Redox equilibria

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  • Created by: r98
  • Created on: 19-04-16 15:35
What does OIL RIG stand for?
Oxidation Is Loss (of electrons) & Reduction Is Gain (of electrons).
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What is a reducing agent?
A species that gives away electrons.
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What is an oxidising agent?
A species that accepts electrons.
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What is the 'oxidation state' of an element in a compound?
It's effectively the number of electrons the element has lost or gained when forming bonds.
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What is the oxidation state of an uncombined element?
= 0.
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When is the oxidation state of hydrogen -1?
In metal hydrides, e.g. NaH.
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When is the oxidation state of oxygen -1? When else is it not +2?
In peroxides it's -1. In compounds with F, e.g. OF2, it's +2.
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What are complex ions?
Ions which have two or more atoms bonded convalently, but where the whole group of atoms has a charge.
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Since we can't measure electrical potential directly, what can be done instead?
Connect together 2 different electrodes and measure the potential difference between them.
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What is used to measure the potential difference of a cell?
A voltmeter.
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What is used to complete the electrical circuit? Why is this used instead of a piece of wire?
A salt bridge. This is used instead of a piece of wire to avoid further metal/ion potentials in the circuit.
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Which is the better reducing agent, if the zinc electrode is more negative than the copper electrode? Explain your answer.
The zinc electrode, because as it's negative, it loses electrons more readily than copper does.
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In a zinc-copper cell, where do the electrons flow to and from?
From the zinc to copper.
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What are the standard conditions when referring to electrodes?
298K, 100kPa, solutions of 1.00moldm^(-3).
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What is the potential of the standard hydrogen electrode defined as?
As 0.
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What is the electromotive? What is the abbreviation for it?
The electromotive force (emf, E), is the measured voltage.
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What can be implied if electrodes have negative 'E standard' values, comparing to hydrogen?
Electrodes with negative 'E standard' values are better at releasing electrons (better reducing agents) than hydrogen.
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At which electrode does oxidation occur?
At the negative electrode.
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At which electrode does reduction occur'?
At the positive electrode.
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How can the voltage, that's obtained by connecting two standard electrodes together, be found?
By calculating the difference between the two 'E standard' values.
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In a cell diagram, what does a vertical solid line indicate?
A phase boundary, e.g between a solid and a solution.
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In a cell diagram, what does a double vertical line indicate?
A salt bridge.
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What is the equation for calculating emf? Use E(R) to represent the emf of right hand electrode, and E(L) to represent the emf of left hand electrode.
emf = E(R) - E(L)
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When representing a cell and giving the value of the emf, what needs to be stated about the right-hand electrode?
The polarity (whether it's positive or negative).
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What is a battery?
A number of cells connected together.
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What are two examples of non-rechargeable cells?
Zinc/copper cells & zinc/carbon cells.
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What principle does the zinc/copper cells work by?
It works on the general principle of electrons being transferred from a more reactive metal to a less reactive one.
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Why was the zinc/copper cell not practical for portable devices?
Because of the liquids it contained.
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In the Leclanche cell (zinc/carbon cell), what is the positive electrode made out of? What does this electrode act like?
The positive electrode is carbon, it acts like the inert platinum electrode in the hydrogen electrode.
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Rather than being a liquid, what is the electrolyte?
A paste.
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What happens to the zinc cannister as the zinc/carbon cell discharges?
The zinc is used up and the walls of the zinc canister become thin and prone to leakage. The ammonium chloride electrolyte is acid and can be corrosive.
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What is a variant of the Leclanche cell? What is the electrolyte of this cell? What is this cell better at doing than the Leclanche cell?
The zinc chloride cell, zinc chloride is the electrolyte. This cell is better at supplying high currents than the Leclanche cell.
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What electrolyte is used in long life alkaline batteries?
Potassium hydroxide.
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Why is powdered zinc used in long life alkaline batteries?
It has a larger surface area, which allows the bsttery to supply high currents.
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How are rechargeable batteries recharged?
By reversing the cells reactions, which is done by applying an external voltage to drive the electrons in the opposite direction.
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What are three examples of rechargeable batteries?
Lead-acid batteries, nickel-cadmium, & lithium ion.
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In what direction will electrons flow, on discharging, if in a lead-acid battery, the positive plate is made of lead coated with lead(iv) oxide, PbO2, and the negative plate is made of lead?
On discharging, electrons flow from the lead plate to the lead(iv) oxide coated plate.
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In what direction will electrons flow, on charging, if in a lead-acid battery, the positive plate is made of lead coated with lead(iv) oxide, PbO2, and the negative plate is made of lead?
On charging the electrons flow from the lead(iv) oxide coated plate to the lead plate.
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Why is it cheaper to buy a nickel/cadmium battery than a zinc/carbon battery?
Because the nickel/cadmium battery can be recharged many times (500 times).
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In a nickel/cadmium battery, is the electrolyte acidic or alkaline?
The electrolyte is alkaline.
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In a nickel/cadmium battery, in which direction do the electrons flow, on discharge?
Electrons flow from Cd to Ni.
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In a nickel/cadmium battery, in which direction do the electrons flow, on charging?
Electrons flow from Ni to Cd.
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Why are lithium batteries light?
Because they are based on lithium rather than heavier metals.
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What electrolyte is used in lithium ion batteries? Why is this electrolyte used?
Polymer electrolytes, means the batteries can't leak.
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What two things does burning hydrocarbons produce?
Carbon dioxide (greenhouse gas) & acidic nitrogen oxides, caused by nitrogen and oxygen combining at high temps.
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What are the 'fuels' used in the fuel cell?
Hydrogen & oxygen.
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What material are the electrodes made of in a fuel cell?
Platinium.
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What does the polymer electrolyte, in a fuel cell, allow to pass through it?
Ions.
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Does the hydrogen/oxygen fuel cell need to be recharged?
No.
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In a fuel cell, what happens at the left hand electrode?
Hydrogen enters and gives up electrons to form H+ ions.
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What is the half equation for the reaction that occurs at left hand electrode in a fuel cell?
H2(g) -----> 2H+(aq) + 2e-
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In a fuel cell, where do the H+ ions flow to?
To a second electrode, where they react with oxygen and electrons from water.
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What is the equation for the reaction that occurs at the right hand electrode in a fuel cell?
4H+(aq) + O2(g) -----> 2H2O(l)
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What is the equation of the overall reaction of the hydrogen-oxygen fuel cell?
2H2(g) + O2(g) -----> 2H2O(l)
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What is the main source of hydrogen right now?
Crude oil (non-renewable).
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What is the problem of making hydrogen by electrolysis of water?
Most electricity is made by burning fossil fuels, which emit carbon dioxide.
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What is a problem with having hydrogen-powered vehicles?
They will need hydrogen filling stations to be built, which also has issues of storing and transporting a highly flammable gas.
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What is a reducing agent?

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A species that gives away electrons.

Card 3

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What is an oxidising agent?

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Card 4

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What is the 'oxidation state' of an element in a compound?

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Card 5

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What is the oxidation state of an uncombined element?

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