1.11 Electrode Potentials Flashcards
(38 cards)
What happens when you place two different metals into a salt solution and join them together
Electrons will flow from more reactive metal to less reactive metal. The current produced can be used to power electrical devices
How can a half cell be constructed
- when a metal is dipped into a solution of its own ions (ions of the same element as the metal)
- or a platinum electrode with two aqueous ions
What happens when you dip Cu into a solution of Cu^2+
Cu -><- Cu^2+ + 2e^-
- equilibrium will take place on surface of solid metal
- The Cu2+ ions dissolve into the solution. This gives the solution a positive charge.
- The electrons collect into the Copper strip, giving it a negative charge
- This means a potential difference (i.e. a voltage) is established between the two.
How does potential difference affect equilibrium
- If there is a LARGE voltage the equilibrium is to the RIGHT
- If there is a SMALL voltage the equilibrium is to the LEFT
How are electrochemical cells formed
- made of two half cells
- to connect these two half cells you need wires, a high resistance voltmeter and a salt bridge
Why is Potassium Nitrate (KNO3) suitable for a salt bridge
KNO3 is unreactive with electrodes and the ions are free to move
The electrons will try to flow from the most reactive metal (which is always the left electrode) to the least reactive metal (which is always the right electrode).
The voltmeter prevents electrons flowing – this enables the voltage to be measured.
What happens if you remove the voltmeter ?
If the volt meter was removed electrons can flow from the left electrode to the right.
If the voltmeter is replaced with an ammeter or a bulb etc., the electrons can flow and a current is produced.
Q1: Why might the current produced by a cell fall to zero after some time?
Q2: What will happen to a cell once the reactants are used up?
A1: All the reactants are used up
A2: stops working OR starts to leak
In electrochemical cells, redox occurs. For the cell undergoing oxidation what can you observe
This electrode will become thinner as more ions are going to be produced to make electrons
True or false, is the left electrode always the negative electrode
True
as electrons are produced there, oxidation always occurs
so electrons flow from left to right
Zn(s) + Cu2+(aq) –> Cu(s) + Zn^2+(aq)
This reaction would continue generating an electrical current until either:
- Cu2+ ions in solution were used up
- solid Zinc was completely reacted
Why is platinum a suitable electrode
Pt is unreactive and conducts electricity
Will the most negative half cell undergo oxidation or reduction
negative = oxidation
positive = reduction
Use of standard hydrogen electrode
The potential of all electrodes are measured by comparing their potential to that of the standard hydrogen electrode.
The standard hydrogen electrode (SHE) is assigned the potential of 0 volts.
Conditions of a standard hydrogen electrode
H2 gas is pumped in at a pressure of 100kPa
2. The electrolyte contains H+ ions of concentration 1moldm-3. (e.g. HCl of conc 1moldm-3/ H2SO4 of 0.5moldm-3)
3. There must be a Platinum electrode
4. The whole system must be at a temperature of 298K
Why will Ag^+ (aq) ions react with Li(s)
Eᶿ Ag+(aq) > Eᶿ Li(s) AND Ag+(aq) is a stronger oxidising agent than Li(s)
What will be observed when Ag+(aq) ions react with Li(s)?
Solid Ag forms
What could happen if conditions of an electrode are changed
equilibrium could shift left or right
Why can some batteries only be used once
the electrochemical reaction taking place inside the cell is irreversible. Once the reactants have been used up then it cannot be used again.
What does a fuel cell do
A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage.
advantages and disadvantages of using fuel cells for energy instead of fossil fuels
advantage :
* greater efficiency
* less pollution and less CO2
Disadvantages :
H2 is difficult to store
expensive
Fossil fuels are combusted to produce hydrogen, which releases carbon dioxide
advantages of fuel cells compared to other type of cells
Voltage is constant, as fuel and oxygen is supplied constantly so concentrations remain constant
In acidic solution, write equations for fuel cell
At the anode (negative electrode)
H2(g) -><- 2H+(aq) + 2e-
At the cathode (positive electrode)
½ O2(g) + 2H+(aq) + 2e-
-><- H2O(l)
Overall: H2(g) + ½ O2(g)
-> H2O(l)
In alkaline solution, write equations for fuel cell
At the anode (negative electrode)
H2(g) + 2OH^-(aq) -><- 2H2O(l) + 2e-
At the cathode (positive electrode)
½ O2(g) + H2O(l) + 2e- -><- 2OH-(aq)
Overall: H2(g) + ½ O2(g) -> H2O(l)
