphthalein8.2 10.0ColourlessRedPink

Choosing an indicator:

Weak Acid + Weak BaseStrong Acid + Strong BaseBromothymol Blue

Strong Acid + Weak BaseMethyl Orange

Weak Acid + Strong BasePhenolphthalein

Titrations

The procedure of adding measured amounts of a solution to a known volume of a second solution until the chemical reaction between them is complete

In the case of acids and bases:One of the substances has an unknown concentration and the other is known. An appropriate indicator must be selected based on the relative strength and weakness of the acid and base.

Procedure for Carrying Out Titrations see txtbk or class notes

Apparatus Preparation of standard solution Conducting the titration Safety Precautions Measures needed to ensure reliable results Interpret the results

Stoichiometric Calculations

cconcentrationmoldm-3vvolumedm3nno. of moles mol(given by balanced equation)

Other Formulas Used:

Tips for performing Stoichiometric Calculations

1. Balance the equation2. Write down the molecular ratio between the substances3. Convert given information into moles4. Use the ratios to calculate the number of moles of the substance needed5. Convert the moles into corresponding quantity needed (volume, concentration, mass etc)

Acid Dissociation & Base Association Constants

Measures the extent to which the acid has ionized at equilibriumA higher value will indicate a strong acid