Further redox Flashcards
(30 cards)
What is the reduction
.Loss of oxygen
.Gain of electrons
.Decrease in oxidation numner
What is oxidation
.Gain of oxygen
.Loss of electrons
.Increase in oxidation number
What is disproportionation
When an element in a single species is simultaneously oxidised and reduced
When is an equilibrium established
When a metal is placed in a solution of is ions
How is a half cell formed
The electron potential between the metal and the solution form a half cell
How is the electron potential measured
Must be measured against a reference electrode
What is the standard electrode potential
.The electromotive force, emf, of a cell
.Where the standard hydrogen electrode is on the left and the half cell is on the right at
.298K
.100kPa
.1.00 moldm-3 concentration of ions
What can the standard hydrogen electrode be used to measure
1) Electrode potential
2) Standard EMF
How is an electrochemical cell made
Two half-cells are combined
What are the two connections needed when two half-cells are combined
1) An external electrical circuit, connecting the two electrodes and allowing electrons to flow from one half-cell to the other
2) A salt bridge, connecting the two aqueous solutions and allowing ions to move between the two half-cells
What is a salt bridge made from
Why
a strip of filter paper soaked in concentrated potassium nitrate solution
This is chosen to stop the possibility of a cell precipitate forming with ions from the half cells
How are half cells represented
using cell diagrams
How do you calculate the emf of a cell under standard conditions
Ecell = Eright -Eleft
What is the standard electrode potential for the standard hydrogen electrode
0V
How is the standard emf value measured
1) Constructing two half-cells
2) Connecting them using a salt bridge and external circuit
3) Ensuring standard conditions
4) Measuring the potential using a high-resistance voltmeter
When are electrochemical reactions feasible
When the Emf (Ecell) is positive
What are the four steps you need t to predict a feasible reaction
1) Write the electrode potential for the two electrode systems side by side, most positive electrode potential on the right
2) The reduction will be on the right, so write the half-equation as seen in the table
3) Oxidation will be on the left, so write the other half-equation in reverse
4) Combine the two half-equations, adjusting for uneven numbers of electrons if needed
You can predict the thermodynamical feasibility of a chemical reaction however the reaction may not happen.WHY
1) The activation energy may be very large
2) The conditions for the reaction may not be standard
Why may non-feasible reactions happen when the conditions are changed from standard
1) Position of the equilibrium of a half cell reaction may change, changing the E
2) This changes the overall Emf which may become positive as a result
When does the position of equilibrium in a half-cell change
When the concentration of a reactant is changed
In the example below what happens if the concentration of mg2+ was decreases
Mg2+(aq) +2e- —>Mg (s)
1) Position of equilibrium moves to the left
2) More electrons are released
3) The electrode becomes more negative
4) E becomes more negative
What are fuel cells
They generate a voltage by the reaction of hydrogen, methanol or other hydrogen-rich fuels like oxygen
What is hydrogen used as in fuel cells
fuel
What does fuel do in fuel cells
1) Produce electricity directly from the reaction between hydrogen and oxygen
2) Produce electricity as long as hydrogen and oxygen are supplied
3) Makes water vapor as their only product
