Acids, Bases and Salts

mineral acids (3)

hydrochloric acid, sulfuric acid, nitric acid

what is an acid also known as and why

a proton donor - they release H⁺ ions when dissolved in water, which are just protons

what is a base

a substance that will react with an acid to produce a salt and water only; a neutralisation reaction

properties of bases (3)

• insoluble in water

• usually metal oxides

• they are proton acceptors - they combine with the H⁺ ions released by acids

what is an alkali

a base that is soluble in water

properties of alkalis (2)

• will form (OH)^- ions when dissolved in water. this neutralises the H⁺ ions from acid

• proton acceptors

ionic equation for a neutralisation reaction

H⁺_(aq) + (OH)^-_(aq) → H₂O

word equations for reactions with acids (4)

• base + acid → salt + water

• metal + acid → salt + hydrogen

• metal carbonate + acid → salt + water + carbon dioxide

• alkali + acid → salt + water

how can an acid be strong

they ionise completely in water - all acid molecules dissociate / ionise to release H⁺ ions, eg HCl is a strong acid, it ionises completely: HCl → H⁺ + Cl^-

how can an acid be weak

they only partially ionise in water. only some acid molecules ionise / dissociate, releasing H⁺ ions, eg CH₃COOH → H⁺ + CH₃COO^-

property of weak acid ionisation

is reversible, setting up an equilibrium

difference between concentrated and dilute acids

concentrated - more amount of acid molecules dissolved in a certain volume
dilute - less amount of acid molecules dissolved in the same volume

if concentration of a strong and weak acid is the same, what is the pH relationship

pH of strong acid is lower than the weak acid

what is pH a measure of

the concentration of H⁺ ions in solution

for every decrease by 1 on the pH scale, the concentration of H⁺ ions rises by a factor of __ (_________ scale), eg pH 5 to 4 means x__ concentration, while pH 5 to 3 means x___ concentration

10, logarithmic, 10, 100

which state is the only state where compounds are split into ions in ionic equations

aqueous

methyl orange indicator in acid, neutral, alkali

red in acid, yellow in neutral, yellow in alkali

phenolphthalein in acid, neutral and alkali

colourless in acid, colourless in neutral, pink in alkali

UI in acid, neutral, and alkali

red in acid, green in neutral, blue in alkali

solubility rules (6)

• all sodium, group 1 and ammonium salts are soluble

• all nitrates are soluble

• most halides, except lead halides and silver chloride, are soluble

• most sulfates are soluble, except for lead sulfate, calcium sulfate and barium sulfate

• most carbonates are insoluble, except from group 1 and ammonium carbonates

• most hydroxides are insoluble, except for group 1 and ammonium hydroxides

way of deciding method to use to get a salt depending on solubility and salt

• is salt soluble?

  • if yes, is salt group 1 or ammonium salt?

    • if yes, use titration method

  • if no, react acid with metal, metal oxide, metal carbonate or metal hydroxide

• if not soluble, use precipitate method, then filter

method to make a pure, dry sample of insoluble salt (4)

• mix two solutions of soluble substance mixed in a beaker - one of them will be a group 1 or ammonium hydroxide. white precipitate formed

• precipitate is filtered through paper and funnel into conical flask

• filtered precipitate on filter paper washed with distilled water

• precipitate scraped off filter paper, left to dry

method to make pure, dry copper sulfate crystals (8)

• measure 40cm³ sulfuric acid into 100cm³ beaker

• set up tripod, gauze and heatproof mat. heat acid gently using bunsen burner until almost boiling. turn off bunsen burner

• use spatula to add copper II oxide powder. stir with glass rod after every spatula. continue to add copper II oxide if it keeps disappearing when stirred.

• stop adding copper II oxide when some of it remains after stirring. allow to cool completely

• set up filter funnel and paper over conical flask. filter contents of beaker

• after filtration, pour conical flask contents into evaporating basin. evaporate gently using water bath (evaporating basin sits on top of a beaker half-full of water, on a tripod, on a gauze, above a bunsen burner). stop heating when crystals start forming

• transfer basin to a cool place for at least 24 hours

• remove crystals and pat them dry

when are titrations used

to find out the volume of acid / alkali is needed to fully neutralise a certain volume of the opposite - alkali / acid

titration method for reaction between NaOH and HCl (9)

• use pipette and pipette filler to put exactly 10cm³ of NaOH solution into conical flask. stand flask on white tile

• clamp burette vertically in clamp stand about halfway up its length. needs to be enough room underneath for flask and tile

• close burette tap. use small funnel to fill burette with dilute HCl

• put 5-10 drops phenolphthalein indicator into flask. swirl to mix and place under burette, on tile

• open tap so HCl flows into flask drop by drop. constantly swirl flask while adding HCl. look for colour change from pink (alkali) to colourless (neutral)

• as solution in flask gets less and less pink, close tap slightly to slow drop rate of HCl

• close tap fully when single drop of HCl turns flask permanently colourless. read burette scale carefully and record volume of HCl added (one titre)

• repeat titration twice more, and record these additional two titres

• calculate mean titre of volume of HCl needed to neutralise 10cm³ of NaOH. record mean titre.