A-LEVEL OCR ChEMISTRY NOTES
Acids and Bases
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- Common acids include:
- Hydrochloric acid, HCl
- Sulphuric acid, H2SO4
- Nitric acid, HNO3
- Acids dissociate in water: HA (aq) ⇌ A- (aq) + H+ (aq)
- This process of HA separating into A¬- and H+ ions is called dissociation.
- The strength of an acid describes how much of it dissociates when it dissolves.
- A strong acid is an acid which dissociates almost completely in water or aqueous solution. HA (aq) → A- (aq) + H+ (aq)
- A weak acid is an acid which is only partially dissociated in water or aqueous solution. HA (aq) ⇌ A- (aq) + H+ (aq)
- Hydrochloric acid is a strong acid: HCl (aq) → H+ (aq) + Cl- (aq)
- Common bases include:
- Sodium hydroxide, NaOH
- Potassium hydroxide, KOH
- Ammonia, NH3
- Alkalis also dissociate in water, forming bases:
- XOH → X+ + OH- e.g. NaOH → Na+ + OH-
- Acids and bases react together in neutralisation reactions:
acid + base → salt + water
- Acid-carbonate reactions:
acid + carbonate → salt + carbon dioxide + water
- Acid-metal reactions:
acid + metal → salt + hydrogen gas
- In each example H+ reacts with OH- to form water.
- Acid-base titrations can be used to find the concentration of a sample of either an acid or a base.
- A known concentration of an acid is gradually added to a known volume of a base of unknown concentration until the solution is neutralised i.e. the titration reaches the ‘end point’
- A burette is used to gradually add the acid, a pipette is used to add a known volume of the base, and an indicator is used to cause a colour change when the reaction reaches the end point
- Calculations are used to determine the concentration of the unknown solution.
- These calculations are those you already know:
- n = c x v m = n x Mr
- Oxidation number is a number representing the number of electrons lost or gained by an atom in a compound.
- Oxidation is a loss of electrons during a reaction or an increase in oxidation number.
- Reduction is a gain of electrons during a reaction or a decrease in oxidation number
- The rules for assigning oxidation numbers:
- An uncombined element has an oxidation number of 0
- A simple ion (of a single element) has an oxidation number equal to the charge on the ion
- The sum of oxidation numbers of the elements in a compound is equal to the overall charge of the compound
- The charge on a complex ion, e.g. NH4+, is equal to the sum of the oxidation numbers
- The most electronegative element in a compound always has a negative oxidation number
- Oxygen is always -2 except in peroxides where its -1
- Hydrogen is +1 except in metal hydrides where its -1
- Oxidation numbers are represented by Roman numerals when naming compounds
- Oxidation numbers can be used to write formulae for a compound