Chemical Changes

Question 1

What is the pH of a neutral substance?

Answer 1

pH 7

Question 2

What ions do acids form in water?

Answer 2

H+ ions - the more hydrogen ions, the more acidic it is so the lower the pH

Question 3

What’s the difference between a base and an alkali?

Answer 3

a base is a substance that reacts with an acid to produce a salt and water - an alkali is a base that is soluble in water

Question 4

What ions do alkalis form in water?

Answer 4

OH- ions - the more hydroxide ions, the more alkaline and so the higher the pH

Question 5

How do you measure the pH of a solution?

Answer 5

using an indicator - add a few drops to the solution and compare to the pH chart

Question 6

What are the types of indicators and what colours do they show?

Answer 6

• universal indicator (red-green-purple)
• litmus (red-purple-blue)
• methyl orange (red-yellow-yellow)
• phenolphthalein (colourless-pink in alkaline)

Question 7

What is neutralisation?

Answer 7

acid + base -> salt + water

Question 8

How do you investigate neutralisation?

Answer 8

1. set volume of dilute hydrochloric acid into a conical flask
2. fixed mass of calcium oxide into hydrochloric acid
3. wait for base to react, then record the pH of solution - using pH probe or universal indicator paper
4. repeat this until all the acid has reacted - when unreacted calcium oxide sits at the bottom
5. a graph can be plotted to see how pH changes

Question 9

How do strong and weak acids ionise in water?

Answer 9

• strong acids ionise almost completely in water - a large proportion of acid molecules dissociate to release H+ ions
• weak acids do not fully ionise in solution - the ionisation of a weak acid is a reversible reaction - setting up an equilibrium

Question 10

What is the difference between strong acids and concentrated acids?

Answer 10

• acid strength is the proportion of acid molecules that ionise in water
• the concentration is how much acid there is in a litre of water

Question 11

What is the strength of an acid?

Answer 11

acid strength is the proportion of acid molecules that ionise in water

Question 12

What is the concentration of an acid?

Answer 12

the concentration is how much acid there is in a litre of water

Question 13

How does changing concentration affect an acid’s pH?

Answer 13

if the concentration of H+ ions increases by a factor of 10 - the pH decreases by 1 (so if it increases by a factor of 100 - the pH decreases by 2

Question 14

When can salts be formed in a reaction?

Answer 14

salts (+water) formed during neutralisation reaction - reaction between acid and base

Question 15

What salt does hydrochloric acid form?

Answer 15

hydrochloric acid - chloride salts

Question 16

What salt does sulfuric acid form?

Answer 16

sulfuric acid - sulfate salts

Question 17

What salt does nitric acid form?

Answer 17

nitric acid - nitrate salts

Question 18

What is produced when an acid + metal oxide reacts?

Answer 18

Acid + Metal Oxide - Salt + Water

Question 19

What is produced when an acid + metal hydroxide reacts?

Answer 19

Acid + Metal Hydroxide - Salt + Water

Question 20

What is produced when an acid + metal reacts?

Answer 20

Acid + Metal - Salt + Hydrogen

Question 21

What is produced when an acid + metal carbonate reacts?

Answer 21

Acid + Metal Carbonate - Salt + Water + CO2

Question 22

How do you test for hydrogen?

Answer 22

test for hydrogen with a lighted splint - a “squeaky pop” will happen if hydrogen is present as hydrogen burns with the oxygen in the air to form water

Question 23

How can you test if a gas is carbon dioxide?

Answer 23

bubble the CO2 through limewater - it will turn cloudy with CO2

Question 24

Which salts are soluble in water?

Answer 24

(S P A N C S)

Sodium salts
Potassium salts
Ammonium salts
Nitrates
Chlorides (except silver/lead nitrate)
Sulfates (except lead/barium/calcium sulfate)
(sodium potassium ammonium carbonates/hydroxides)

Question 25

Which salts are insoluble in water?

Answer 25

(CHCS) Carbonates Hydroxides Chlorides of silver and lead Sulfates of lead, barium and calcium

Question 26

How can you make pure insoluble salts?

Answer 26

Using a precipitation reaction - react 2 soluble salts to form an insoluble salt e.g. lead nitrate (soluble) + sodium chloride (soluble) -> lead chloride (insoluble) + sodium nitrate

Question 27

What is the method of making insoluble salts? (imp)

Answer 27

1. spatula of lead nitrate (or other soluble salt) to a test tube with water (deionised so no other ions are present) - shake so dissolved 2. in another tube do the same with a spatula of sodium chloride (or the other soluble salt) 3. mix the two solutions together in a new beaker - lead chloride should form as a precipitate (or other insoluble salt depending on the two soluble salts) 4. put filter paper in a filter funnel and stick the funnel into a conical flask - filter out the precipitate (including swill out the beaker once empty to get out all the precipitate) 5. rinse contents of filter paper to get out all the soluble solution 6. scrape lead chloride onto fresh paper and leave to dry in oven or desiccator:)

Question 28

How can you make pure soluble salts?

Answer 28

by reacting an acid + insoluble base (containing the ions you want to form in the soluble solution) - this is often a metal oxide + metal hydroxide - by reacting an acid and an alkali

Question 29

What is the method of making soluble salts? (imp)

Answer 29

1. heat acid in a water bath in a fume cupboard (just to speed up reaction) 2. add the insoluble base to the acid which will react to form a soluble salt - you’ll know when the acid has been neutralised because there will be excess solid 3. filter off the excess solid to get a solution containing only salt and water 4. heat solution to slowly evaporate some of the water - leave to cool and allow the salt to crystallise - filter this off and leave to dry

Question 30

How can you make soluble salts using acid/alkali reactions? (method)

Answer 30

1. measure out set amount of acid into a conical flask and add indicator 2. slowly add alkali to the acid using a burette until the acid has been exactly neutralised (indicator changes colour) 3. carry out reaction using exactly the same volumes to completely neutralise the solution but without the indicator (so not contaminated) 4. solution will now contain only salt and water - evaporate some water and leave the solution to crystallise - filter off solid and dry it (it is not a pure dry salt

Question 31

What is electrolysis?

Answer 31

electrolysis is the breaking down of a substance using electricity

Question 32

How does electrolysis work? (basic)

Answer 32

an electric current is passed through an electrolyte (molten/dissolved iconic compound) causing it to decompose

Question 33

What happens to positive and negative ions during electrolysis?

Answer 33

- positive ions (cations) in the electrolyte/compound move towards the cathode (negative electrode) and are reduced (gain electrons) - negative ions (anions) in the electrolyte mode towards the anode (positive electrode) and are oxidised

Question 34

What happens once substances have reached the anode/cathode in electrolysis?

Answer 34

the ions gain or lose electrons to form uncharged substances and are discharged from the electrolyte

Question 35

What do half equations show?

Answer 35

they show how electrons are transferred during electrons (e.g. show what happens at electrodes during electrolysis)

Question 36

How do you write half equations?

Answer 36

1. put the thing being oxidised/deduced on one side and the thing it’s becoming on the other side 2. balance the equations as usual 3. add electrons (e-) on to one side to balance up the equations e.g. Na -> Na+ + e- 2H+ + 2e- -> H2

Question 37

What is an electrochemical cell?

Answer 37

a circuit with an anode, cathode, electrolyte, power sourde and wires to connect the two electrodes

Question 38

How do you set up an electrochemical cell for a solution?

Answer 38

1. get two inert electrodes e.g. graphite/platinum 2. clean electrode surfaces with emery paper 3. place electrodes into a beaker with electrolyte 4. connect the electrodes to power supple using crocodile clips and wires (so current can pass through)

Question 39

How do you set up an electrochemical cell for a molten ionic substance?

Answer 39

1. put the solid ionic compound in a crucible 2. heat crucible in fume cupboard until solid is molten 3. dip two clean inert electrodes into the electrolyte 4. attach electrodes to power supply and current will run through the cell

Question 40

Why can't ionic solids be electrolysed?

Answer 40

because the ions are in fixed positions - molten compounds have free ions so conduct electricity

Question 41

What are the products at the anode and cathode in molten solution?

Answer 41

the positive metal ions are reduced at the cathode - so METALS are usually at the CATHODE and the other thing (e.g. chloride -> chlorine) at the anode

Question 42

What are extra ions present in electrolysis of aqueous solutions?

Answer 42

``` hydrogen ions (H+) hydroxide ions (OH-) - these come from the water ```

Question 43

What happens at the cathode of electrolysis in a solution?

Answer 43

cathode: metal ions (just like in molten solution) and H+ ions from the water are present WHAT HAPPENS: if the metal is MORE reactive than hydrogen, then HYDROGEN gas is given off - if the metal is LESS reactive than hydrogen then pure METAL solid will be produced

Question 44

What happens at the anode of electrolysis in a solution?

Answer 44

anode: (usually) halide ions (like in molten solution) and OH- ions from the water are present WHAT HAPPENS: molecules of chlorine, bromine or iodine are formed (from the halide ions e.g. Cl-, Br-, I-) HOWEVER sometimes halide ions aren't present in the solution to begin with - in which case oxygen is formed

Question 45

What happens in the electrolysis of copper sulfate using inert electrodes?

Answer 45

1. copper sulphate solution contains: Cu(2+), SO4(2-), H(+), OH(-) (*brackets only there to make it more clear) 2. cathode: Cu(2+) and H+ - copper is less reactive than hydrogen so copper solid is produced 3. anode: SO4(2-) and OH(-) - there are no halide ions present here so oxygen is formed (you will see it bubbling)

Question 46

What happens in the electrolysis of copper sulfate using NON-inert electrodes?

Answer 46

using copper electrodes (instead of inert ones e.g. graphite/platinum): 1. as the reaction happens - copper (Cu2+) is transferred from the copper anode to the cathode (so the mass of the anode will decrease and the mass of the cathode will increase) - this may take longer to happen e.g. 30 mins 2. the electrical supply pulls electrons off copper atoms at the anode - turning the copper atoms into 2+ ions - the electrical supply then offers electrons st the cathode to nearby Cu2+ ions TO FIND THE MASS CHANGE: you can measure how the mass in electrodes change by finding the difference between the masses at the start and end - make sure they are dry before weighing

Question 47

When is electrolysis of copper sulfate with non-inert electrodes used?

Answer 47

1. copper is extracted from its ore by reduction with carbon - the copper made here is impure 2. electrolysis can then be used to purify the copper 3. during this: the anode starts as a lump of impure copper and the cathode starts as a thing piece of pure copper 4. the electrolyte (solution) used is copper sulfate 5. the impure copper at the anode is oxidised (gains electrons) - the anode will dissolve into the electrolyte to form copper ions 6. the copper ions are reduced (lose electrons) at the cathode - forming a layer of pure copper around the cathode 7. any impurities from the impure copper anode sink to the bottom of the cell and form a sludge