Electrochemistry Flashcards
What are electrochemical cells?
Batteries
What are electrochemical cells made up of?
Two half cells
What if a half cell usually made up of?
- Solid metal (x) electrode- 1.0 mol.dm^-3 solution of (x) ions (aq)
What is set up in a half cell?
An equilibrium between x(s) and x ions (aq)
What can happen to a half cells equilibrium?
It can shift depending on the other half cell
What happens in the forward reaction of a half cell?
- Reduction- Ions form the solid- Electrons enter from the other half cell
What happens in the reverse reaction of a half cell?
- Oxidation- Solid forms the ions- Electrons leave to go to the other half cell
What can two half cells be combined to create?
A potential difference
How can you work out which half-cell is oxidised?
- Put the most negative half cell on top- Top equation is oxidised
How can you work out which half-cell is reduced?
- Put the most negative half cell on top- Bottom equation is reduced
Which half cell belongs on the LHS?
The one being oxidised
What happens at the LHS?
- Oxidised- Annode- Negative
Which half cell belongs on the RHS?
The one being reduced
What happens at the RHS?
- Reduced- Cathode- Positive
What conditions are half-cells combined at to form an electrochemical cell?
- 298K- 100 kPa
What is needed to complete an electrochemical cell?
A salt bridge
What does a salt bridge do?
Completes the circuit
What is the REDOX power of a half-cell?
Relative
What is the REDOX potential of all the half-cells measured against?
The standard hydrogen electrode
How do you calculate the electrode potential?
Connect a standard hydrogen electrode to another half cell and the potential difference is the other half cells electrode potential
In the standard hydrogen electrode what is the electrode made of?
Platinum
Why is the standard hydrogen electrode made of platinum?
Platinum is unreactive and porus and allows for easy electron transfer
What is the electrode potential of the standard hydrogen electrode?
0.00v
Why is the electrode potential of the standard hydrogen electrode 0?
Because it is compared to itself
What happens if one half cell contains two ions in solution?
- Both ions are present in the solution- Use a platinum electrode
In the electrochemical series what is the most negative electrode potential?
Strongest reducing agent
In the electrochemical series what is the most positive electrode potential?
Strongest oxidising agent
What happens as electrode potential decreases?
- Reducing power decreases- Further left the equilibrium lies- Easier it is to lose electrons- Easier to oxidise
What happens as electrode potential increases?
- Reducing power increases- Further right the equilibrium lies- Harder it is to lose electrons- Easier to reduce
In electrochemical series what happens to the half-cell with the most negative electrode potential?
Oxidised
In electrochemical series what happens to the half-cell with the most positive electrode potential?
Reduced
How is EMF calculated?
Most positive electrode potential - most negative electrode potential or oxidised half cell value - reduced half cell value
Which half cells will have the greatest EMF value?
Half cells with the largest difference in electrode potential
Which half cells will have the smallest EMF value?
Half cells with the smallest difference in electrode potential
With half-cells, which one will be the easiest to oxidise?
- Most negative- Strongest reducing agent
With half-cells, which one will be the easiest to reduce?
- Most positive- Strongest oxidising agent
What are the steps in using half cells to predict reactions?
1 - Write the half equations with the most negative on top2 - Add anticlockwise arrows ( left = oxidise, right = reduced )3 - Write equations in the correct direction4 - Balance equations5 - Combine
What can the electrode potential be used to predict?
If a reaction is feasible
How do you use electrode potentials to calculate the feasibility of a reaction?
1 - Most negative on top2 - Anticlockwise rule 3 - Underline reactants4 - If the reactants are on the top right and bottom left then the reaction will occur under standard conditions
What are the limitations of using electrode potentials to calculate feasibility?
- All electrode potential values are based off standard conditions- If conditions change the electrode potential values will change- Kinetics may be so slow that no real reaction is seen- Some reactions may not happen under standard conditions but will happen with higher concentrations or temperatures
What are the standard conditions of an electrochemical cell?
298K, 100kPa, 1.0mol.dm^-3
What can be used to make predictions when concentration or temperature is changed
Le chateliers principle
How can a change in concentration effect the electrochemical cell value?
- Increasing conc and/or decreasing product shifts the equilibrium to the right and the electrochemical cell value will increase - Decreasing conc and/or increasing product shifts the equilibrium to the left and the electrochemical cell value will decrease
What is the deltaH value for the majority of electrochemical cells?
Negative - exothermic
How can an increase in temperature effect exothermic cells?
- Shift to the left- Favours endothermic- Against predicted route- Decrease electrochemical cell value
How can a decrease in temperature effect exothermic cells?
- Shift to the right- Favours exothermic- With predicted route- Increase electrochemical cell value
