Chapter 23.4 Till 23.6 Half Cells Potential Etc

Question 1

Convention for half equation direction in half cells?

Answer 1

Forward reaction is REDUCTION, so gain in electrons

Question 2

How are standard half cells made?

Answer 2

With a METAL ROD dipped in its AQEUOUS METAL SOLUTION

So like copper metal dipped in Cu2+ solution

Question 3

What about ion / ion half cells? How are they made.(2)

Answer 3

Both ion soltuons are put in, and this time an INERT METAL ROD of PLATINUM IS USED

This acts as the thing that allows electrons to move around ti complete the circuit

Question 4

So what’s the idea behind usign two half cells to generate electricity?

Answer 4

We want a flow of electrons, so one half cell that oxidised and loses the electrons, and another half cell that reduces and gains these electrons, which causes a FLOW of electrons

Question 5

What is the electrode potential definition (1)

How to remember

Answer 5

This is the tendency to be REDUCED and gain electrons

Can remember this bevause we know more negative becomes oxidised, so yeah the higher the more reduced.

Question 6

What is the STANDARD electrode potential of something, like copper, by definition

Answer 6

Is the emf of a half cell measured when connected to a STANDARD hydrogen half cell with everything used in STANDARD CONDITIONS

Question 7

How is a standard hydrogen cell used
- describe all features

Answer 7

• a glass tube with holes in it
• here hydrogen gas is bubbled into it
• then there is a wire going in the tube and on the end is inert PLATINUM ELECTRODE
  -SOLUTION is h+ ions

Everything is in STANDARD CONDTIONS

Question 8

What are the standard condtions used when doing standard electrode experiments

3 points

Answer 8

Every liquid = 1moldm-3

TEMPERTAURE = 298k 25°

And the pressure , like of the hydrogen gas = 100KPA

Question 9

As hydrogen is set as the standard, by definition, what is the electrode potential of hydrogen and then what will happen when you connect other things

Answer 9

By definition hydorgen has a potential of 0v

Everything else, based on their relative tendency to be reduced, will be negative or psotiobe compared to hydrogen

Question 10

Structure if any half cell set up?
What extra stuff they need

What condtions shojld everything be set up as

Answer 10

1) connected by a wire and voltmeter to allow FLOW OF ELECTRONS, and to measure emf

2) SALT BRIDGE needed to ALLOW FLOW OF IONS

3) , standard conc if it’s metal / ion
If it’s ion/ion then SAME CONC

• everything standard temp and if gas used standard pressure

Question 11

What is the salt bridge made from and why neeeded

Answer 11

Made from strip of filter paper soaked in AQEUOUS POTASSIUM NITRATE KNO3

Needed to allow flow of IONS

Question 12

More negative vs positive electrode potential means what will reduce and what will oxidise?
2) so using data of electode potentials how to balance equation

Answer 12

More negative = lose = oxidation tendency

And positive = reduction

2) check which is more negative, choose rhe oxidation, and then balance

Question 13

What are normally the type that gain electrons and what normally lose , metals v non metals

Answer 13

Metals are normally the ones that LOSE electrons and thus negarive

Non metals normally GAIN electrons and thus positive

Question 14

So what does more negative and more positive mean for reactivity wise for metals and non metals

Answer 14

If there is a MORE negative metal compared to another, it means it is MORE REACTIVE in losing electrons

And if there is more POSITIVE non metal , means more reactive in GAINING ELECTRONS

Question 15

How to decide which electrodes are the positive or negative ones quickly

Answer 15

More positive electrode potential = that’s gonna be the one that is positive

Why? Well because this is reduced, so it’s gaining electrons, and these are coming from a MORE negative one to a positive r one, that’s why the negative o potential is negative electrode and positbe positbe

Question 16

Which ekectode Porte tail gives rise to biggest reducing or oxidising agent = how to find it out

Answer 16

Biggest reducing agent is the one that gets OXIDISED RHE MOST

This one would be the MOST NEGATIVE electode potential

Same way other way around

Question 17

How to find the standard CELL potential from standard electrode potentials

Where will this show?

Answer 17

Would be the positive - negative

So positive + mod negative ,
2) this is what will show on voltmeter

Question 18

How ti justify why one thing will be oxidised or reduced exam technique (proper wording)

Answer 18

Say it has a more negative electode potential, so it has greater tendency to LOSE electrons and become oxidised compared to the other one, which will be reduce d

Question 19

What are the three ish LIMITATIONS of using standard electrode potential to predict the cell potential?

Answer 19

1) does not take into account kinetic behaviour such as Rates
2) concentrations used may be higher or lower changing e cell
3) other conditions may not be standard

Question 20

How does having a higher or lower concentration of the substance effect e cell
1) what does it do to the EQUILIBIRUM
2) what does that do depending on which way equilibrium went on the electrode potential?

IMPORTSNT

Answer 20

1) first decide WHERRE a higher or lower conc will Shift the equilibrium, left or right , based on le Chatellier
2) if it causes electrons to be USED UP, the cell will become LESS NEGATIVE.
3) if it causes electrons to be gained, becomes more negative,

Based on which cell became more negative/ less, e cell will go up or down

Question 21

Full breakdown into 3 things that are limitations

Answer 21

1) doesn’t take into account like gibbs the kinetics of the system, might have reallt slow rate so e cell might be different
2) if conc is not 1moldm-3, it won’t be the same e cell

3) if not standard condtions, won’t be either

Question 22

What are the three type of modern day cells

Answer 22

Primary
Secondary
Fuel cells

Question 23

What are primary cells
1) what technology is used here
Why do they run flat? And how

Answer 23

Primary cells are NIN RECHARGABLE one time use cells
- they run on same technology as two half cells

2) problem is as chemicals are used up, voltsge falls and battery goes flat

Question 24

Why in primary cells and a normal half cell setup will the battery go flat?

Answer 24

NEED TO ANSWER

Question 25

What are secondary cells
1) rechargeable or not
2) what tehcnogly do the use ()

Answer 25

These ARE rechargable

2) technology is that the cell reaction producing the energy can be REVERSED during recharging, whereas primary cell it can’t

Common rechargeable battery = lithium

Question 26

Why can a secondary cell be recharged?

Answer 26

This is because the cell reaction that produces energy can be reversed during RECHARGE

Therefore chemicals needed regenerated and it can produced electrical energy again

Question 27

What are benefits and risks of using batteries, such as lithium or without

MARKSCHEME QUESTION

Answer 27

Benefit
- that you can use them in small spaces, and make compact machines etc

Risk
- lithium batteries very reactive and have IGNITED IN THE PAST
- some chemicals are toxic in batteries in disposal

Question 28

What is a fuel cell

Key point cover

Answer 28

A fuel cell is a cell that uses the energy from a fuel burning with OXYGEN to create a voltage

Question 29

What are the features of a fuel cell 3

Answer 29

Fuel + oxygen flow into the fuel cell and products flow out, but the electrolyte stays in the cell

2) fuel cells can keep on working provided replaced with fuel and oxygen

3) there is no RECHARGE process that reverses the reaction, can be CONTINOUSLY USED

Question 30

What fuel is most typically used for fuel cells and why

Answer 30

Typically it’s hydrigen because by product is just water

But anything can be used

Question 31

Cells summary

Answer 31

Primary = non rechargeable, one time use, same technology as two half cells, once chemicals used up goes flat

Secondary = rechargeable , process producing energy can be reversed to regenerated chemicals needed in reaction, lithium most common but dangerous as can ignite

Fuel= uses energy produced with fuel react with oxygen, fuel and oxygen in fuel cell products release but electrolyte remains, can continuously work with fuel and oxygen being provided (DOESNT need to be recharged)

Most common one is hydorgen for its environmentally good by products