Chemistry 3.1 - Redox and Standard Electrode Potential

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Define 'oxidation'.
1. The loss of electrons. 2. The addition of oxygen. 3. The removal of hydrogen.
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Define 'reduction'.
1. The gain of electrons. 2. The loss of oxygen. 3. The addition of hydrogen.
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If an atom is oxidised, what happens to its oxidation number?
It INCREASES.
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If an atom is reduced, what happens to its oxidation number?
It DECREASES.
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Oxygen almost always has an oxidation state of -2. State the exception.
In peroxides, it is -1.
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Hydrogen almost always has an oxidation state of +1. State the exception.
In hydrides it is -1.
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How can you tell that a species is an oxidising agent?
The species that is reduced will accept electrons from another atom/group, causing it to lose electrons and therefore be oxidised
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How can you tell that a species is a reducing agent?
The species that is oxidised will donate electrons to another atom/group, causing it to be reduced.
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When creating a half equation, you add electrons to the side with...
the highest oxidation state.
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If the electrons are on the left of the half equation, what has occurred?
REDUCTION.
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If the electrons are on the right of the half equation, what has occurred?
OXIDATION.
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How do you balance electrons in a half equation?
Use the difference in oxidation states, alongside the balancing numbers.
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Electrode
A metal dipped in a solution of its own ions.
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What would happen if a piece of metal was placed in water?
The metals atoms have a tendency to dissociate and form positive ions, leaving their electrons on the metal. This gives it a negative charge. The metal ions in solution would be attracted back to the metal and so some metal atoms may reform.
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Dynamic equilibrium.
When the rates of the forward and reverse reaction are equal. In this case, the rate at which ions form equals the rate in which they rejoin with electrons to form atoms.
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If an equation were to be written to show what was happening at the metal, how would it be shown?
AS REDUCTION (electrons and ions to the left hand side). Also, a double-headed arrow.
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How do you measure the potential of a cell?
It cannot be measured directly, and so two electrodes are connected and the potential difference between them is measured using a voltmeter.
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What is the salt bridge made of?
Potassium nitrate on agar.
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What is the purpose of the salt bridge?
It completes the circuit by allowing ions to move without the solutions mixing.
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What is the purpose of the wire?
It allows electrons to flow from the electrode with the greatest negative electrode potential (most electrons on the metal).
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What is the purpose of the voltmeter?
To measure the potential difference in the cell.
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When drawing a cell, the concentrations of the ions must be displayed. Why?
If they were different the dynamic equilibrium would be altered.
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What does the standard electrode potential measure?
A half cell's ability to gain or lose electrons.
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A hydrogen half cell is used as its E standard value is 0. How must it be set up?
UNDER STANDARD CONDITIONS.
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If the E standard value is less than zero for another electrode...
It forms a greater negative electrode potential. It is more easily oxidised (more easily loses its electrons and so its metal has a greater negative electrode potential).
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If the E standard value is greater than zero for another electrode...
It forms a weaker negative electrode potential. It is more easily reduced.
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What are the standard conditions for the standard hydrogen electrode?
1 atmosphere pressure, concentration of 1moldm-3 and 298K.
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What actually happens at a standard hydrogen electrode?
As the hydrogen gas flows over the platinum an equilibrium is established between the gas molecules and the ions in solution. This is catalysed by the inert platinum.
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Zinc has a standard electrode potential value of -0.76V. What does this show?
Zinc has a greater negative electrode potential than the Pt: it has a greater build up of electrons. Its position of equilibrium is further to the left.
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When writing a cell, which half cell goes on the left?
The one with the most negative electrode potential.
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The species in the centre are those that are...
most easily oxidised.
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The half cell of iodine requires inert Pt, although it is a solid. Why?
Iodine will not conduct electricity (no freely moving electrons).
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Give the two equations for calculating the EMF of a cell.
E°= E°RHS - E°LHS OR E° = E°RED - E°OXID
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A positive EMF value means the reaction is...
FEASIBLE!!
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What does a positive EMF value show?
It demonstrates that the species that is reduced (has the more positive E°) is a strong enough oxidising agent to oxidise the metal atoms into ions. The react is feasible.
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Step 1 of a hydrogen fuel cell.
The hydrogen gas is passed over inert platinum, which acts as both a catalyst and the anode. THE HYDROGEN IS OXIDISED AT THE ANODE.
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Step 2 of a hydrogen fuel cell.
The hydrogen ions produced migrate across the semi-permeable membrane, which does not allow the flow of electrons.
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Step 3 of a hydrogen fuel cell.
The hydrogen ions react with oxygen at the cathode to form water. This generates a flow of electrons in an external circuit, which results in a current.
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Advantages of a hydrogen fuel cell.
1. Only releases water - no harmful emissions. 2. The energy efficiency is much higher than standard fuel systems. 3. It is a convenient way of storing and releasing energy.
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Disadvantages of a hydrogen fuel cell.
1. Expensive procedure. 2. Hydrogen is often produced from fossil fuels. 3. Hydrogen and oxygen gases are difficult to store. 4. Pt is an expensive catalyst.
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Define the standard hydrogen electrode.
It has an inert Pt electrode in a 1moldm-3 solution of H+ ions with hydrogen gas at 1 atm, all at a temperature of 298K.
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Define the standard electrode potential.
The EMF measured on a high resistance voltmeter when a half cell containing all solutions at 1moldm-3 and all gases at 1 atm is connected to the standard hydrogen electrode at 298K.
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Other cards in this set

Card 2

Front

Define 'reduction'.

Back

1. The gain of electrons. 2. The loss of oxygen. 3. The addition of hydrogen.

Card 3

Front

If an atom is oxidised, what happens to its oxidation number?

Back

Preview of the front of card 3

Card 4

Front

If an atom is reduced, what happens to its oxidation number?

Back

Preview of the front of card 4

Card 5

Front

Oxygen almost always has an oxidation state of -2. State the exception.

Back

Preview of the front of card 5
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