Electrochemistry Flashcards
Electrochemistry
Redox reactions can be studied electrically using…?
electrochemical cells
What is an electrochemical cell?
an electrochemical cell is a simple system that generates a voltage
Draw a diagram to show an electrochemical cell made up of 2 half-cells connected by a salt bridge
What are the metal strips?
these are called electrodes (metallic conductors)
What does each beaker contain?
each beaker contains a half-cell made up of redox couple
What does a redox couple consist of?
a redox couple consist of metal atoms in equilibrium with it’s aqueous solution of ions
In this experiment, which two things need to be connected in order for a reaction to occur?
No reaction can occur until the 2 half cells are connected by an external circuit (and a salt bridge)
What is the external circuit?
The external circuit is a conducting wire through which electrons can pass from one electrode to the other.
What is the salt bridge?
The salt bridge is a strip of filter paper soaked in a solution of soluble ionic compound (e.g potassum nitrate or potassium chloride)
What is the salt bridge usually soaked in?
Potassium Nitrate or Potassium Chloride
Electrode Potentials
What is the external Circuit?
The external circuit is a conducting wire through which electrons can pass from one electrode to the other
What is the salt bridge?
the salt bridge is often a strip of filter paper soaked in a solution of a soluble ionic compound(e.g potassium nitrate or potassium chloride) that does not react with the contents of either beaker
What is the role of the salt bridge?
- the salt bridge enables the cell to work by completing the circuit without the 2 solutions mixing
- the salt bridge allows ions to move between the 2 half-cell compartments
- it prevents ion saturation at the electrodes that would stop the cell from working
when does a voltmeter measure a reading?
when there is a potential difference between the 2 half-cells
What is a voltmeter reading called?
A cell potential
Draw and label a cell diagram
The cell diagram can be written in a shorthand notation. Write the shorthand notation for RH magnesium electrode and a LH copper electrode. Describe what each thing notation represents?
- Positive electrode = written/drawn on the right hand side
- single vertical lines = represent phase (state) boundaries
- double vertical lines = salt bridge
- metal electrodes = always on the outside
- oxidation occurs in the Left hand half-cell and reduction occurs in the Right hand half-cell
Describe the flow of electrons according to the cell diagram and the shorthand cell diagram?
electrons flow from the left hand half-cell to the right hand half-cell via the external circuit
What type of reaction occurs in the Left-hand half cell and what type of reaction occurs in the right hand half-cell?
Left = Oxidation
Right = reduction
Factors that affect the cell potential
What are the factors that affect the cell potential?
- cell current
- cell concentration
- cell temperature
- cell pressure
What is the cell current/ what is it measure by?
a high resistance digital voltmeter must be used to measure the true cell potential
What does a high resistance voltmeter provide?
a high resistance voltmeter provides zero-current conditions
What is the electromotive force (e.m.f)?
is the cell potential measured under zero-current conditions (Ecell)
What happens when current if lowing to the voltmeter reading?
the cell potential reading drops
What are the standard conditions used when measuring cell potentials?
- Cell concentration = 1 mol dm-3
- Cell temperature = 298K
- Cell pressure = 100KPa
- (Cell current = Zero-current)
The cell pressure is significantly affected by what type of electrodes?
Gas electrodes
What are gas electrodes?
gas electrodes consist of an inert metal surrounded by gas in equilibrium with a solution of it’s ions
The inert metal is usually what metal?
Platinum
What does the inert metal/ platinum act as?
Acts as a source or a sink for electrons
What is the most important gas electrode?
Hydrogen electrode
What redox couple does the hydrogen electrode correspond to?
H+ (aq) / H2(g)
draw the left and right shorthand diagram to show the hydrogen cell in a cell diagram?
Left: Pt (s) / H2 (g) / H+ (aq)
Right: H+ (s) / H2 (g) / Pt (s)
What is the oxidation half equation for hydrogen?
H2 (g) —-> 2H+ (aq) + 2e-
What is a redox electrode?
at a redox electrode, 2 oxidation states of a given element undergo a redox reaction at an inert metal surface
For example the redox couple Fe3+ (aq) / Fe2+, write out the right and left shorthand notation for this Iron gas electrode?
Left: Pt (s) / Fe2+ or Fe3+ (aq)
Right : Fe2+ or Fe3+ (aq) / Pt (s)
The Standard Hydrogen Electrode and The Standard Electrode Potentials
What are standard potentials?
Cell potentials are called standard potentials when they are carried out under standard conditions
What are standard conditions?
Standard conditions are:
- 1 mol dm-3
- 298K
- 100 KPa
- Zero current
What can standard potentials for particular electrochemical cell are given by what symbol?
E°cell
What is the electrode potential?
What is it a measure of?
- the electrode potential is the potential difference in volts between the metal and a solution of metal ions.
- it is a measure of the tendency of the metal to lose electrons
What is it not possible to measure the potential of?
it is not possible to measure the potential of a single electrode
So how are potentials measured?
Instead electrode potentials are listed relative to a standard reference electrode.
What is the Primary Standard electrode reference?
the primary standard electrode used is the Standard Hydrogen Electrode (SHE)
What value has the Standard Hydrogen electrode been given?
a value of Zero E°= 0.00V by scientists
So all electrode potentials are what to the Standard Hydrogen potential?
all other electrode potentials are listed relative to the Standard Hydrogen electrode
What is the standard Hydrogen Electrode?
the standard Hydrogen electrode is the hydrogen electrode operating under standard conditions (298K, 100 KPa, 1 mol dm-3)
Describe the conditions surrounding the standard hydrogen electrode?
Draw a diagram to show this.
in the hydrogen electrode pure hydrogen gas at 100KPa pressure is bubbled across an inert platinum electrode placed in 1M solution of H+ at 298K
Write the notion used to represent the standard hydrogen electrode?
Pt (s) / H2(g) / H+(aq)
Draw the reduction half equation to show this reaction?
2H+ (aq) + 2e- ⇌ H2 (g)
The direction of the above reaction depends on what?
the direction of the above reaction depends on the other half cell to which the hydrogen electrode is connected
The standard electrode potential of any redox couple can be predicted, how?
by connecting the redox couple to the standard hydrogen electrode
Why must these reactions be carried out in standard conditions
Standard conditions so that E°cell values for different redox couples can be compared
Describe the flow of electrons in the redox couple below:
Pt (s) / H2(g) / H+ (aq) // Cu2+ (aq) / Cu (s)
electrons flow from the hydrogen electrode to the Cu2+ (aq) / Cu (s) redox couple
What is the positive electrode?
Cu (s)
So the E °Cell for Cu2+ (aq) / Cu (s) redox couple is also…?
State the E ° Value of this redox couple ?
- Positive -
- the E ° value for the copper half cell = + 0.34 V
If the copper half cell is replaced by Magnesium half cell Mg(s) /Mg2+ (aq), describe the flow of electrons now?
the electrons will flow from the magnesium redox couple towards the hydrogen electrode
So the magnesium electrode will be..?
So the E ° value for the magnesium half cell would be ?
- Negative
- E ° value = -2.36V
How are standard electrode potential of redox couples determined normally?
Many SEP of redox couples have been determined experimentally (using the SHE or secondary standard) and are listed in chemistry data books
What can these values be used to calculate..?
standard e.m.f values practically
Why are standard electrode potentials always quoted for the reduction process?
by convention
All electrode reactions are..?
reversible equilibrium reactions
the direction in which the forward and backward reaction in a half cell will go depends on..?
which half-cell it is connected to
What is the standard convection?
the standard convection is to place the half-cell with the more positive E° on the right hand side (positive electrode) and the half cell with the more negative E° on the left hand side (negative electrode)
This results in electrons flowing?
From the left hand half cell to the right hand half cell
Using this convention an expression can be applied to any pair of redox couples, what is the expression?
E°Cell = E°right - E°left
Calculate the standard e.m.f of a cell with Zinc and Copper electrodes in solutions of 1.0M Zinc sulphate and Copper (II) sulphate respectively
Zn2+ (aq) + 2e- ⇌ Zn (s) E° = -0.76V
Cu2+ (aq) + 2e- ⇌ Cu(s) E° = +0.34V
- use expression: E°Cell = E°right - E°left
- remember the reaction with the more positive value is the right hand half-cell
- E°Cell = +0.34 - (-0.76)
- E°Cell = +1.10V
Consider the cell made by connecting
Zn2+ (aq) / Zn (s) to Mg2+ (aq) / Mg (s)
a) write the equation for the reaction that occurs (the spontaneous or feasible reaction) and draw the cell notation
b) calculate the standard e.m.f of the cell
a) Two half equations are:
Zn2+ (aq) + 2e- ⇌ Zn (s) E° = -0.76V
Mg2+ (aq) + 2e-⇌ Mg(s) E° = -2.38V
- zinc is the positive electrode because E° = is more positive
- electrons are received here at this redox couple from the external circuit
- so the half cell reaction for this reaction goes FORWARD:
Zn2+ (aq) + 2e- ——–> Zn (s)
- Magnesium is the negative electrode
- electrons leave this electrode and enter the external circuit
- so magnesium half equation = in reverse
Mg (s) ——-> Mg2+ + 2e-
- so overall equation:
Zn2+ (aq) + Mg (s) —-> Zn (s) + Mg2+(aq)
- Cell representation:
- Mg (s) / Mg2+ (aq) // Zn2+ (aq) / Zn (s)
b) = +1.62V
Secondary Standards
What are the disadvantages of using the hydrogen electrode?
the hydrogen electrode is bulky, difficult to use and a little hazardous
How do we resolve this?
so sometimes a different standard is used instead of the hydrogen electrode. Called the secondary standard
Why is using a secondary standard better?
secondary standards are more convenient to use than the hydrogen electrode