emf Flashcards
(34 cards)
Why use a high resistance voltmeter?
The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit. In this state it is possible to measure the maximum possible potential difference (E).
The reactions will not be occurring because the very high resistance voltmeter stops the current from flowing.
how salt bridge made
a piece of filter paper (or material) soaked in a salt solution, usually potassium nitrate.
why not use wire as salt bridge
the metal wire would set up its own electrode system with the solutions.
purpose of salt bridge
The salt bridge is used to connect up the circuit. The free moving ions conduct the charge.
Why does a voltage form? use example w zn and copper
When connected together the zinc half-cell has more of a tendency to oxidise to the Zn2+ ion and release electrons than the copper half-cell. (ZnZn2+ + 2e-)
More electrons will therefore build up on the zinc electrode than the copper electrode.
A potential difference is created between the two electrodes.
The zinc strip is the negative terminal and the copper strip is the positive terminal.
This potential difference is measured with a high resistance voltmeter, and is given the symbol E. The E for the above cell is E= +1.1V.
What happens if current is allowed to flow?
If the voltmeter is removed and replaced with a bulb or if the circuit is short circuited, a current flows. The reactions will then occur separately at each electrode. The voltage will fall to zero as the reactants are used up.
The most positive electrode will always undergo reduction.
Cu2+ (aq) + 2e- Cu(s) (positive as electrons are used up)
The most negative electrode will always undergo oxidation. Zn(s) Zn2+ (aq) + 2e- (negative as electrons are given off)
Systems that do not include metals.
If a system does not include a metal that can act as an electrode, then a platinum electrode must be used and included in the cell diagram. It provides a conducting surface for electron transfer.
why use pt
it is unreactive and can conduct electricity.
Measuring the electrode potential of a cell
- It is not possible to measure the absolute potential of a half electrode on its own. It is only possible to measure the potential difference between two electrodes.
- To measure it, it has to be connected to another half-cell of known potential, and the potential difference between the two half-cells measured.
- by convention we can assign a relative potential to each electrode by linking it to a reference electrode (hydrogen electrode), which is given a potential of zero Volts
In a cell diagram the hydrogen electrode is represented by:
Pt |H2 (g) | H+ (aq)
Components of a standard hydrogen electrode. 4
- Hydrogen gas at pressure of 100kPa
- Solution containing the hydrogen ion at 1.0 mol dm-3 (solution is usually 1 mol dm-3 HCl)
- Temperature at 298K 4. Platinum electrode
The hydrogen electrode equilibrium is:
H2 (g) <=> 2H+ (aq) + 2e-
why pt used in she
(The platinum black is used because it is porous and can absorb the hydrogen gas.)
why use Secondary standards
The standard hydrogen electrode is difficult to use, so often a different standard is used which is easier to use.
These other standards are themselves calibrated against the SHE. This is known as using a secondary standard - i.e. a standard electrode that has been calibrated against the primary standard. The common ones are:
silver / silver chloride E = +0.22 V calomel electrode E = +0.27 V
The standard conditions are : 4
*all ion solutions at 1 mol dm-3 *temperature 298 K
*gases at 100 kPa pressure *no current flowing
Calculating the EMF of a cell
Ecell= Erhs - Elhs
= Ered - Eox
how to know which half cell reduces/oxidises
NO Problem
e cell for spontaneous change is pos/neg
POS
The most powerful reducing agents will be found at the most
negative end of the series on the right (i.e. the one with the lower oxidation number).
The most powerful oxidising agents will be found at the most
positive end of the series on the left (i.e. the one with the higher oxidation number).
Effect of concentration on Ecell
So increasing concentration of ‘reactants’ would increase Ecell and decreasing them would cause Ecell to decrease.
Effect of temperature on Ecell
Most cells are exothermic in the spontaneous direction so applying Le Chatelier to a temperature rise to these would result in a decrease in Ecell because the equilibrium reactions would shift backwards.
will a reaction occur if ecell is pos
it indicates a reaction might occur. There is still a possibility, however, that the reaction will not occur or will occur so slowly that effectively it does not happen.
If the reaction has a high activation energy the reaction will not occur.
Electrochemical cells
used as a commercial source of electrical energy
