Acid-Base Equilibria


Edexcel A level Chemistry. Topic 12: Acid-Base Equilibria
Francesca Wittmann
Flashcards by Francesca Wittmann, updated more than 1 year ago
Francesca Wittmann
Created by Francesca Wittmann over 7 years ago

Resource summary

Question Answer
Arrhenius Acid Dissolves in water to form H+ ions
Arrhenius base Dissolves in water to form OH- ions
Bronsted-Lowry acid proton donor
Bronsted-Lowry base Proton acceptor
Monotropic acid can donate one proton per molecule e.g. HCl where as H2SO4 is ditropic
Weak Acid Only partially dissociates into ions when dissolved in water
Strong Acid Fully ionises in water (dissociates into its ions)
Conjugate Base A substance which has donated a proton and could now accept a proton
Conjugate acid A substance that has accepted a proton that could now donate a proton
Define Acid-Base conjugate pairs A pair of molecules that can be transformed from one to another by the loss or gain of a proton
pH equation -log[H+]
equation for [H+] in terms of pH = 10^-pH
Things to assume when dealing with strong acids The [H+] is the same as the concentration of the acid. i.e. [HCl] = [H+] in an equation
What is Ka and the Equation for it? Ka= acid dissociation constant = [H+][A-]/[HA] units= mol/dm3
Assumptions for calculating pH of weak acids? (2 things) 1. Dissociation= so small that [HA] at equilibrium= [HA] of undissociated acid 2. [H+]=[A-] (formed in equal amounts)
Equation for Ka of a weak acid Ka= [H+]^2/ [HA]
What is pKa and the equation for it? pKa= -log[Ka]
Size of Ka and pKa for a strong acid Large Ka therefore small pKa
Equation for the ionic product of water Kw= [OH-][H+]
Ionic product for water at room temp 1.00x10^-14 mol2/dm6
equation combining pH and pOH 14 =pH + pOH 14= -log[H+] -log[OH-]
Define End point The point in a titration at which a colour change of pH change indicated that just enough reactant has been added from the barrette to react with the chemical in the flask
Define Equivalence point The point in a titration when the amount of a reactant has been added to exactly react with the amount of chemical in the flask according to the stoichiometry of the chemical equation
Titration curve of a strong acid and strong base sharp zig zag with equivalence point at pH=7
Titration curve for strong acid and weak base Zig Zag with slight curve at lower part. Equivalence point ph<7
Titration curve of strong base and weak acid Curved lower part where solution forms buffer. Equivalence point pH>7
Titration curve of a weak acid and weak base S shaped: pH change around the equivalence point is gradual
Assumptions when calculating pH of a buffer solution Assume all [A-] comes from conjugate based formed in reaction. Assume all weak acid [HA] is undissociated
Methyl Orange: Colour change and pH (from acid to alkali) from Red to Yellow at 3.2-4.4
Methyl Red: Colour change and pH (from acid to alkali) Yellow to Red at 4.2-6.3
Bromothymol Blue: Colour Change and pH (from acid to alkali) Yellow to Blue at 6.0-7.6
Phenolphthalein: Colour Change and pH Colourless to Pink at 8.2-10.0
How does temperature affect the ionic product of water? Remains neutral as [H+]=[OH] Position of eq shifts to the right therefore [H+] increases so pH decreases
Define Standard enthalpy change of neutralisation the enthalpy change for the reaction when an acid neutralises an alkali under standard conditions to form one mole of water.
Standard enthalpy of neutralisation for a strong acid and strong base -57.5 kJmol-1
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