Electrochemistry

Question 1

What is oxidation?

Answer 1

The loss of electrons

Question 2

What is reduction?

Answer 2

The gain of electrons

Question 3

What are electrochemical cells?

Answer 3

Cells that use the transfer of electrons during redox reactions to produce electrical energy

Question 4

What are electrochemical cells made of?

Answer 4

Two half cells that are connected via an external circuit like a conducting wire. Electrons flow accross the wire
A salt bridge, where filter paper is soaked in potassium chloride or potassium nitride, allowing ions to flow accross.
Under standard conditions

Question 5

What are the standard conditions that electrochemical cells are under?

Answer 5

All solitions are 1moldm3
100kpa
298K

Question 6

Describe a standard copper zinc cell

Answer 6

Tye copper electrode is the positive electrode and on the right, the zinc electrode is the negative and on the left. Electrons flow from zinc to copper

Question 7

Descrobe the flow lf electrons in an electrochemical cell, and where does oxidation and reeuction occur?

Answer 7

Electrons flow from the negative to positive electrode. Oxidation occurs at the negative electrode. Reeuctipn at the positive

Question 8

What is a half cell?

Answer 8

A cell that contains two species of the same element, with different oxidation states.

Question 9

What is the standard electrode potential?

Answer 9

The e.m.f of a half cell compared to a standard hydrogen half cell, under standard contitions

Question 10

What is the primary standard ekectrode potential?

Answer 10

The standard hydrogen electrode potential (0.00V) and is used to determine the potential lf other half cells

Question 11

Describe a hydrogen half cell

Answer 11

Contains 100kpa H2 gass
1 moldm3 H+ ions (HCl)
Pt electrode

Question 12

Describe the conventional representation of an electrochemical cell

Answer 12

A double verticle line indicates a salt bridge
A single,e verticle line indicates a phase boundary (change in state)
The species with the highest oxidation state for each half cell is written next to the salt bridge
Positive electrode is on the right and negative on the left (unless hydrigen is used in which case hydrogen is always on the left)

Question 13

What happens in an electrochemical cell if a half cell contains species whoch are both aqueous solutions?

Answer 13

A Pt electrode is used to ensure there is both an oxidised and reeuces species in the half cell

Question 14

In the electrochemical series, which species are the best reducing agents?

Answer 14

Species with the most negative potential on the right hand side

Question 15

In the electrochemical series, which are the best oxidising agents?

Answer 15

The species with the highest potential on the left hand side

Question 16

How do you calculate an electrochemical cells potential when hydrogen is not used?

Answer 16

Potential of the right species- potential of the left species

Question 17

When reading an electrochemical series, how do you know is two element will react with eachother?

Answer 17

Pne must be the best oxidising reagent, and another must be a best reducing reagent.
A species must be more negative than another to be able to reduce it
Or a species must be more positive than another to oxidise it

Question 18

How does a change in mass of electrodes predict redox reactions?

Answer 18

If the mass decreases, oxidation, electrode looses electrons so looses mass
If mass increases, reduction, electrode gains electrons so gains mass

Question 19

Does the concentration of a solution affect e.m.f?

Answer 19

Yes, of not standard conditions (1moldm-3) it will increase or decrease the emf

Question 20

What happens when the conc. at the positive electrode is > 1moldm-3?

Answer 20

Equilibrium shifts to the right
E becomes more positive because the forward reaction gains more electrons
Emf increases because the difference in E of both electrons increases

Question 21

What happens when the conc. at the positive electrode is < 1moldm-3?

Answer 21

Equilibrium shifts to the left
E of electrode is less positive because the forward reaction gains less electroms
Emf decreases because the difference between the E of both electrodes decreases

Question 22

What happens when the conc. at the negative electrode is > 1moldm-3?

Answer 22

Equilibrium shifts to the right
E of electrode is less negative because forward reaction looses fewer electrons
Emf decreases because difference of both electrodes E decreases

Question 23

What happens when the conc. at the negative electrode is <1moldm-3?

Answer 23

Equilibrium shifts to the left
E of electrode is more negative because the forward reaction looses more electrons
Emf increases because the difference in E of both electrodes is greater

Question 24

What are limitations for using electrode potentials for predicting redox reactions?

Answer 24

Predictions only tell us about the equilibrium, not how slow the rate is or how high the activation energy may be

Many lab reactions dont use standard conditions, so the electrode potential values are affected

Question 25

State three types of electrochemical cells

Answer 25

Non-rechargeable cells Rechargeable cells Fuel cells

Question 26

What are non-rechargeable cells?

Answer 26

Cells that can only be used once. The reactions cannot be reversed so eventually the chemicals are used up and the emf falls fo zero

Question 27

Give examples of non- rechargeable cells

Answer 27

Alkaline based zinc and manganese dioxide cells In smoke detectors and clock batteries

Question 28

What are rechargeable cells?

Answer 28

Cells than can be used more than once because the reactions can be revrsed so the chemicals in the reaction can be recharged

Question 29

Give examples of three rechargeable cells and where theyre used

Answer 29

Lithium ion cells in phones and laptops Lead-acid car batteries Ni-Cd cells in torches

Question 30

Why are lithium ion cells good to use in phones and laptops?

Answer 30

Because lithium is the least dense metal, so is very light

Question 31

What are fuel cells?

Answer 31

Electrochemical cells that use energy from the reaction between a duel and oxygen to create a voltage

Question 32

Descrobe the how a alkaline hydrogen- oxygen fuel cell works

Answer 32

Uses two poroud platinum electrodes, separated by a partially permeable membrane. Uses the electrolyte sodium hydroxide solution Hydrogen enters at the negative electrode: H2 + 2OH- —> 2e- + 2H2O Electrons flow to the positive electrode Oxygen enters at the positive electrode: 4e- + O2 + 2H2O —> 4OH- Overall eqation: 2H2 + O2 —> 2H2O

Question 33

What are the two half equations of an acid hydrogen oxygen fuel cell?

Answer 33

H2 —> 2H+ + 2e- O2 + 4H+ + 4e- —> 2H2O

Question 34

Why do alkaline and acid hydrogen hydrogen oxygen duel cells both have the same emf?

Answer 34

Because they have the same overall eauation 2H2 + O2 —> 2H2O

Question 35

What are three advantages of fuel cells?

Answer 35

They are the most efficient, as they convert more of their available energy into kinetic energy They do not produce Carbon dioxide They do not need to be recharged and produce a continuous supply of energy

Question 36

What are three disadvantages of fuel cells?

Answer 36

Hydrogen must be produced, whoch is done via the electrolysis of water which uses electricity from fossil fuel Hydrogen is highly flammable There is no technology to provode hydrogen fuel cells for cars

Question 37

What is the electrochemical series?

Answer 37

A list of electrode potentials in numerical order