Electrode Potentials Flashcards
What happens when a rod of metal is dipped into a solution of its own? (1)
An equilibrium is set up between the solid metal and the aqueous metal ions
What is an electrode/half cell? (1)
A strip of metal dipped into a solution of its own ions
What does the potential difference show and how? (3)
Show where the position of the equilibrium lies
- If there is a LARGE voltage the equilibrium is to the RIGHT
- If there is a SMALL voltage the equilibrium is to the LEFT
What is the simplest salt bridge made of? (1)
Filter paper soaked in Potassium Nitrate solution
Why are salt bridges necessary? (3)
Complete the circuit
Allows ion movements to balance the charge
Do not react with electrodes
Why is KNO3 a suitable solution for a salt bridge? (2)
KNO3 is unreactive with the electrodes AND the ions are free to move
Why can the voltmeter give a voltage reading? (1)
Prevents electrons flowing
Why might the current produced by a cell fall to zero after some time? (1)
All the reactants are used up
What will happen to a cell once the reactants are used up? (1)
Stops working OR starts to leak
Why is platinum a suitable electrode? (2)
Pt is unreactive AND conducts electricity
When is a platinum electrode used? (1)
When there is no solid metal in the reaction, such as when there are metal ions of two different charges in the same solution
What is the voltage of the Standard Hydrogen Half Cell? (1)
Zero
Describe a standard hydrogen electrode (4)
Hydrogen/H2 gas/bubbles
1.0 mol dm–3 H+
At 298K and 100kPa
Pt (electrode)
When is the the standard electrode potential value given? (1)
If the cell connected to the standard hydrogen electrode is also in standard condition
What is an electrochemical series? (1)
A series of chemical elements arranged in the order of their standard electrode potential
The standard electrode potential of Cu2+/Cu is 0.37 V. Why might the electrode potential of the following cell not be 0.37 V? (1)
The concentration of the CuSO4 solution is not 1 mol dm-3.
Why might other standard electrodes be used occasionally? (2)
Cheaper/easier/quicker to use and provide just as good a reference
Platinum is expensive
What does a vertical solid lines indicate in a cell notation diagram? (1)
Phase boundary
What does a double vertical line indicate in a cell notation diagram? (1)
Salt bridge
Equation for Emf (1)
more positive – least positive
What is the function of the platinum electrode? (1)
To allow transfer of electrons/provide a reaction surface
Give one reason why the emf of this cell changes when the electrodes are connected and a current flows? (3)
Concentration of the ions change
OR
Are no longer standard
OR
The emf is determined when no current flows
State why the electrode potential for the standard hydrogen electrode is equal to 0.00V? (1)
By definition
What factors will change E note value? (2)
Concentration of ions
Temperature
What happens if you reduce the concentration of ions in the left hand half cell? (4)
Equilibrium moves to the left to oppose the change of removing ions
This releases more electrons
The E note of the left hand half cell becomes more negative
So the emf of the cell increases
When is a reaction feasible? (1)
Ecell value is positive (above zero)
How are cells recharged? (2)
Reactions are reversible
Reversed by running a higher voltage through the cell then the cell E note value
Where are lithium ion cells used? (2)
Mobile phones
Laptops
Give an environmental advantage of using rechargeable cells (1)
Metals are reused
Give an environmental disadvantage of using rechargeable cells (2)
Mains electricity is used to recharge, which may come from combusting fossil fuels, which releases CO2(g)
What is a fuel cell? (3)
Generates an electrical current from electrochemical reactions.
A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
Does not require electrical recharging
Advantages of using fuel cells for energy instead of fossil fuels (2)
Greater efficiency than burning hydrogen in a combustion engine
Less-polluting as water is the only product
Disadvantages of using fuel cells for energy instead of fossil fuels (2)
H2 is difficult to store
Fossil fuels are combusted to produce the hydrogen, which releases carbon dioxide
Advantages of fuel cells compared to other types of cell (2)
Voltage is constant, as fuel and oxygen is supplied constantly
So concentrations of reactants remain constant.
Reactions that take place in an alkali hydrogen fuel cell (2)
At the anode (negative electrode):
H2(g) + 2OH-(aq) –><– 2H2O(l) + 2e-
At the cathode (positive electrode)
O2(g) + 2H2O(l) + 4e- –><– 4OH-(aq)
Reactions that take place in an acid hydrogen fuel cell (2)
At the anode (negative electrode):
H2(g) –><– 2H+(aq) + 2e-
At the cathode (positive electrode):
½ O2(g) + 2H+(aq) + 2e- –><–H2O(l)
How can the emf of a cell be kept constant (3)
Reagents supplied constantly
So the concentrations of ions are constant
E note value remains constant
Give an equation for the reaction that occurs at the positive lithium cobalt oxide electrode
Give an equation for the reaction that occurs at the negative lithium electrode (2)
Positive:
Li+ + CoO2 + e– → Li+[CoO2]–
Negative electrode:
Li → Li+ + e–
