acids&bases summary
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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
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