IB Chemistry S3.2

Half Equation

An equation that happens to one reactant, in terms of electrons.

Oxidation

gain of oxygen, loss of electrons, loss of hydrogen or increase in oxidation number

Reduction

loss of oxygen, gain of electrons, gain of hydrogen or decrease in oxidation number

Redox

Reactions involving Oxidation and Reduction.

Oxidation Number

The measure of electron control atoms in molecules have relative to atoms of the pure (non-ionic) element. Often the same as ionic charge

Oxidising Agent

A reactant that is reduced so that another can be oxidised

As you go up group 7...

the halogens become stronger oxidising agents because their reactivity increases

Reducing Agent

A reactant that is oxidised so that another can be reduced

The more reactive a metal...

the better reducing agent it is because it lose valence electrons more readily

Displacement Reaction

A reaction where an ion in a solution is displaced through oxidation of an element added to the solution.

Aside from a displacement reaction how can you know if one metal is more reactive than other?

Measure the temperature because displacement reactions are exothermic

Spectator Ion

An ion that takes part in displacement reactions, but isn't chemically changed

Electrode Potential

The charge difference between metal ions and the solution containing them.

Voltaic Cell (Battery)

A type of electrochemical cell that converts chemical energy from a spontaneous reaction into electrical energy

Anode

The electrode where oxidation occurs.

VAN

voltaic anode negative

What charge is the anode in an electrolytic cell?

positive

1 multiple choice option

Cathode

The electrode where reduction occurs. It's positive in a voltaic cell and negative in an electrolytic cell.

What are the typical components of a voltaic cell?

Anode, cathode, salt bridge, wires and voltmeter

In what direction do electrons flow in a voltaic cell?

Anode → Cathode

Salt Bridge

A glass tube or paper strip that prevents charge buildup so the solutions remain neutral, ions can be transferred and voltage can be generated

How can the voltage generated by a voltaic cell be altered?

Use different metals (changes difference in reactivity), different concentrations, change temperature or pressure if gases are present

In a cell diagram, what does a single line represent?

a phase boundary

In a cell diagram, what does a double line represent?

a salt bridge

What is a secondary cell?

cells that involve redox reactions that can be reversed using electrical energy which makes the cell rechargeable

Give an example of a secondary cell?

Lead-acid car batteries that power the engine's motor. The anode is lead and the cathode is lead oxide which both sit in sulfuric acid

How is the reaction reversed in a lead-acid battery?

the chemical energy from combustion

What kind of electrochemical cell is a lithium-ion battery?

a secondary cell

Evaluate secondary cells

Compared to voltaic cells, they satisfy higher current demands but have a higher rate of self-discharge

What are fuel cells?

Fuel cells convert methanol, ethanol or hydrogen into water, carbon dioxide and heat

Evaluate fuel cells

Very efficient as more energy is converted into kinetic energy. Little pollution. But they aren't rechargeable so they require a constant supply of fuels. It's also expensive to transport and store these fuels which can require energy to make e.g hydrogen

What is an electrolytic cell?

Cells that converts electrical energy into chemical by bringing about non-spontaneous reactions with an external power source

Electrolysis

Breaking down a compound by inducing an electrical charge.

Electrolyte

A substance that dissolves in water to make a solution with ions that conduct electric current

Where does the electrical energy come from in an electrolytic cell?

The electrodes are suspended in an electrolyte which is typically a molten or aqueous ionic compound. Therefore, the ions are mobile which allows current to flow

What are the components of an electrolytic cell?

two inert electrodes, an electrolyte, direct current power source

What materials can inert electrodes be made of?

platinum or carbon e.g graphite

In molten electrolysis, what happens at the anode?

Anions are oxidised into atoms

In molten electrolysis, what happens at the cathode?

Cations are reduced into atoms

In the electrolysis of aqueous solutions, what happens at the anode?

the non-metal ion or water will be oxidised

In the electrolysis of aqueous solutions, what happens at the cathode?

the metal ion or water will be reduced

Which 3 factors determine what products are formed during the electrolysis of aqueous solutions?

- concentrations of the ions in the electrolyte

- nature/material of the electrodes

- E° values of the ions

How do the E° values of the ions affect the products formed during the electrolysis of aqueous solutions?

At the cathode and the anode, the reaction with the more positive electrode potential will occur

Selective Discharge

The process in an electrolytic cell by which only one ion at an electrode is oxidised or reduced, and only one product is given off. This depends on which is oxidised or reduced more easily.

The Standard Hydrogen Electrode

An electrode with an electrode potential of 0.00V to act as a reference point by which from electrode potentials can be compared.

What are the features of the standard hydrogen electrode?

platinum electrode, 1.00 mol/dm^-3 acid, hydrogen gas, 100kPa at 298K

2H⁺(aq) + 2e⁻ ↔ H₂(g)

The reaction occurring in the Standard Hydrogen Electrode.

Standard half-Cell

A half cell in standard conditions.

Standard Electrode Potential (E°)

the potential (voltage) of the reduction half-equation under STP relative to SHE

Negative standard electrode potentials indicate that...

reduction is unlikely to occur but it can act as a reducing agent

Positive standard electrode potentials indicate that...

reduction is more likely to occur than the reduction of hydrogen ions so it can act as an oxidising agent

How is E^Θcell calculated?

= E^Θhalf-cell of reduction - E^Θhalf-cell of oxidation

If E^Θcell is positive...

the reaction is spontaneous

If E^Θcell is negative...

the reaction isn't spontaneous but the reverse reaction will be

Electroplating

The process of coating one metal (the cathode) in another through electrolysis because the metal at the anode will erode due to oxidation. These ions will move to the cathode and become solid atoms again

What's an example of using electrolysis to purify a metal?

Impure copper as the anode and pure copper as the cathode in a solution of copper sulphate

What are common oxidising agents and why do they work well?

acidified potassium manganate and potassium dichromate because they are transition metals with lots of oxygen

How are oxidation agents denoted in equations?

[O]

What happens when a primary alcohol is oxidised by distillation?

It becomes an aldehyde and water is released

Describe how a primary alcohol is oxidised by distillation?

The alcohol and oxidising agent are gently heated in a flask to the boiling point of the aldehyde so that it becomes gaseous. This vapour passes through the condenser so it condenses and the liquid is collected in a flask

What happens when a primary alcohol is oxidised by reflux?

An aldehyde forms initially, then it becomes carboxylic acid

Describe the process of oxidation by heating under reflux?

The reflux condenser condenses any gases and forces them back into the reaction vessel. This puts the aldehyde back in the flask to be oxidised again.

What happens when a secondary alcohol is oxidised by reflux?

it becomes a ketone because the hydroxyl is oxidised into a carbonyl and water is lost

Can tertiary alcohols be oxidized?

No

How can the oxidation of alcohols be reversed?

Using reducing agents which are good sources of hybride ions (H-)

How are hybride ions represented?

[H]

How are alkenes and alkynes reduced?

Because they are unsaturated, the addition of hydrogen, in the presence of a transition metal catalyst e.g nickel will reduce carbon's oxidation state and the degree of unsaturation

What is formed when an alkyne is reduced?

An alkene is formed

What is formed when an alkene is reduced?

An alkane is formed

How can alkynes be directly reduced to alkanes?

Reacting them with excess hydrogen in the presence of transition metal catalyst