Studied by 0 peopleWhat colour does litmus paper turn in acid/alkaline solutions?
ACID: Red
ALKALI: Blue
What range pH would be classified as strongly or weakly acidic?
Strongly acidic: 0-3
Weakly acidic: 4-6
What range of pH would be classified as strongly or weakly alkaline?
Strongly alkaline: 11-14
Weakly alkaline: 8-10
What colour does phenolphthalein turn in acidic/alkaline solutions?
ACID: Colourless
ALKALI: Pink
What colour does methyl orange turn in acidic/alkaline solutions?
ACID: Red
ALKALI: Yellow
What colour does universal indicator turn in acidic/alkaline solutions?
ACID: Red
NEUTRAL: Green
ALKALI: Purple
What is an acid in aqueous solution?
A source of H+ ions
What is an alkali in aqueous solution?
A source of OH- ions
How would you carry out a titration?
Use pipette to put 25 cm3Ā sodium hydroxide in conical flask and add indicator (must have sharp colour change)
Place conical flask on white tile, tip of burette in flask
Pour acid into burette using a funnel and take initial reading
Perform rough titration by running the acid in quite quickly, swirling the flask
Record end reading, make sure meniscus is at eye level
Repeat with new sodium hydroxide
Add acid one drop at a time near the end point
Stop as soon as colour of the indicator in the sodium hydroxide changes
Record final volume to nearest 0.05cm3
Repeat until two concordant results (within 0.2 cm3Ā of each other) to increase accuracy
Why must you constantly mix the flask when performing a titration?
To ensure everything reacts before more solution is added
Why do you put the conical flask on a white tile in a titration?
To clearly identify when the colour change has occurred
Why would you perform a titration?
To make a soluble salt. You can find out the exact volume of one solution it takes to neutralise an accurately measured known volume of another solution.
How would you repeat a titration to produce a pure soluble salt?
Repeat without indicator and add the concordant value of the volume required to react with the solution
HCl (aq) + NaOH (aq) ā NaCl (aq) + H2O (l)
How would you make this an ionic equation?
H+ + OH- ā H2O
What happens in a neutralisation between acids and bases?
If the base is insoluble it cannot release OH- ions in solution. A proton is transferred from the acid (proton donor) to the base (proton acceptor.)
Which salts are always soluble?
Sodium, potassium, ammonium salts and nitrates
Are chlorides soluble?
Yes except for silver and lead (II) chlorides
Are sulphates soluble?
Yes, except for barium, calcium and lead (II) sulphates
Are carbonates soluble?
No, apart from the ones that are always soluble (Na, K, NH4+).
What is an ammonium ion?
NH4+
Are hydroxides soluble?
No, except the ones that are always soluble (Na, K, NH4+). Calcium hydroxide is slightly soluble.
What is a precipitation reaction?
When two soluble salts are mixed
Acid + metal ā
Acid + metal ā Salt + hydrogen
Acid + metal oxide ā
Acid + metal oxide ā Salt + water
Acid + metal hydroxide ā
Acid + metal hydroxide ā Salt + water
Acid + metal carbonate ā
Acid + metal carbonate ā Salt + water + carbon dioxide
Examples of bases
Metal oxides, hydroxides and metal carbonates
What is a precipitate?
An insoluble product of two solutions reacting
How would you make an insoluble salt?
Precipitation reaction:
Mix together two separate aqueous solutions (one with required positive ion and one with negative ion)
Filter and keep precipitate
Rinse with distilled water
Leave to dry in a warm place
How would you make a soluble salt (not Na, K or NH4+)?
Excess base:
Acid in beaker, gently heat with tripod, gauze and Bunsen burner
Add an excess of the insoluble base (metal, metal oxide, hydroxide, carbonate) - it is in excess when no more disappears (dissolves) upon stirring
Filter off excess solid
Transfer filtrate to evaporating basin, heat until left with a saturated solution
Allow solution to cool and crystals to form
Filter out crystals, rinse with a little distilled water (not too much as it is soluble)
Dry crystals in warm place
Why do crystals appear as the saturated solution cools in the excess base method?
If the solution is saturated while hot, less can dissolve as it cools so crystals start to form
Why do we rinse the salt at the end?
To remove soluble impurities
Why is the mixture filtered in the excess base method?
To remove the excess of the excess base
Why shouldnāt you heat crystals to complete dryness?
The water of crystallisation would evaporate and the crystals would be anhydrous
Why add an excess of the base?
To ensure that all of the acid has reacted to obtain pure crystals
Why should you use indicator paper rather than aqueous indicator when preparing salts?
The indicator is an impurity
Why is effervescence (fizzing) observed when a metal or metal carbonate reacts with an acid?
Metal + acid ā Salt + hydrogen
Hydrogen is a gas so appears as bubbles
Ammonia (NH3) + acid ā
Ammonia (NH3) + acid ā Ammonium salt (NH4)
Note
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