Redox Reactions (electron transfer)

Unit 2

Redox

Reduction and Oxidation occurring at the same time. Electron transfer reactions.

Oxidation

loss of electrons

Reduction

 gain of electrons

Oxidant/ Oxidising agent

causes oxidation of another substance, and is reduced. 

Reductant/Reducing agent

cause reduction of chemical, and is oxidised.

Conjugate REDOX pairs

two species differ by certain no. of electrons. 

Oxidation no’s

charges of a substance.

How to determine whether a reaction is a redox reaction, and if an element has been oxidised/reduced?

using oxidation numbers 

Element

oxidation no. of 0 e.g. Zn metal, H2 (g)

Neutral compound

overall oxidation no. of 0 - adds up to 0 e.g. CO2 (g)

Elemental ions

oxidation no. according to charge on the ion. Determined by: looking at what group the element belongs to. E.g. chloride ion, CI- has oxidation no. of -1.

Polyatomic ions

overall oxidation no. = to overall charge of ion. E.g. sulfate ion, SO4^2-, overall oxidation no. of -2.

Hydrogen

part of a compound, has oxidation no. of +1. Exception: when hydrogen is part of a metal hydride. E.g. LiH

Oxygen

part of a compound, oxidation no. of -2. Exception: oxygen part of peroxide, or bonded to fluoride.

Transition metal

can have variable oxidation numbers e.g. iron(II) io, has oxidation no. of +2. Iron (III) ion - oxidation no. of +3

Reactivity / electrochemical series

metals ranked in a list according to their reactivity/ability to act as a reducing agent.

Lists metals in order of how readily they react with oxygen, water, steam, dilute acids and salt solutions, and is useful for predicting reactions.

reactivity series rules

right: metal from least to most reactive top to bottom. Stronger reductants, oxidise easier, lose e’s.

left: cations from bottom to top, increasing in greater e attraction, stronger oxidants, reduce easier.

Metal displacement reaction

series allows you to predict which metals displace other metals from solutions of their ions.

More reactive metal OXIDISED, and donate e’s to cation of less reactive

How would a spontaneous redox reaction occur in reactivity series?

cations above metal (cation higher up, oxidant will react with lower metal, reductant).

Redox reactions seen in…

primary cells/batteries, corrosion, corrosion protection

Redox reactions

spontaneous reactions that involve flow of e’s, energy released from reaction.

primary vs secondary cell

Primary cell - single-use battery.

Secondary cell - rechargeable battery.

Galvanic cell

electrochemical cell converts chemical to electrical energy.

Anode (-)

 where oxidation occurs

Cathode (+)

where reduction occurs.

Salt bridge

maintains electrical neutrality, provides ions to balance ions consumed or produced in each half-cell. 

Primary cell features:

- more reactive metal gives e’s to ions of less reactive

-e’s always flow to the cathode (+)

-electrode polarity - sign of electrodes 

-Galvanic cell: Anode (-), Cathode (+)

-Electrodes inert like graphite (carbon) / platinum.