C4: Chemical Changes

Question 1

define oxidation and reduction

Answer 1

oxidation: when a substance gains oxygen / loses electrons
reduction: when a substance loses oxygen/ gains electrons

Question 2

what is the reactivity series of metals?

Answer 2

A list of elements ordered by their reactivity

Question 3

what are the trends in reactivities of metals in reactions with acids/water?

Answer 3

• metals above hydrogen in reactivity series react with acid to produce hydrogen
• The more reactive the metal is, the quicker and more violent reaction with acid occurs
• metals below H2 dont react with acids
• not all metals above H2 react with water

Question 4

Write the reactivity series from
most reactive to least reactive

Answer 4

potassium
sodium
calcium
magnesium
aluminium
carbon
zinc
iron
tin
lead
hydrogen
copper
silver
gold
platinum

Question 5

what is a displacement reaction?

Answer 5

• a reaction where a more reactive metal displaces a less reactive metal from a compound

Question 6

how are unreactive metals found in Earth?

Answer 6

in their natural state

Question 7

Explain why zinc can be
extracted from zinc oxide with
carbon but magnesium cannot
be extracted from magnesium
oxide with carbon

Answer 7

Magnesium is more reactive than carbon (1), Zinc is less reactive than carbon (1), Carbon can therefore remove oxygen from zinc oxide but not magnesium oxide (1

Question 8

What process is used to extract
metals more reactive than
carbon

Answer 8

Electrolysis

Question 9

how can metals less reactive then carbon be extracted?

Answer 9

• reduction with carbon
• Carbon displaces the metal in a metal oxide
• Carbon gets oxidised to form carbon oxides .
• Metal from the metal oxide gets reduced to the pure metal
• eg Copper oxide + carbon → copper + carbon dioxide

Question 10

what is the general equation for a reaction between metals and acids? what type of reaction is this?

Answer 10

metal + acid –> salt + hydrogen
redox reaction, also a displacement reaction

Question 11

which metals in the reactivity series will react with acid?

Answer 11

those above hydrogen

Question 12

what is the general equation for a neutralisation reaction?

Answer 12

Base + acid –> salt + water

Question 13

what is the general equation for the reaction between metal carbonate and acid?

Answer 13

• metal carbonate + acid—>salt + water + carbon dioxide

Question 14

what is the general equation for the reaction between metal oxides and acids?

Answer 14

metal oxide + acid—> salt + water

Question 15

what is a redox reaction?

Answer 15

• a reaction where both oxidation and reduction occurs

Question 16

explain in terms of gain or loss of electrons which species has been oxidised and which species has been reduced when magnesium reacts with hydrochloric acid

Answer 16

magnesium has lost electrons and thus has been oxidised (Mg to MG2+)
- the hydrogen in HCL has gained electrons and thus has been reduced (H+ to H2)

Question 17

how is a soluble salt formed?

Answer 17

1. react the excess acid with some insoluble chemical (e.g. metal oxide)
2. filter off the leftovers
3. Crystallise the product

Question 18

what do acids and alkalis produce in aqueous solutions?

Answer 18

• acids produce hydrogen ions, alkalis produce hydroxide ions

Question 19

define bases , acids and alkalis?

Answer 19

• bases are compounds that neutralise acids
• acids produce hydrogen ions in aqueous solutions
• alkalis are soluble bases - produce hydroxide ions in aqueous solutions

Question 20

what is the pH scale and what does a pH of 7 show?

Answer 20

• the measure of acidity/ alkalinity of a solution
• a neutral solution

Question 21

state the general equation for a neutralisation reaction in a short, ionic form

Answer 21

H* + OH- –> h20

Question 22

what is a strong acid? what is a weak acid?

Answer 22

• strong acid is completely ionised in aqueous solution
• weak acid is only partially ionised in aqueous solution

Question 23

what happens to pH as concentration of H+ increases?

Answer 23

• the pH decreases

Question 24

what is a concentrated acid and what is a dilute acid? is this the same as a strong and weak acid?

Answer 24

• concentrated acid has more moles of acid per unt volume than dilute acids
• it is not the same - strength refers to whether the acid is completely ionised in water

Question 25

As the pH is decreased by one unit , what change is seen in the hydrogen ion concentration

Answer 25

increases by a factor of 10

Question 26

name the following salts: - LiNO3 - K2CO3 - MgBr2 - BaSO4

Answer 26

- lithium nitrate - potassium carbonate - magnesium bromide - barium sulfate

Question 27

what is electrolysis?

Answer 27

- the passing of an electric current through ionic substances that are molten to break them down into elements - ions are discharged (they lose/gain electrons) at electrodes to produce these

Question 28

what is an electrolyte?

Answer 28

the liquid/solution which conducts electricity

Question 29

what is a cathode and what is an anode?

Answer 29

cathode is the negative electrode anode is the positive electrode

Question 30

what occurs at the cathode and what occurs at the anode during electrolysis?

Answer 30

reduction occurs at the cathode oxidation occurs at the anode

Question 31

in aqueous electrolysis, which element is discharged at the cathode?

Answer 31

- the less reactive element discharges at the cathode. - hydrogen is produced unless there is a less reactive metal - in which case the said metal is produced

Question 32

how is aluminium manufactured? why is it expensive?

Answer 32

- aluminium is made through the electrolysis of aluminium oxide and cryolite - lots of energy is needed to produce the current in electrolysis which makes this process expensive

Question 33

what are the half equations in the extraction of aluminium?

Answer 33

cathode: Al3* + 3e- --> Al(cathode) anode: 2O 2- → O 2 + 4e -( anode) oxygen reacts with Carbon of the anode producing CO2-

Question 34

why is cryolite used in manufacturing of aluminium

Answer 34

- it lowers the melting point of aluminium oxide, reducing energy costs

Question 35

what are the half equations in electrolysis of the aqueous Na2SO4?

Answer 35

2H* + 2E- --> H2 (cathode) 4OH- --> 2H20 + 02 + 4e- (anode)

Question 36

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Answer 36

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Question 37

what are the half equations in electrolysis of the aqueous CuBr2

Answer 37

Cu2* + 2e- ---> Cu (cathode) 2Br- --> Br2 + 2e-

Question 38

define an ore

Answer 38

material containing enough metal in it for it to be economically worthwhile to extract the metal.

Question 39

give the empirical formulae for - hydrochloric acid - sulfuric acid - nitric acid

Answer 39

- HCl - H₂SO₄ - HNO₃

Question 40

describe what occurs at the anode in the electrolysis of aqueous solutions

Answer 40

- negatively charged OH– ions and non-metal ions are attracted to the positive electrode - If halide ions (Cl-, Br-, I-) and OH- are present then the halide ion is discharged at the anode, loses electrons and forms a halogen (chlorine, bromine or iodine) - If no halide ions are present, then OH- is discharged at the anode, loses electrons and forms oxygen

Question 41

describe what occurs in the cathode in the electrolysis of aqueous solutions

Answer 41

- Positively charged H+ and metal ions are attracted to the negative electrode - If the metal is above hydrogen in the reactivity series, then hydrogen will be produced and bubbling will be seen at the cathode