10.3 Strong and Weak Bases. Na + OH - (s) + (aq) Na + (aq) + OH - (aq) completely ionised NH 3 (g) + (aq) NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq)

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10.3 Strong and Weak Bases

Na + OH - (s) + (aq) Na + (aq) + OH - (aq) completely ionised NH 3 (g) + (aq) NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) ⇌ equilibrium lies well to left

W A strong base is a base which is completely ionised in aqueous solution W A weak base is a base which is partially ionised in aqueous solution

Sodium hydroxide is a strong base NaOH (s) + H 2 O (l) Na + (aq) + OH - (aq) Other strong bases are:- potassium,lithium,magnesium, calcium and barium hydroxides

Ammonia solution is a weak base NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH - (aq) Aqueous amine (Unit 2) solutions are also weak bases eg ethyl ammonium hydroxide C 2 H 5 NH 4 + OH - lies well to the left

The solution of the weak base has a lower pH as it contains fewer hydroxide ( OH - (aq) ) ions mA10 mA The solution of the weak base has a lower conductivity as it contains fewer ions The solutions must be equimolar so that the comparison is fair

The concentration of hydroxide ions in 1 mol l -1 NaOH is about 250 times greater than 1 mol l -1 NH 3 solution AimTo compare the quantity of precipitate produced when iron(III) nitrate solution is added to equimolar solutions of a strong and weak base

Apparatus 1 mol l -1 NaOH (aq) 10 cm 3 syringe 1 mol l -1 NH 3 (aq) 5 cm 3 iron(III) nitrate Method 1.5 cm 3 of iron(III) nitrate was added to each test tube 2.10 cm 3 of NaOH (aq) was added to one test tube and 10 cm 3 of NH 3 (aq) was added to the other and the quantity of precipitate was compared

Results The quantity of precipitate in each tube looked the same Fe 3+ (NO 3 - ) 3 (aq) + 3 Na + OH - (aq) Fe 3+ (OH - ) 3 (s) + 3 Na + NO 3 – (aq) Fe 3+ (NO 3 - ) 3 (aq) + 3 NH 4 + OH - (aq) Fe 3+ (OH - ) 3 (s) + 3 NH 4 + NO 3 - (aq) The stoichiometry (moles reacting and moles formed) is the same for both the strong and weak bases 3 mol 1 mol

3 NH 3 (aq) + 3 H 2 O (l) 3 NH 4 + (aq) + 3 OH – (aq) ⇌ Fe 3+ (aq) + Fe 3+ (OH - ) 3 (s) equilibrium moves to the right to replace removed OH - ions This continues until all of the molecules have dissociated

Check Test 10.3