Topic 14 Redox 2 Flashcards
What does redox stand for?
Reduction & oxidation
What is reduction? oxidation?
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.
What is an oxidation number?
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
What is stochiometry?
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.
What is an oxoanion?
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⁻
What is an oxidising agent? Reducing agent?
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.
What is a primary standard? What characteristics must they have?
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.
What is a standard solution? How do you prepare it?
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).
What is an electrochemical cell?
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.
What is the electromotive force (e.m.f)?
The maximum voltage produced by an electrochemical cell. The symbol for e.m.f is E and its SI unit is the volt (V).
What is the standard hydrogen electrode (SHE)?
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
What is the standard electrode (reduction) potential?
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.
What is a reference electrode?
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.
What does the term feasibility mean?
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.
What is a disproportionation reaction?
A reaction in which one of the reactants simultaneously increases and decreases its oxidation number (it both oxidises and reduces).
What is a storage cell?
An electrochemical cell that is based on reversible chemical changes so that it can be recharged by an external electricity supply.
What is a battery?
Two or more electrochemical cells connected in series.
What is a fuel cell?
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.
What are the rules of oxidation numbers?
- 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.
What is the equation for the e.m.f of a cell?
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
How do you write cell notation?
- 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)
What is a salt bridge?
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.
