Topic 14 Redox 2 Flashcards

1
Q

What does redox stand for?

A

Reduction & oxidation

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2
Q

What is reduction? oxidation?

A

Reduction - This is the gain of electrons or the decrease in oxidation number of an atom, ion or molecule.

Oxidation - This is the loss of electrons or the increase in oxidation number of an atom, ion or molecule.

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3
Q

What is an oxidation number?

A

A number assigned to an atom or ion to describe its relative state of oxidation or reduction.

E.g.
in H₂O –> H = +1 O = -2
in H₂O₂ –> H = +1 O = -1

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4
Q

What is stochiometry?

A

The amount of molecules of the reactants and products that are shown in the balanced equation for the reaction. A stochiometric reaction is one that uses up reactants and produces products in amounts exactly as predicted in the balanced equation.

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5
Q

What is an oxoanion?

A

An ion with the general formula AₓOᵧᶻ⁻, where A represents any element (metal or non-metal) and O represents an oxygen atom.

E.g. CO₃²⁻ or MnO₄⁻

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6
Q

What is an oxidising agent? Reducing agent?

A

Oxidising agent - A reactant which takes electrons and is reduced when they react.

Reducing agent - A reactant which gives up electrons and is oxidised when they react.

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7
Q

What is a primary standard? What characteristics must they have?

A

A chemical which can be weighed out accurately to make up a standard solution.

A primary standard must be very pure, not gain or lose mass when exposed to the air, have a relatively high molar mass so weighing errors are minimised and react exactly and rapidly as described by the chemical equation.

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8
Q

What is a standard solution? How do you prepare it?

A

A solution with an accurately known concentration.

It is prepared by dissolving a weighed sample (primary standard) in water and then make the solution up to a definite volume in a graduated flask (A volumetric flask is most commonly used).

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9
Q

What is an electrochemical cell?

A

A cell which produces an electrical potential difference (voltage) from a redox reaction.

In an electrochemical cell, two separate redox reactions occur in separate half cells. The electrons flow from on cell to the other through a wire which connects the electrodes. The electric circuit is completed by using a salt bridge which connects the two solutions.

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10
Q

What is the electromotive force (e.m.f)?

A

The maximum voltage produced by an electrochemical cell. The symbol for e.m.f is E and its SI unit is the volt (V).

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11
Q

What is the standard hydrogen electrode (SHE)?

A

A half-cell in which a 1.00 mol dm⁻³ solution of hydrogen ions is in equilibrium with hydrogen gas at 100 kPa pressure on the surface of a platinum electrode at 298 K.

This has the cell notation of:
Pt(s) | H₂(g) | H⁺(aq) || X

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12
Q

What is the standard electrode (reduction) potential?

A

The E⌀ of a standard half-cell is the e.m.f of that half-cell relative to a SHE under standard conditions.

These are sometimes called standard redox potentials.

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13
Q

What is a reference electrode?

A

A half-cell that is used to measure electrode potentials instead of using the SHE.

A silver/silver chloride electrode or calomel (mercury/mercury chloride) electrode are often used in place of the SHE.

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14
Q

What does the term feasibility mean?

A

A reaction becomes feasible if it is one that naturally tends to occur, even if it is very slow because it has a high activation energy.

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15
Q

What is a disproportionation reaction?

A

A reaction in which one of the reactants simultaneously increases and decreases its oxidation number (it both oxidises and reduces).

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16
Q

What is a storage cell?

A

An electrochemical cell that is based on reversible chemical changes so that it can be recharged by an external electricity supply.

17
Q

What is a battery?

A

Two or more electrochemical cells connected in series.

18
Q

What is a fuel cell?

A

An electrochemical cell which is continuously supplied with fuel and oxidising agent. A fuel cell produces electric power from a fuel directly, without having to burn it.

19
Q

What are the rules of oxidation numbers?

A
  • The oxidation number of an element in its standard state is 0.
  • A simple monoprotic ion has an oxidation number equal to its charge.
  • The sum of the oxidations numbers in a neutral formula is 0.
  • The sum of the oxidation number in an ion adds up to the charge on that ion.
20
Q

What is the equation for the e.m.f of a cell?

A

e.m.f = E⌀ (of right half-cell) - E⌀ (of left half-cell)

e.g. Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
e.m.f = 0.34 - -0.76
= 1.10 V

21
Q

How do you write cell notation?

A
  • There needs to be a solid at each end of the notation. If there is no solid substance in the half equation for a cell, platinum (Pt (s) ) is used in addition.
  • Remember ROOR (Reduction, Oxidation, Oxidation, Reduction).
  • The SHE is always on the left hand side.
  • Any change in phase must be represented by a
    line ( | ).
  • A salt bridge must be displayed by a double
    line ( || ).
  • The anode half cell is always on the left (where oxidation occurs)
22
Q

What is a salt bridge?

A

A salt bridge is used to complete the circuit in an electrochemical cell. It can be made in two different ways:

  • A piece of filter paper soaked in a solution of unreactive ions
  • A tube containing unreactive ions in an agar gel

KNO₃ is generally used as K⁺ & NO₃⁻ ions are unreactive.