chemical changes

what is an acid?

a substance that dissociates into H+ ions when dissolved in water or a substance that forms an aqeous solution with a pH less than 7

what is a base?

any substance with a pH greater than 7 that can react with an acid to produce a salt and water

what is an alkali?

a type of base that dissolves in water to form OH- ions

what are the two ways we can measure pH by?

- by using an indicator

- by dipping a pH probe in the solution and connecting it to a pH meter, this is more accurate as it eradicates human error by having to judge by the colour the indicator changes to

what pH is neutral/ pure water?

pH of 7

what colour is methyl orange in acidic and alkaline?

alkaline = yellow

acidic = red

what colour is litmus blue in alkaline and acidic?

alkaline = blue

acidic = red

what colour is phenolpthalein in acidic and alklaline?

alklaline = pink

acidic = colourless

Explain why all alkalis are bases but not all bases are alkali

Alkalis are soluble bases, but only some bases are soluble in water, so these aren't alkalis

Why do acid-alkalis neutralisation reactions form water

Because it's a reaction between H+ ions and OH- ions to form water

What's a neutralisation reaction

The reaction between an acid and a base to form a salt +water

What are the products formed in the reaction between an acid and a metal carbonate

A salt, water and carbon dioxide

What salt does hydrochloric acid form

A chloride salt

What salt does sulfuric acid form

A sulfate

What salt does nitric acid form

A nitrate salt

Why don't chemists make salts by reacting pure metals with acids

- the reaction between metals and acids forms flammable hydrogen, very dangerous

- unreactive metals such as copper don't react with dilute acids and very react

What's a strong acid

acids that ionise completely, all particles completely dissociate to release H+ ions

What's a weak acid

Acids that are only partially ionised, only some particles dissociate to release H+ ions, meaning it's a reversible reaction

What happens at a low pH that makes low pH so acidic

The acid dissociate fully so more H+ ions are released, so higher concentration of H+ ions

Each decrease of ___ on the pH scale represents the conc of H+ ions increasing by a factor of __

One, 10

metal oxide/hydroxide + acid —>

salt + water

metal carbonate + acid —>

salt +water + carbon dioxide

neutralisation steps

1. place dilute acid ( HCl) in a beaker and gently heat

2. then keep on adding insoluble base ( copper oxide) a little bit at a time

3. at first it will keep dissapearing as it reacts to form aqeous copper chloride and water

4. eventually the base will stop dissapearing so we know it’s in excess and now has neutralised all the acid

5. filter out the excess copper oxide and what we have left is the dissolved form of the soluble salt

What's reactivity

A measure of how vigorously a substance will react

What's formed when a metal reacts with water

a metal hydroxide and hydrogen gas

A metal below hydrogen in the reactivity series will/will not react with a dilute acid

Will not

What's formed when a metal reacts with a dilute acid

A metal salt and hydrogen

What is oxidation

The gain of oxygen or loss of electrons

What is reduction

The loss of oxygen or gain of electrons

what are redox reactions

reactions in which oxidation and reduction both take place

What are displacement reactions

When a more reactive metal displaces a less reactive one from its compound

What changes would you observe in a displacement reaction between magnesium and copper sulphate

- The blue colour of the solution fades as blue copper sulphate solution is replaced by colourless magnesium sulphate solution

- copper at the bottom of the beaker

How do you write an ionic equation

- Write out all the ions in each compound separately

- remove the spectator ion(s) from the aqueous solution

When do we extract a metal from its compound using carbon and how

If it's lower than carbon in the reactivity series and we reduce the metal oxide by reacting it with carbon to form pure metal and carbon dioxide

why do metals such as gold not need to be extracted

it's unreactive so it’s found uncombined in nature (on its own)

What's an ore

A rock that contains enough of a metal or metal compound to make extracting the metal worthwhile

What are the products formed when a metal oxide is reduced with carbon

Pure metal and carbon dioxide

What four metals are found uncombined in nature

Gold,platinum, copper, silver

When do we extract a metal using electrolysis

If it's higher than carbon in the reactivity series

what is electrolysis?

the breaking down of an ionic compound using electricity

why must the ionic compound be molten?

so that the ions are free to move and carry a charge

what happens at the anode for the electrolysis of molten aluminium oxide Al₂O₃

- the negative oxygen ions are attracted to the positive anode

- oxidation takes place as they lose electrons to form oxygen gas as bubbles are observed

- the electrons travel along the wire to the cathode

what happens at the cathode for the electrolysis of molten aluminium oxide Al₂O₃

- the positive aluminium ions are attracted to the negative cathode

- reduction takes place as the aluminium ions gain electrons to form pure molten aluminium

when is electrolysis used to extract metals?

if the metal is too reactive to be reduced with carbon or if the metal reacts with carbon

why must the graphite electrodes be replaced regularly in electrolysis of aluminium oxide?

because the oxygen discharged at the anode reacts with the carbon in the graphite to form carbon dioxide causing the carbon anode to burn away

why must aluminium be mixed with cryolite before it can be separated using electrolysis?

aluminium oxide has a very high melting point so it’a first dissolved in molten cryolite producing an electrolyte with a much lower melting point, this reduces the energy and therefore cost needed for the procedure

why must the electrode be made of graphite in electrolysis of aluminium oxide

- graphite is inert

- graphite can conduct electricity as each carbon atom has a delocalised electron which is free to flow and carry charge through the structure

why does electrolysis of aqeous solutions contain hydrogen ions and hydroxide ions?

because aqeuous solutions contain water, and water dissociates to form H+ and OH-

what is the rule for the ion that gets discharged at the cathode for electrolysis of aqeous solutions?

the ions of the least reactive element are formed so either a metal or hydrogen gas is formed

what is the rule for the ion that gets discharged at the anode for electrolysis of aqeous solutions?

halide ions are discharged, if there isn’t a halide ion present then the hydroxide is discharged

what is the half equation for if the the hydroxide ion was discharged at the anode for electrolysis of aqeous solutions?

4OH- —> 2H2O + O2 + 4e-

half equation at anode of electrolysis of alumnium oxide

2O^2- → O₂ + 4e^-

half equation at cathode of electrolysis of aluminim oxide

Al³⁺ + 3e^- → Al