Section 3- Equations, Calculations and Electrolysis P2

Question 1

What is the concentration of a solution?

Answer 1

Concentration is the ‘Amount of stuff’ per unit volume

Question 2

What is the unit for measuring concentration?

Answer 2

moles per dm³ (mol/dm³)

-sometimes measured in grams per dm³ (g/dm³)

Question 3

What is the formula used to work out concentration?

Answer 3

concentration (mol/dm³) =

number of moles ÷ volume of solution (dm³)

Question 4

How do you convert from mol/dm³ to g/dm³?

Answer 4

• work out RFM of the chemical in question

- convert the concentration in moles into concentration in grams -> mass (g) = moles x RFM

Question 5

What does Avogadro’s Law state?

Answer 5

That one mole of any gas occupies 24 dm³ at room temperature and pressure

Question 6

What is the 1 dm³ in cm³?

Answer 6

1000 cm³

Question 7

What formula can you use to work out volume?

Answer 7

volume (dm³) = moles of gas x 24

Question 8

How can you calculate volumes in reactions if you know the masses?

Answer 8

• write out the balanced equation
• work out the RFM of the substances in question
• calculate the number of moles [moles = mass ÷ RFM]
• look at the mole ratio to work out the number of moles of the elements of the equation
• calculate the volume [volume = moles x 24]

Question 9

What does electrolysis involve?

Answer 9

Electrolysis involves oxidation and reduction

Question 10

What is electrolysis?

Answer 10

Electrolysis is the breaking down of a substance using electricity.
-an electric current is passed through an electrolyte causing it to decompose

Question 11

Define an electrolyte:

Answer 11

a molten or dissolved ionic compound

Question 12

Whats happens in electrolysis?

Answer 12

oxidation and reduction
-cations go to the cathode and are reduced
-anions go to the anode and are oxidised
as ions gain or loose electrons they form uncharged substances and are discharged from the electrolyte

Question 13

What is the cathode?

Answer 13

the cathode is the negative electrode

Question 14

What is the anode?

Answer 14

the anode is the positive electrode

Question 15

What are ionic half equations?

Answer 15

They are equations which show how electrons are transferred during a reaction (shows oxidation or reduction)

Question 16

How do you write a half equation?

Answer 16

• put one of the things being oxidised or reduced on one side of the arrow, and the thing it gets oxidised or reduced to on the other
• balance the number of atoms like in a normal equation
• add the electrons (e⁻) to one side or the other to balance the charges

Question 17

Why can molten ionic compounds be electrolysed?

Answer 17

Because the ions can move freely

Question 18

What happens to molten ionic compounds in electrolysis?

Answer 18

• the +ve ions are attracted to the -ve cathode, at the cathode the ions gain electrons and are reduced
• the -ve ions are attracted to the +ve anode, at the anode the ions lose electrons and are oxidised

Question 19

What are the electrodes used in electrolysis made of?

Answer 19

They are made from an inert (unreactive) material so they don’t take part in the reaction

Question 20

Describe a practical for how to set up an electrochemical cell for an aqueous solution:

Answer 20

• get two inert electrodes, and clean them using emery paper
• from now on don’t touch the surface of the electodes with your hands (so that they don’t become messy)
• place both electrodes into a beaker filled with your electrolyte
• connect the electrodes to a power supply using crocodile clips, when you turn the power supply on a current will flow through the cell

Question 21

What is an electrochemical cell?

Answer 21

an electrochemical cell is a circuit made up of the anode, cathode, electrolyte, a power source and the wires that connect the two electrodes

Question 22

What are examples of materials used for the electrolysis of an aqueous solution?

Answer 22

graphite

platinum

Question 23

What does the electrolysis of an aqueous solution also involve?

Answer 23

ions from water

H⁺ and OH⁻ ions

Question 24

What happens at the cathode for the electrolysis of an aqueous solution?

Answer 24

• the H⁺ ions and the metal ions are present
• hydrogen gas will be produced if the metal is more reactive the hydrogen, otherwise if the metal is less reactive than hydrogen it will form at the cathode

Question 25

What happens at the anode for the electrolysis of an aqueous solution?

Answer 25

• the OH⁻ ions and the anions are present

- if the anion present is a halide ion it will be formed at the anode, otherwise oxygen will form at the anode

Question 26

What is the half equation for oxygen at the anode during electrolysis?

Answer 26

4OH⁻ -> O₂ + 2H₂O + 4e⁻
or it can also be written as
2H₂O -> 4H⁺ + O₂ + 4e⁻