IB SL Chemistry - Reactivity 3.2

Redox reaction

The oxidation and reduction of species

What are the 4 ways to “define” redox reactions?

1) The loss and gain of oxygen, 2) the los and gain of hydrogen, 3) electron transfer, and 4) oxidation state.

Disproportionation

A species can be both reduced and oxidized in the same reaction.

Ex: 4KClO₃ → 3KClO₄ + KCl

True or False: The combustion of metals to form meta oxides is an oxidation reaction.

True- oxidation is a reaction where a substance combines with oxygen, while reduction is a reaction where oxygen is removed from a substance.

True or False: Oxidation reactions can be considered as a gain of hydrogen, while reduction reactions can be considered as a loss of hydrogen.

False- Oxidation loses hydrogen atoms, while reduction gains hydrogen atoms.

What does LEO and GER mean when it comes to electron transfer?

LEO = Loss of Electrons in Oxidation

GER = Gain of Electrons in Reduction

Oxidation state

Represents the charge that an atom would have in a compound if the compound were composed of ions.

A compounds is oxidized if the oxidation state of an atom in that compound…

..increases.

A compounds is reduced if the oxidation state of an atom in that compound…

…decreases.

In and acidic solution, what species are added to balance the number of oxygen and hydrogen atoms?

H₂O_(l) is added to balance any oxygen and H⁺_(aq) is added to balance any hydrogen.

In a basic solution, what species are added to balance the number of oxygen and hydrogen atoms?

OH^-_(aq) is added to balance any oxygen and H₂O_(l) is added to balance any hydrogen.

Which way does reactivity increase for halogens?

Reactivity increases up the group!

True or False: Halogens can act as oxidizing agents in redox reactions.

True- they themselves are reduced, but cause other species to be oxidized.

Which way does reactivity increase for group 1 metals?

Reactivity increases down the group!

True or False: Group 1 metals can act as reducing agents in redox reactions.

True- they themselves are oxidized, but cause other species to be reduced.

To test the reactivity of other metals, when placing a pure metal into a solution of ions of a different metal, what does it mean if a reaction occurs or if no reaction occurs?

If a reaction occurs: the pure metal is more easily oxidized.

If no reaction occurs: the metal in the solution (initially) is more easily oxidized.

What are the two types of electrochemical cells?

1) Primary (voltaic) cells, secondary (rechargeable) cells, and fuel cells- energy is produced by spontaneous chemical reactions to generate electrical energy.

2) Electrolytic cell- electrical energy is used to drive forward non-spontaneous chemical reactions.

True or False: Nearly all spontaneous redox reactions are exothermic.

True- Hint: the energy released in these chemical changes can be used to produce electrical energy.

What does “an ox and a red cat” mean?

In the ANode, OXidation occurs. In the CAThode, REDuction occurs.

What is the purpose of the salt bridge that connects the two half-cells?

The salt bridge is used to complete the electrical circuit. It consists of an ionic salt solution that allows the cations to flow toward the slightly negative cathode and the anions flow toward the slightly positive anode. Note: This is necessary because if the cell becomes polarized, the redox reaction stops.

What is the general template to write cell diagrams for metal-ion primary cells?

anode being oxidized | product of oxidation || species being reduced | product of reduction/cathode

Battery

A series of two or more electrochemical cells.

List characteristics of primary (voltaic) cells

Not rechargeable and can not experience the reversible reaction.

Polarization causes a build up of H_2(g) bubbles on the surface of the anode, which increases internal resistance and reduces electrical output.

This cell is only good for low currents.

List characteristics of secondary (rechargeable) cells

The reversible reaction can occur by applying electrical current to the cell.

Can sustain high current demands and is self-dischargeable.

List characteristics of fuel cells

Converts hydrogen, methanol or ethanol, and oxygen into water, carbon dioxide, and heat.

Produces little pollution and is very efficient.

Not rechargeable, but they require a steady supply to continue.

Hydrogen fuel cell

H_2(g) is supplied to the anode while O_2(g) is supplied to the cathode.

Produces water, not any greenhouse gases.

Direct methanol fuel cell (DMFC)

Methanol (CH₃OH) is supplied to the anode.

Produces carbon dioxide and water.

What is the purpose of an electrolyte or separator?

To keep components from mixing.

What is a proton exchange membrane (PEM)?

It is a type of electrolyte/separator that allows H⁺ ions to diffuse through by prevents the diffusion of other ions, electrons or molecules.

Electrodes

Made of a catalyst. There is an oxidizing electrode (anode) and a reducing electrode (cathode).

Bipolar plate

Conducts the electrical current from cell to cell and ensures uniform distribution of the fuel gas.

List the problems associated with a hydrogen fuel cell

1) Often made of platinum which is very expensive.

2) The hydrogen gas must be pure, but there are only two sources:

a) the electrolysis of water or b) from the reaction

CH₄ + H₂O → 3H₂ + CO

What is the electrolysis of water?

Electrolysis of water is using electricity to split water into oxygen and hydrogen gas.

Electrolysis

When electrical energy is used to make a non spontaneous redox reaction react.

Electrolyte

Ionic salt with free moving cations and anions.

True or False: The negative terminal of the DC power source is connected to the anode.

False- the negative terminal is connected to the cathode, therefore allowing the electrons to flow to the cathode and reduce the cations in the electrolyte.

In the presence of an oxidizing agent (in this case on oxygen atom donor), the hydroxyl group in secondary alcohols can be oxidized to a…

…carbonyl group, forming a ketone. (Draw this!)

In the presence of an oxidizing agent (in this case on oxygen atom donor), the primary alcohols can be oxidized to…

…carboxylic acids in a two-step reaction. (Draw this!)

What are the two steps that primary alcohols have to undergo during oxidation to produce carboxylic acid? (Draw this!)

1) The hydroxyl group is oxidized to a carbonyl group, forming an aldehyde.

2) The carbonyl group is oxidized to a carboxyl group, forming a carboxylic acid.

What is the difference between reflux and distillation?

Reflux: used to continuously heat a solution without significant loss of solvent, reactants, or products. Ensures a reaction proceeds fully by trapping vapors.

Distillation: used for separating components of a mixture based on their boiling points. Isolates a specific component or purifies a substance. 

True or False: Tertiary alcohols can be oxidized in the same wasy as secondary and primary alcohols.

False- tertiary alcohols can not be oxidized in the same way because oxidation requires the removal of the hydrogen atom connected to the carbon in the hydroxyl group.

True or False: When reducing organic compounds, the reduction agent provides a source of hydrogen atoms.

True

What is the two-step process for the reduction of carboxylic acid?

Carboxylic acid → aldehyde → primary alcohol. (Draw this!)

In the presence of a reducing agent, the carbonyl group in a ketone can be reduced to a…

…hydroxyl group, forming a secondary alcohol. (Draw this!)

True or False: Unsaturated compounds (alkenes and alkynes) can be reduced by the addition of hydrogen to the multiple bond.

True: Alkynes + H_2(g) → Alkenes and Alkenes + H_2(g) → Alkanes