Redox equilibria

Question 0

Electrochemical cells info

Answer 0

Can be made from 2 different metals dipped in salt solutions of their own ions and connected by a wire( the external circuit). The solutions are connected using a salt bridge. There are always 2 reactions within the cell, one is an oxidation and the other reduction reaction = redox process. Electrons flow through the wire from the most reactive metal to the least. A voltmeter in the external circuit shows the voltage between the 2 half-cells, called cell potential or e.m.f

Question 1

Electrochemical cells make…

Answer 1

Electricity

Question 2

Explain what happens in the zinc/copper electrochemical cell

Answer 2

Zinc loses electrons more easily than copper, so zinc (from the zinc electrode) is oxidised to form Zn2+ ions, releasing electrons into the external circuit. In the other half-cell the same number of electrons are taken from the external circuit reducing the Cu2+ ions to copper atoms

Question 3

What is a salt bridge used for, what is it made from and what does it do

Answer 3

It is used to connect 2 half-cell solutions, it is made from filter paper soaked in potassium nitrate (KNO3). It allows ions to flow through and balance out the charges

Question 4

Another example of half-cells

Answer 4

Involve solutions of 2 aqueous ions of the same element such as Fe2+/Fe3+. The conversion happens on the surface of the electrode. The electrode is made of platinum as it is an inert metal

Question 5

How easily a metal is oxidised is measured using…

Answer 5

Electrode potential. A metal that’s easily oxidised = very negative potential, one that’s harder to oxidise has less negative/ positive potential

Question 6

Important conventions when drawing electrochemical cells include…

Answer 6

The half-cell with more negative potential goes on left

The oxidised forms go in the centre of cell diagram

Question 7

Cell potential calculation

Answer 7

Electrode potential of right hand side - electrode potential of left hand side

Question 8

Why will the cell potential always be positive

Answer 8

Because you are subtracting the more negative value from the more positive vale

Question 9

Half-cell reactions are reversible. This means the equilibrium position is affected by changes in…

Answer 9

Temp
Pressure
Concentration

Question 10

Changing the equilibrium position in half-cell reactions changes the… How do you get around this

Answer 10

Cell potential

Get around it by using standard conditions to measure the electrode potentials, using these conditions means you will always get the same value for the electrode potential and you can compare values for different cells

Question 11

What are electrode potentials measured against

Answer 11

Standard hydrogen electrodes

Question 12

What is the standard electrode potential of a half-cell

Answer 12

The voltage measured under standard conditions where the half-cell is connected to a standard hydrogen electrode

Question 13

In half-cells, when using a standard hydrogen electrode, what is the electrode made of and why, where is the electrode found and what are the standard conditions

Answer 13

Made of platinum as you can’t have a gas electrode

electrode is always shown on the left

Standard conditions…
Any solution = concentration of 1.00moldm-3 eg HCl or HNO3
Temp must be 298k
Pressure must be 100kPa

Question 14

The standard hydrogen electrode half-cell has an electrode potential of…

Answer 14

0 volts

Question 15

What example provides evidence that electrons are transferred

Answer 15

The electrochemical cell, due to oxidation and reduction reactions and the flow of electrons through the external circuit

Question 16

What is the anticlockwise rule used for

Answer 16

To predict whether a reaction will occur and to show which direction it will go in

Question 17

Electrochemical cells are used as….

Answer 17

Batteries

Question 18

What is a common type of non-rechargeable cell and where are they found

Answer 18

Dry cell alkaline battery, found in things like TV remotes, torches, smoke alarms- gadgets that don’t use a lot of power or are only used for short periods of time

Question 19

Why is it not practical to reverse the half equations/ recharge non-rechargeable batteries

Answer 19

They can be made to run backwards under the right conditions, but trying to do this can make it leak or explode. This is because the zinc anode forms the casing of the battery, so becomes thinner and zinc is oxidised. Another reason is that the ammonium ions produce hydrogen gas which escapes from battery. Without hydrogen the ammonium ions cannot be reformed by reversing the reactions

Question 20

Examples of where rechargeable batteries are found, 3 types and how to recharge them

Answer 20

Mobile phones, laptops and cars.

Lead-acid, NiCad (nickel-cadmium) and L ion (lithium ion)

To recharge, a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions . This is possible as none of the substances escape or are used up.

Question 21

Advantages of non-rechargeable batteries

Answer 21

Cheaper than rechargeable batteries

Will work for longer than a rechargeable battery

Can be recycled, metal in them removed and used again

Less likely to contain toxic metals such as lead and cadmium = less hazardous in landfill if contents leak out and pollute water sources

Question 22

Disadvantages of non-rechargeable batteries

Answer 22

Have to be replaced every time they run out, so rechargeables cheaper in long run

Have to be disposed of once used as cannot be recharged

Can’t supply as much power, no use for devices that use a lot of power

More produced as they only last once so use more resources creating more waste, ends up in landfill as hardly any recycled

Question 23

Fuel cells info

Answer 23

In most cells the chemicals that generate the electricity are contained in the electrodes and the electrolyte that form the cell. In fuel cell, chemicals stored separately outside the cell and fed in when electricity required. Example is the hydrogen-oxygen fuel cell, can be used to power electric vehicles.

Question 24

How does the hydrogen-oxygen fuel cell work

Answer 24

Hydrogen and oxygen gases are fed into 2 separate platinum containing electrodes. Electrodes separated by an ion-exchange membrane that allows protons (H+ ions) to pass through it, but stops electrons going through it. Hydrogen is fed to the negative electrode. Electrons flow from negative electrode through external circuit to positive electrode. H+ ions pass through ion-exchange membrane towards positive electrode. Oxygen fed to positive electrode. Overall effect is that H2 and O2 react to make water

Question 25

What is the major advantage with fuel cells

Answer 25

They don’t need electrical recharging. As long as hydrogen and oxygen is supplied, the cell will continue to produce electricity. Another benefit = only waste product is water, so no nasty toxic chemicals to dispose of and no CO2 emissions from cell itself.

Question 26

Disadvantage of fuel cells

Answer 26

You need energy to produce the supply of hydrogen and oxygen. Can be produced from the electrolysis of water, requires energy which is normally generated from fossil fuels so not carbon neutral. Also hydrogen highly flammable so needs to be handled carefully when stored or transported

Question 27

What is a reducing agent

Answer 27

A species that donates electrons

Question 28

What is an oxidising agent

Answer 28

An agent that accepts electrons

Question 29

Why are hydrogen fuel cells not as ‘green’ as they appear

Answer 29

Most hydrogen is made from crude oil. Could be made by electrolysis of water but most electricity is made by burning fossil fuels which emit carbon dioxide. Expensive to store and transport hydrogen as specialist equipment needed

Question 30

The Roman numerals in a chemical names represent…

Answer 30

The oxidation number

Question 31

The oxidation state for an atom will ……. by 1 for each electron lost

Answer 31

Increase

Question 32

The oxidation state will ……. by 1 for each electron gained

Answer 32

Decrease

Question 33

Identify a solution that could be used in an electrochemical cell to connect the half cells. Give 2 reasons why this solution is suitable for its purpose

Answer 33

Potassium nitrate

It allows ions to flow to balance charges
AND
It doesn’t react with the solutions

Question 34

Explain environmental advantages of rechargeable cells compared with non-rechargeable cells

Answer 34

The compounds are reused, the supplies don’t deplete

Question 35

Suggest why ethanol is seen as a carbon neutral fuel

Answer 35

CO2 released by combustion is the same amount taken in through photosynthesis when the plant was growing