IB Chemistry Topic 9/19

What is oxidation

Gain of oxygen

Loss of hydrogen

Loss of electrons

What is reduction

Loss of oxygen

Gain of hydrogen

Gain of electrons

What is an oxidation state

The apparent change that an atom has in a molecule or ion

What is the oxidation state of a free, uncombined element.

Zero

What is the oxidation state of a simple ion

the same as the charge on the ion

what is the oxidation state of a neutral compound

All oxidation states add up to zero

What is the oxidation state of a polyatomic ion

All oxidation states add up to the charge on the polyatomic ion

What are the usual oxidation states

Halogens = -1

Alkali Metals = +1

Hydrogen = +1

Oxygen = -2

What does the oxidation number show

The higher the oxidation number, the more oxidised it it

The lower the oxidation number, the more reduced it is

What is an oxidising agent

The reactant which accepts electrons (reduced)

What is a reducing agent

The reactant which loses electrons (oxidation)

What does the activity series show

The strength of reducing agents. The element at the top is the strongest reducing agent.

What trends are there in strength of oxidising agents

The strength of an oxidising agent decreases down a group

What are the features of a redox titration

Redox reaction between oxidising and reducing agents

Electrons are transferred from reducing agent to oxidising agent

What is BOD

Biological oxygen demand

Defined as the amount of oxygen used to decompose organic matter in a sample of water over a certain period of time at a certain temperature

Used as a means of measuring pollution

How is the Winkler Method used

Dissolved oxygen in water is “fixed” by the addition of a manganese salt. The reaction causes manganese to be oxidised

Acidified iodine ions are added, and are oxidised by manganese to produce Iodine in a diatomic molecule

Iodine is titrated against sodium thiosulfate

For every molecule of O₂ used, 4 moles of sodium thiosulfate are used.

What are the two types of electrochemical cells

Voltaic (galvanic) cells generate electricity from chemical reactions

Electrolytic cells drive chemical reactions using energy

How is the simplest half cell created

By putting a strip of metal into a solution of its ions

What will occur in a half cell (zinc as example)

zinc atoms will form ions by releasing electrons.

This will make the surface of the metal negatively charged with respect to the solution.

This causes a charge seperation (electrode potential)

What is electrode potential

When a charge seperation occurs. However it is a redox reaction, so the position of equilibrium determines the size of the electrode potential. The equilibrium position is determined by the reactivity of the metal.

How does the reactivity of the metal affect the equilibrium and how does this affect electrode potential.

Less reactive metals have their equilibrium to the right as they are less likely to lose electrons. Consequently there are less electrons on the surface of the metal, less charge difference so a less negative electrode potential

How is a voltaic cell constructed

Combining two half cells (electrodes) by an external wire.

What occurs in the anode and cathode of the voltaic cell.

Oxidation occurs at the anode (negative charge)

Reduction occurs at the cathode (positive charge)

What must the connection between the half cells be to produce a complete circuit

An external electronic circuit which connects to both electrodes.

And a salt bridge.

What is a salt bridge

A glass tube or piece of absorptive paper which contains an aqueous solution of ions. Movement of these ions neutralizes any charge buildup and maintains potential difference.

What are the conventions of drawing a cell diagram

The anode is on the left of the equation and the cathode on the right. Electrons flow left to right

A double line in the middle of the equation represents the salt bridge

A single line represents state boundaries eg. gas and liquid

What affects the potential difference of a voltaic cell

The difference in electrode potentials between the two species, which is affected by reactivity

What determines which species is the anode in a voltaic cell

The one with the lowest electrode potential (the greater reducing power). The more negative electrode potential. Oxidation occurs here.

What movement occurs through the salt bridge

Anions migrate from the cathode to the anode

Cations migrate from the anode to the cathode

What is the electromotive force (EMF)

The potential difference created by the cell. The magnitude of this is detemined by the difference in the ability to be reduced of both electrodes.

What is standard hydrogen electrode (SHE)

Used as a baseline for comparing electrode potentials of other half cells. Platinum is used as the conducting metal as it is fairy inert in the cell. This is assigned an electrode potential of 0 which allows us to compare other electrode potentials.

What are the standard conditions for measuring electrode potentials

Concentration of 1 mol/dm

All gases at 100 kPa

Temperature at 298 K

All substances must be pure

If the half cell does not contain a solid metal, platinum is used.

What does a positive electrode potential mean

The half cell has a greater tendency to be reduced than hydrogen.

What does a negative electrode potential mean

The half cell is less likely to be reduced than hydrogen.

What are some important points about standard electrode potential

All standard electrode potentials refer to the species being reduced

The standard electrode potentials do not depend on the number of electrons so are not scaled up/down according to the co-efficient

The more positive the electrode potential, the more readily it is reduced

How is EMF calculated from electrode potentials

EMF = Electrode potential of half cell which is reduced (cathode) - Electrode potential of half cell which is oxidised (anode)

How is spontaneity determined by EMF

If EMF is positive then it is spontaneous

If EMF is negative then it is non-spontaneous and the reverse reaction is spontaneous

How are Gibbs free energy and EMF linked in an equation

∆G = -nFE.

n is the number of moles of electrons transferred in the reaction.

F is the Faraday constant

What is the relationship between EMF, Gibbs free energy and spontaneity

EMF = positive and Gibbs = negative

spontaneous

EMF = negative and Gibbs = positive

non-spontaneous

When they both = 0 the reaction is at equilibrium

What is an electrolytic cell

An external source of electricity drives non-spontaneous redox reaction

What is the electrolyte

The reactant in the process is present in the electrolyte. The electrolyte is a liquid either molten/a solution of an ionic compound

What are electrodes made of

An inert, conducting substance such as graphite

What is the cathode in an electrolytic cell

the negative electrode

the power source pushes electrons towards the cathode

What is the anode in an electrolytic cell

The positive electrode

Charge flows through the ions in the electrolyte from the cathode to the anode where they return to the power source.

Which ions are attracted to which electrode in an electrolytic cell

Cations (postive ions) are attracted to the cathode (negative electrode) and are reduced

Anions (negative ions) are attracted to the anode (positive electrode) and are oxidised

What occurs in the electrolysis of molten salts

There is no solvent so only 2 ions present, cations go to the cathode and anions go to the anode.

What are the rules for electrolysis of an aqueous solution at the anode

If a halogen is present then that is oxidised

If not then OH-

What are the rules for electrolysis of an aqueous solution at the cathode

The species which is the lowest down in the reactivity series is reduced

which is the same as the species with the most positive electrode potential value

How can we calculate mass of product on a electrode in electrolysis

1. Calculate the charge of the circuit using Q= It meaning charge = current x time

2. Use Faradays constant to work out the number of Faradays which is equal to number of moles number of F = charge/faraday constant

3. The use the mole ratio in the half equation to determine moles of product

4. Mass = moles x mr

What is the Faraday Constant

The amount of charge carried by 1 mole of electrons.

What is electroplating

the process of using electrolysis to deposit a layer of a metal on top of another metal or other conductive object

How does electroplating work

The cathode is made out of the metal which is going to be electroplated

The electrolyte has the ions which are used to be plated

Anode is made of the same metal which is to be coated because it may be oxidised to replenish the supply of ions in the electrolyte.