Acids & Alkalis Flashcards
What are acids? (4 marks)
They have a pH of 1-6
They turn the universal indicator yellow, orange and red
Can neutralise alkalis
Produces hydrogen ions (H+) in aqueous solutions
Why do you write (aq) next to an acid? (2 marks)
Acid are aqueous solutions - which means they are dissolved in water
What is an example of a strong acid? (1 mark)
Hydrochloric acid, sulfuric acid
What is an example of a weak acid? (1 mark)
Ethanoic acid (vinegar) - CH3COOH, hydrofluoric acid
What are alkalis? (4 marks)
Have a pH of 8-14
Turn universal indicator blue or purple
Can neutralise acids
They form negative hydroxide ions (OH-) in aqueous solutions
What are bases?
a substance that reacts with an acid to neutralise it and produce a salt
They are usually metal oxides
or metal hydroxides
What is an example of a strong alkali? (1 mark)
Sodium hydroxide (NaOH)
What is an example of a weak base? (1 mark)
Ammonia (NH3)
What number means neutral on the pH scale?
7
What are the methods to measure pH? (2 marks)
an indicator or a pH probe
What is an indicator? (1 mark)
A substance that changes colour depending on the pH of the solution to which it is added
What happens during the neutralisation reaction of acids and alkalis?
the H+ ions react with the OH- ions to produce water
this is the net ionic equation of all acid-base neutralisation
H+ + OH- —> H₂O
What is a universal indicator useful for estimating the pH of?
of an unknown solution
operates across a broad range
acid + metal —> …. + …..
acid + metal —> salt + hydrogen
acid + metal oxide —> …. + ….
acid + metal oxide —> salt + water
acid + metal carbonate —> …. + …. + ….
acid + metal carbonate —> salt + water + carbon dioxide
acid + alkali —> … + ….
acid + alkali —> salt + water
Formula for hydrochloric acid?
HCl
Formula for sulfuric acid?
H₂SO₄
Formula for nitric acid?
HNO₃
Formula for sodium hydroxide?
NaOH
Formula for ammonium?
NH₄⁺
Formula for nitrate?
NO₃−
Formula for sulfate?
SO₄²-
Formula for carbonate?
CO₃²-
Formula for sulfide?
S²-
What can ammonium nitrate be used as?
a fertaliser
What are the steps involved in making a pure, dry sample of a soluble salt?
(To prepare crystals of copper sulfate)
To prepare crystals of copper sulfate
- add copper oxide (one spatula at a time) to sulfuric acid with gentle heating until no more will dissolve/ copper oxide is in excess
- filter the mixture to remove unreacted copper oxide
- pour the filtrate into an evaporating basin and heat the solution until crystals just start to appear
- allow to cool to room temperature and leave to evaporate slowly to obtain blue crystals of copper sulfate
- ensure to wear eye protection
What is the method for titration?
- Put 25cm³ of …. hydroxide (alkali) into a conical flask and add a few drops of indicator
- Put …. acid into a burette
- Add the acid slowly to the alkali, with swirling, until the indicator just changes colour
- Note the amount of acid you added
- repeat this process until you receive 2 similar results (and calculate an average)
- Now knowing the amount of acid required to neutralise the alkali, add this amount to 25cm³ of a new sample of alkali, without the indicator
- You now have salt + water
- Heat the solution until crystals just start to appear, then leave to cool and evaporate the water slowly to obtain the crystals
What is ammonium nitrate made up of (titration)?
ammonia + nitric acid
What is sodium chloride made up of (titration)?
sodium hydroxide + hydrochloric acid
Why isn’t potassium added to nitric acid to make potassium nitrate?
potassium is very reactive
reaction would be extremely violent
Why isn’t copper added to sulfuric acid to make copper sulphate?
copper is unreactive
wouldn’t react with acid
Why is sulfuric acid stronger than ethanoic acid?
Sulfuric acid fully ionises in water
(this results in a larger concentration of H⁺ ions and a very low pH)
ethanoic acid only partially ionises
Why is litmus not suitable for titration?
It is a wide range indicator and therefore does not give a sharp enough colour change at the endpoint
What are titrations?
Methods of analysing the concentration of solutions
What is a neutralisation reaction?
A reaction between an acid and alkali
What is a salt?
a compound that is formed when the hydrogen atom in an acid is replaced by a metal
Formula of phosphate?
PO₄³⁻
Why do salts have no overall charge?
Since the sum of the charges on the ions is equal to zero
What is an insoluble substance?
a substance (solid) that will not dissolve in a solvent even after mixing
What is a solvent?
a substance that does the dissolving (e.g. liquid)
Why is a carbonate not used in making soluble salts?
any carbon dioxide gas produced would have been released into the atmosphere
What is the titration equation?
concentration = moles / volume in dm³
How do you work out the volume in dm³
volume (cm) / 1000
magnesium + hydrochloric acid reaction?
effervescence, temperature increase, pop (splint test)
magnesium oxide + hydrochloric acid reaction?
effervescence, red (UI)
sodium hydroxide + hydrochloric acid reaction?
green (UI)
potassium carbonate + hydrochloric acid reaction?
effervescence, cloudy (limewater)
What is corrosion?
corrosion occurs when a metal continues to oxidise. The metal may become weaker and eventually, all of it may become metal oxide.
Formula for ammonia?
NH₃
Formula for ethanoic acid?
CH₃COOH
Why are the types of indicators
Methyl orange
Phenolphthalein
Universal indicator
litmus paper
Why does potassium carbonate not cause limewater to bubble when heated?
Potassium carbonate (and other group 1 metals apart from lithium carbonate) does not decompose to produce carbon dioxide / a gas
What evidence is there for a chemical reaction?
colour change
formation of a precipitate (solid)
formation of a gas (effervescence)
temperature change
Why is it important for the percentage atom economy of a reaction to be as high as possible?
Economic reasons
Waste products may be pollutants / greenhouse gases
Discuss the difference in pH between a weak acid and strong acid of the same concentration
A strong acid completely ionises (dissociates) in aqueous solutions where is weak acids only partially ionise (dissociate). This means the strong acid has a much lower pH than the weak acid. As a result the strong acid has a higher concentration of H+ ions as a greater number of acid molecules ionise (dissociate)
What is produced when acids react with metals that are not in group 1?
Acids react with metals to produce salts and hydrogen
How are solid salts produced from salt solutions?
salt solutions can be crystallised to produce solid salts
If the pH decreases by one unit, by what factor does the ion concentration increase by?
10
What is one way to speed up a reaction between a metal oxide and acid?
Heat the mixture
If the acid was the limiting reactant, how could you tell if all the acid was used up?
If only metal oxide remains
How could a student obtain a solution from a reaction mixture, when it has finished reacting?
Through filtration
Describe how salt crystals are produced from salt solution?
Crystalise the salt solution - which involves heating the solution until crystals just start to appear and allowing it to cool to obtain crystals - to form salt crystals
