Acids, Bases, and Salts Flashcards
What are acids?
Substances that form hydrogen (H+) ions when dissolved in water
What are bases?
Any substance that can neutralise an acid
Either soluble or insoluble
Soluble bases release hydroxide (OH-) ions in liquids - these are called alkalis
Insoluble bases are metal oxides and metal carbonates
What happens when an acid and alkali react?
Hydrogen ions react with hydroxide ions to form the neutral solution: water
H+ + OH- = H2O
What does the strength of an acid depend on?
How much it ionises in water (how many acid molecules break up to form H+ ions)
What do weak acids do?
Do not fully ionise in water and so reactions with weak acids are much slower and less exothermic
Most acids are weak
E.g.
Ethanoic acid
What do strong acids do?
Fully ionise in water so reactions are faster and more exothermic Strong acids include: Hydrochloric acid Sulphuric acid Nitric acid
What are examples of strong bases?
Aqueous sodium hydroxide + Potassium hydroxide
What is an example of a weak base and why is it weak?
Ammonia - Only a few NH3 molecules react with water molecules to form ions
What is the difference between strength and concentration of an acid?
Strength = How much of the acid ionises in water Concentration = How many moles of H+ ions are in a unit volume
What are examples of indicators and how can we tell whether something is an acid or alkali by using them?
Litmus paper:
Red in acid
Blue in alkali
Methyl orange:
Red in acid
Yellow in alkali
What does a reaction between an acid and a metal produce?
Salt + Hydrogen
What does a reaction between an acid and a base produce?
Salt + Water
What does a reaction between an acid and a metal carbonate produce?
Salt + Water + Carbon dioxide
Same as bases but with added carbon dioxide, because a carbonate is also a base and so neutralises an acid
What are examples of bases that neutralise acids?
Oxides and hydroxides of metals e.g. Sodium hydroxide Potassium hydroxide Copper (II) oxide
What names do different acids give salts?
Sulfuric acid - Sulfate
Hydrochloric acid - Chloride
Nitric acid - Nitrate
What is produced when a base is reacted with ammonium salt?
Salt + Ammonia gas + Water
How can we solve the problem of a soil’s pH level being too low?
Add a suitable base e.g. slaked lime to control the acidity of the soil so it is the optimum pH for plant growth
What can be observed when an acid reacts with a metal carbonate?
Fizzing occurs (from carbon dioxide produced) Turns limewater milky
How would you test for a carbonate?
Add acid - Results in fizzing
Gas turns limewater milky
How would you test for an acid?
Add any carbonate - Fizzing occurs
How would you test for sulfate ions?
Add barium chloride to a SOLUTION of the sulfate - White precipitate is formed
What are redox reactions?
Reactions which involve a reduction reaction (one reactant gains electrons) and an oxidation reaction (one reactant loses electrons)
e.g. In reaction between Mg and HCl, magnesium atoms are oxidised, hydrogen atoms are reduced, and chlorine atoms remain unchanged
How do we prepare soluble salts?
Through a reaction between acids and solid insoluble substances like metals, metal oxides, metal hydroxides, or metal carbonates
How would you produce blue copper sulfate crystals?
Sulfuric acid + Copper carbonate OR Copper oxide
Copper carbonate is a green powder, copper oxide is a black powder
What are the steps to producing a dry sample of a soluble salt? E.g. Making copper sulfate crystals
When using copper carbonate:
- Keep adding the metal to the acid until fizzing stops and powder is left over (in excess)
- Filter out excess copper carbonate so filtrate contains only copper sulfate solution
- Remove water by putting solution in an evaporating dish OR speed up process by removing ~1/3 of the water with heat
How would you separate different mixtures?
Mixture of insoluble solid and liquid - Filtration
Mixture of soluble solid and liquid (solution) - Crystallisation
Mixture of liquids - Simple distillation/ Fractional distillation
Mixture of dissolved solids - Chromatography
What do the sections on the pH scale mean?
1-6 = Acidic 7 = Neutral 8-14 = Alkaline
How would you measure the pH of a solution?
Universal indicator - Changes to a particular colour depending on the pH of the solution
pH probe - Returns a number which represents the pH of the solution
What exactly is the pH scale?
A measure of the concentration of H+ ions in a solution
What does titration measure?
The volumes of acid and alkali solutions that are needed in a neutralisation reaction - making us able to work out the concentration of either the acid or the alkali if we know the concentration of the other
Which indicators can be used in titration and what is their colour change?
Phenolphthalein: Pink in alkali, colourless in acid
Methyl orange: Yellow in alkali, red in acid
Why do we need indicators in titration?
To show the end point of a titration by changing colour when the solution changes from acidic to alkaline or from alkaline to acidic
What are the steps in titration?
- Measure a known volume of alkali into a flask
- Add indicator
- Put acid of known concentration into a burette and take reading (doesn’t need to be 0)
- Add acid to alkali until it has neutralised
- Read burette to discover how much acid it took to neutralise the alkali
- This is the rough titration, ignore this result and repeat 3 more times and average these results
Why do we do a rough titration?
To know roughly where the neutralisation point is so we can be more accurate the next time
How would you use titration to make a soluble salt?
- Carry out titration as normal, however this solution is not pure as it has been contaminated with indicator
- Repeat with fresh alkali in beaker but do NOT add indicator
- Add volume of acid you know will neutralise it exactly
- Evaporate the water to recover dry salt
