Reduction and oxidation reactions
Reduction is the loss of oxygen atom from a molecule or the gaining of one or more electrons. A reduction reaction is seen from the point of view of the molecule being reduced, as when one molecule gets reduced another gets oxidised. The full reaction is known as a Redox reaction. This is a good way of remembering it.
This can be remembered with the term OIL RIG when speaking about electrons.
- Oxidation Is Loss
- Reduction Is Gain
In the case of Organic Chemistry it is usually the case of the gaining/loss of Oxygen/Hydrogen
- Oxidation is a process where a substance
- Loses one or more electrons
- Gains an oxygen atom or Electronegative atoms
- Loses a hydrogen atom or Electropositive atoms
- Gains an increase in its oxidation number
- Reduction is the opposite!
Electrochemistry plays an important part in our everyday lives. It is responsible for the rusting of iron, it allows us to purify many metals or plate common metals with silver or gold, it explains how batteries power iPods (and also how we can recharge the batteries), and is used in countless other technologies.
In order to understand electrochemistry, we must first examine Reduction/Oxidation reactions (also known as REDOX reactions).
Some elements have a higher affinity for electrons than others. When a material comes into contact with a material with a lower electron affinity, it will remove electrons from the other material. The process in which a substance loses an electron in a chemical reaction is called oxidation. The lost electron cannot exist on its own and must be gained by a second substance. The substance that gains the electron is said to be reduced (a simple trick to help remember this is the acronym "LEO (lose electrons - oxidized) went GER (gain electrons - reduced)" Or an alternative way of remembering oxidation and reduction in a substance is to remember "OILRIG"- ( OIL = Oxidation Is Loss of electrons and RIG= Reduction Is Gain of electrons).
Redox and electrochemistry
- Oxidation is a combination of elements with oxygen. It's also a reaction of losing electrons and gaining positive charge. The atoms that lost electrons are said to be oxidized. Atoms can be oxidized by nonmetals.
- Reduction is gain of electrons and thus gaining of negative charge. The atom that acquired electrons is said to be reduced.
When electrons are lost by one atom, they must be gained by another element. Therefore oxidation and reduction cannot occur alone. If one occurs, the other must occur also. Reactions involving oxidation and reduction are called redox reactions.
- The oxidation number (step) of an element is a convenient way to keep track of electron transfer. It defines the number of electrons that are lost or gained by an element.
- The metals of group 1 (1A) always have oxidation number +1.
- The metals of group 2 (2A) always have oxidation number +2.
- Fluorine always has an oxidation number of -1.
- The rest of the halogens (Cl, Br, I) usually have oxidation numbers of -1, except in their compounds with fluorine and oxygen, where they will be positive.
- Oxygen usually has an oxidation number of -2, unless paired with fluorine (where it will be positive).
- Hydrogen has an oxidation number of +1 in all its compounds except hydrides of active metals. Its oxidation number in metal hydrides is -1.
2-When vegetable oil is reduced, we get hydrogenated oil.
Changes in oxidation number
Redox reactions can be recognized, as the change of the oxidation number of some of the atoms. If the oxidation number of an element increases, then the element is oxidized. If it is decreased, then the element is reduced.
- The oxidation number is in the range between -7 and +7. It shows the number of electrons lost with regards to the neutral atom.
- The reducing agent is an element or compound that can lose an electron (undergo oxidation).
- The oxidizing agent is an element or compound that can gain an electron (undergo reduction).
Every redox reaction consists of two parts, the oxidation and the reduction. Each one separately is called a half - reaction.
- During the redox reaction there is a transfer of electrons from the substance being oxidized to the substance being reduced. In a voltaic cell, these reactions happen in separate vessels.
- The vessels are connected by a salt bridge, which allows ions to move from one solution to another. The salt bridge does NOT transfer electrons. The electrons are transferred through a wire connecting the two vessels. This flow constitutes an electric current. The flow of charge continues due to the migration of the ions through the salt bridge. The complete system is called an electrochemical cell, or simply a chemical cell. Each vessel in which half reaction takes place is called a half cell.
- The flow of electrons through the wire is caused by differences in electric potential between the electrodes. This potential, measured in volts, is proportional to the free energy change. The flow of current carries electrical energy, which is used to do work. A voltaic or galvanic cell is a device that directly converts chemical energy to electrical energy. Cells that convert electrical energy into chemical energy are called electrolytic cells.Using ecs we can predict which oxidising agent can oxidise other reducing agent excellent approach
It's not possible to measure the potential of a half reaction by itself. Instead, it is necessary to measure the difference in potentials between two half reactions, when they are paired in a chemical cell. The voltage developed between any given half reaction, is called standard electrode potential ( E 0 ) of a half reaction. the measure of ( E 0 ) made under 1 atm pressure with 1 molar (M) solutions at 25 degrees C.
Oxidation and Redox Potentials
For a half-reaction with a given oxidation potential, its reduction potential will be opposite in sign. The overall potential of a redox reaction is the sum of the reduction and oxidation half-reaction potentials.
- The potential of a chemical cell is a sum of the potentials of the half reactions.
- When adding the half-cell potentials, make sure that there is a reduction and an oxidation taking place.
- The positive electrode is called the cathode, and the negative electrode is called the anode.
Balancing the redox reactions
- Assign the oxidation numbers to the elements.
- Write half reactions for the oxidation and reduction.
- Multiply the half reaction numbers by a number, that will equalize
The number of electrons
The oxidation number in reactants and products must be balanced to zero. After this step we will define which element is oxidized and which element is reduced.