Electrochemistry Flashcards
What is electrochemistry?
Deals with the interconversion of electrical energy and chemical energy, electrical effects occur as a result of the movement of electronically charge either as ions in solution or electrons in a conductor
Electric current?
The rate at which charge passes a point
When does an electric current flow?
If there is a potential difference between two points in a circuit, the potential difference represents the driving force that pushes the electrons or ions around the circuit
Ohms law?
V = I x R
potential difference = current x resistance
Resistance in a cicutty?
For a constant potential difference the lower the resistance in a circuit the higher the current
Emf?
If an electrochemical cell is used to produce a high current the voltage falls as the reactants are used up, however if the resistance of the current is his the current is low and the voltage does not fall so quickly, when the resistance is extremely high an infinitesimal small current flows and the voltage is at its maximum, the maximum voltage is called the zero current cell potential also known as the electromotive force and given the symbol Ecell
Reductions?
When electrons are gained
Oxidations?
When electrons are lost
Electrolyte?
Substance that when dissolved gives a solution that conducts electricity, most are ionic substances, in an electrolyte solution the charge is carried between the electrodes by dissolved ions
Conductivity cell?
The properties of electrolyte solution can be studied in a conductivity cell, consist of two platinum electrodes held apart at a fixed distance in a solution, platinum is used since it is inert and very reactive, a voltage is applied between the electrodes and the resistance R is measured the resistance is a measure of how difficult the motion of ions is between the electrodes, to prevent any chemical reactions due to electrolysis it is usual to make conductivity measurements with an alternating current
What does conductivity depend on?
The number of ions in solution, a solution with a higher concentration of ions has a higher conductivity
Strong electrolytes?
KCl or Na2SO4 are fully ionised in solution and so are strong electrolytes
Limiting molar conductivity?
For a strong electrolyte measurement using a conductivity cells shows that the molar conductivity decreases as the solution becomes more dilute, the interactions between ions in a solution affect the movement of ions through solution, these interactions are less important in dilute solution so molar conductivity increases with dilution, eventually the molar conductance ceases to increase with dilution and approaches a limiting value called the limiting molar conductivity
What happens when the solution is very dilute?
The ions are far enough apart that they do no interact with each other and so behave independently of one another, each ion makes its own independent contribution to the molar conductivity
Ionic conductivity values?
Known as limiting values since they are only valid in very dilute solutions at infinite dilution, most singly charged anions or cations have similar values of ionic conductivity values since each ion carries the same amount of charge, divalent ions carry more charge so they have higher values
Anomalous values of H+ and OH-
Grotthus mechanism, due to swapping of H+ from now molecule to another, charge moves a large distance even though each individual H+ ion moves only a small distance it is the electrons in the bonds that move, also explains the unusually high conductivity of OH- ion
What are weak electrolytes?
They dissolve in water but do not completely ionise
What is a salt bridge?
Completes the circuit, the charge is conducted by ions in solution, a strip of absorbent materials saved in solution of sodium sulfate
Metal ion electrodes?
When the metals that form the electrodes also take part in the cell reactions
Standard hydrogen electrode?
Reaction chosen to be the standard, under standard conditions a hydrogen gas pressure of 1 bar and [H+(aq)] = 1.00 meld-3 at 298K the potential of this half cell is set at 0.00V
What is the standard reduction potential?
The potential of a reduction half cell relative to the standard hydrogen electron set at E = 0.00V at 298K and 1 bar with all species in their standard states
How are half reactions written?
As reductions and are listed in order of decreasing E. The list is in order of the tendency to accept electrons relative to the hydrogen half reaction. the value of E give a measure of how strongly the reduction is driven, half reactions with highly positive E values accept electrons readily so are good oxidising agents. Half reactions with highly negative E values donate electrons readily and are good reducing agents
Values of half reactions for oxidising agents?
The value of E give a measure of how strongly the reduction is driven, half reactions with highly positive E values accept electrons readily so are good oxidising agents. A half reaction with a high positive E value has a stronger tendency to occur in the forward direction tan one with a less positive value that is a strong oxidising agent
Values of half reactions for reducing agents?
Half reactions with highly negative E values donate electrons readily and are good reducing agents, the more negative the value the stronger the tendency for the half reaction to proceed int he reverse reaction and to act as a reducing agent
How to calculate E cell?
E cell = E cathode - E anode
Sign of E cell?
Will always be positive if the cell reduction occurs spontaneously, if the value is negative it is non spontaneous it will occur spontaneously in the opposite direction to that in the cell diagram
Gibbs energy for E cell?
A reaction with a positive cell potential has a negative value for Gibbs energy changes and it therefore is spontaneous under standard conditions
If E cell is positive what is the Gibbs energy change, K and the direction of the reaction?
If E cell is positive > 0, Gibbs energy change of reaction is negative < 0, K is > 1 and the reaction is spontaneous in the forward reaction
If E cell is zero what is the Gibbs energy change, K and the direction of the reaction?
If E cell is zero, Gibbs energy change of reaction is zero, K is equal to 1 and the reaction has no tendency to occur it is at equilibrium
If E cell is negative < 0 what is the Gibbs energy change, K and the direction of the reaction?
If E cell is negative < 0, Gibbs energy change of reaction is positive > 0, K is < 1 and the reaction is spontaneous in the reverse reaction
Nernst equation?
Can calculate the cell potential under any set of conditions, can also calculate the concentration of a species involved in an electrochemical cell
Concentration cells?
Because reduction cell potentials change with concentration it is possible to construct an electrochemical cell using two half cells with the same chemical reaction but with different concentrations, such cells are called concentration cells
Value of Ecell for concentration cells?
Usually quite small, when current is drawn from the cell the cell potential decreases as the reaction proceeds, the concentrations in the two half cells tend to equalise when the concentration are the same in both half cells then the reaction comes to equilibrium and Ecell = 0
Where are concentration cells important?
Where solutions of electrolytes with different concentration are separated by a porous membrane, if one of the ions can pass through the membrane but other cannot then passage of ions will occur tending to equalise the concentrations which leads to an excess of charged ions on one side of the membrane, the imbalance of ions on each side leads to a potential difference across the membrane which slows down and stops the movement of ions, this is known as he membrane potential or the Nernst potential
What is electrolysis?
Electrolysis involves driving a chemical reaction by suppling it with electronically energy, the input of energy means that non spontaneous reactions can be made to occur and this principle forms the basis of a number of industrially very important reactions
