Chapter 23.4 Till 23.6 Half Cells Potential Etc Flashcards
Convention for half equation direction in half cells?
Forward reaction is REDUCTION, so gain in electrons
How are standard half cells made?
With a METAL ROD dipped in its AQEUOUS METAL SOLUTION
So like copper metal dipped in Cu2+ solution
What about ion / ion half cells? How are they made.(2)
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
So what’s the idea behind usign two half cells to generate electricity?
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
What is the electrode potential definition (1)
How to remember
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.
What is the STANDARD electrode potential of something, like copper, by definition
Is the emf of a half cell measured when connected to a STANDARD hydrogen half cell with everything used in STANDARD CONDITIONS
How is a standard hydrogen cell used
- describe all features
- 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
What are the standard condtions used when doing standard electrode experiments
3 points
Every liquid = 1moldm-3
TEMPERTAURE = 298k 25°
And the pressure , like of the hydrogen gas = 100KPA
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
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
Structure if any half cell set up?
What extra stuff they need
What condtions shojld everything be set up as
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
What is the salt bridge made from and why neeeded
Made from strip of filter paper soaked in AQEUOUS POTASSIUM NITRATE KNO3
Needed to allow flow of IONS
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
More negative = lose = oxidation tendency
And positive = reduction
2) check which is more negative, choose rhe oxidation, and then balance
What are normally the type that gain electrons and what normally lose , metals v non metals
Metals are normally the ones that LOSE electrons and thus negarive
Non metals normally GAIN electrons and thus positive
So what does more negative and more positive mean for reactivity wise for metals and non metals
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
How to decide which electrodes are the positive or negative ones quickly
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
Which ekectode Porte tail gives rise to biggest reducing or oxidising agent = how to find it out
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
How to find the standard CELL potential from standard electrode potentials
Where will this show?
Would be the positive - negative
So positive + mod negative ,
2) this is what will show on voltmeter
How ti justify why one thing will be oxidised or reduced exam technique (proper wording)
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
What are the three ish LIMITATIONS of using standard electrode potential to predict the cell potential?
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
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
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
Full breakdown into 3 things that are limitations
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
What are the three type of modern day cells
Primary
Secondary
Fuel cells
What are primary cells
1) what technology is used here
Why do they run flat? And how
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
Why in primary cells and a normal half cell setup will the battery go flat?
NEED TO ANSWER
What are secondary cells
1) rechargeable or not
2) what tehcnogly do the use ()
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
Why can a secondary cell be recharged?
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
What are benefits and risks of using batteries, such as lithium or without
MARKSCHEME QUESTION
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
What is a fuel cell
Key point cover
A fuel cell is a cell that uses the energy from a fuel burning with OXYGEN to create a voltage
What are the features of a fuel cell 3
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
What fuel is most typically used for fuel cells and why
Typically it’s hydrigen because by product is just water
But anything can be used
Cells summary
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
