Unit 4 - Chemical changes Flashcards
(19 cards)
What is a displacement reaction?
When a more reactive metal displaces a less reactive metal from a compound
What do metals react with acids to form?
MASH - Metal + acid makes a salt + hydrogen
What do metal carbonates react with acid to form?
A salt, carbon dioxide and water.
What is the reactivity series?
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What happens when an acid reacts with an alkali?
A salt and water is produced
What happens when a substance dissolves?
The ions partially dissociate
How can we obtain crystals of a dissolved salt?
Evaporating the water
What does universal indicator measure?
Concentration of H+ and OH- ions, giving a pH
How does the pH scale work?
It is logarithmic, meaning it is non-linear. A change of 1pH is a 10x change in concentration of ions
What does a strong acid do?
Dissociates more completely
Why do we use titrations?
To measure what volume of acid/alkali is needed to neutralise to work out a concentration.
What are the steps for a titration?
- Use a glass pipette to measure 50cm3 of alkali of known conc and put in conical flask with known indicator (e.g. methyl orange, phenolphthalein)
- Put acid in burette up to the 0cm3 mark and let the acid drip into the flask.
- Once colour change has occurred (e.g. pink to colourless for phenolphthalein) stop and swirl. If it stays, neutralisation has occurred.
- Use moles to calculate the unknown concentration.
What is electrolysis?
Passing an electrical current through a substance to cause oxidation and reduction at the two electrodes. It can be used to obtain pure metals from a compound/ore.
Why do ionic compounds need to be molten or in solution for electrolysis?
The ions in the compounds must be free to move to the electrode, which only occurs when molten or in solution.
What happens to cations and anions in electrolysis of MOLTEN compounds?
Cations are always reduced at the cathode to produce metal atoms. Anions are oxidised at the anode
Why do the anodes need to be replaced when oxygen is produced?
The oxygen produced at the anode reacts with the carbon in the anode, producing CO2
Why is cryolite added?
To reduce the melting point, making it cheaper.
What happens to the cations in electrolysis of SOLUTIONS?
If they are more reactive than hydrogen, hydrogen ions will move to the cathode to be reduced, producing hydrogen atoms. If less reactive, the metal is produced (making an acid).
What happens to the anions in electrolysis of SOLUTIONS?
If there is a halide ion present, for example chloride, it is always oxidised at the anode. If the anion isn’t a halide, oxygen is oxidised and oxygen gas is produced.
