Topic 3 - Chemical Changes

Question 1

What ions are present in all acids?

Answer 1

H+

Question 2

What ions are present in all alkalis

Answer 2

OH-

Question 3

Whats the link between pH and and the concentrations of ions in acids and alkalis

Answer 3

When pH is 7 the concentration of hydrogen ions is equal to hydroxide ions

Question 4

What is the link between hydrogen ion concentration and pH

Answer 4

higher concentration of hydrogen ions means more acidic so pH decreases.

Question 5

Whats the difference between a dilute and concentrated acids?

Answer 5

Concentrated is how much much acid there is (in mol dm-3) in 1dm3 of water
-large number of acid molecules compared to volume of water means more concentrated.

Question 6

Whats the difference between a strong and weak acid?

Answer 6

strong acids ionise almost completely in water, large proportion of acid molecules release H+ ions bringing down pH
weak acids dont ionise fully or much in water so only small amount of H+ ions released so higher pH

Question 7

How does a base react in a neutralisation reaction

Answer 7

when a base neautralises the acid is makes the number of H+ ions = OH- which forms H20 (water)

Question 8

What happens when an acid reacts with a metal oxide? with a word equation example

Answer 8

Salt and water are produced

Hydrochloric acid + copper oxide -> copper chloride +water

Question 9

How do you make a soluble salt from an acid and an insoluble salt

Answer 9

• heat acid in water bath to speed up reaction in fume cupboard
• add the insoluble base to the acid
• these will react until finished and excess has sunk
• filter the excess solid out
• heat solution gently until see crystals form and then filter and leave to dry.

Question 10

(In terms of a base) what is an alkali

Answer 10

a soluble base

Question 11

What reaction takes place between an acid and an alkali and what is formed?

Answer 11

Neutralisation forms a salt and water

Question 12

What happens to the ions during neatralisation?

Answer 12

The H+ ions from the acid and the OH- ions from the alkali both neautralise each other and join to form H2O

Question 13

why is titration used to prepare soluble salts?

Answer 13

because you can add a set volume of acid to the cm2 to an alkali in a neautralisation reaction. when the indicator turns light pink you know it has neautralised. this solution can then be evaporated to give a soluble salt

Question 14

What is produced by reacting an acid and a metal

Answer 14

salt and hydrogen

Question 15

what is produced when reacting an acid and a metal carbonate?

Answer 15

salt water and carbon dioxide

Question 16

how can we test for hydrogen

Answer 16

squeaky pop test

Question 17

how do you test for carbon dioxide

Answer 17

Lime water going cloudy

Question 18

How do you make a sample of insoluble salt

Answer 18

Add together two soluble salt solutions that are dissolved in water
then for example if using leadnitrate and sodium chloride the lead chloride should precipitate out. then you filter out the mixture (swill with deionised water to get all precipitate out) then scrape insoluble salt off the paper

Question 19

What is an electrolyte

Answer 19

a molten or dissolved ionic compound

Question 20

What happens in electrolysis

Answer 20

The breaking down of a substance using electricity so an electric current is passes through and electrolyte causing it to decompose

Question 21

which electrode do metals go to during electrolysis

Answer 21

to the cathode - negative anode

Question 22

what forms at the anode

Answer 22

non metals or if its a sulfate or if no halide ions present then oxygen formed at the anode

Question 23

Are the anions oxidised or reduced

Answer 23

oxidised they lose electrons

Question 24

are the cations oxidised or reduced

Answer 24

reduced they gain electrons and together with anions create a charge through the circuit

Question 25

What is the half eqaution at the cathode for a metal

eg lead

Answer 25

Pb2+ + 2e- -> Pb

Question 26

what is is the half equation for bromine a non metal at the anode

Answer 26

2Br- -> Br2 +2e-

Question 27

How do you set up electrolyis of a solution

Answer 27

get two inert electrodes and clean with emery paper
place electrodes into beaker filled with electrolyte
connect electrodes to power supply with crocodile clips and wires so current flows through electrochemical cell

Question 28

How do you carry out electrolysis on a molten ionic substance

Answer 28

Put solid ionic substance into crucible and heat with a bunsen burner until molten and carry out in fume cupboard to stop fumes escaping
when molten put two clean inert electrodes into the electrolyte and connect to power supply.

Question 29

What products are formed by electrolysis of:
copper chloride solution
sodium chloride solution
sulfate solution
acidified water
molten lead bromide

Answer 29

Copper chloride = Copper at cathode , chlorine at anode
sodium chloride solution = sodium at cat ,chlorine at an

sulfate solution = metal at cathode, oxygen at an
acidified water= hydrogen at cath , oxygen at anode

molten lead bromide= Lead at cathode, bromine at an

Question 30

How can electrolysis of copper sulfate using copper electrodes be used to purify copper

Answer 30

(Copper is less reactive than hydrogen so is produced at cathode)

• anode starts off as lump of copper and cathode is thin
• impure copper anode is oxidised and dissolves into electrolyte to form copper ions
• copper ions are reduced at copper cathode as layer of pure copper
• any impurities sink to bottom from anode as sludge

Question 31

Which electrode does oxidation occur at

Answer 31

anode

Question 32

Which electrode does reduction occur at

Answer 32

cathode (ions form atoms)

Question 33

Describe some common metals reacting with acid and water

Answer 33

Squeaky pop test for hydrogen
Magnesium has a loud squeaky pop in Dilute HCl
Aluminium
Zinc
Iron
Copper has no squeaky pop as it doesnt react

Question 34

Describe what happens in a reaction with a metal in salt solution

Answer 34

If the metal is more reactive that the metal in the salt it displaces it
eg) Copper sulfate + iron = iron sulfate + copper