Physical - Oxidation, Reduction and Redox Equations Flashcards
What are redox reactions?
The word redox is short for reduction-oxidation. By definition, reduction and oxidation take place simultaneously and we can show an overall equation or two half equations for redox reactions.
Historically, oxidation was used for reactions in which oxygen was added. The oxygen added is called the oxidising agent. If oxygen is removed, it is called reduction.
Reduction described a reaction in which oxygen was removed. As hydrogen was often used to remove oxygen, the addition of hydrogen was called reduction. The hydrogen added is called the reducing agent. If hydrogen is removed, it is called oxidation.
What is oxidation?
The loss of electrons.
What is reduction?
The gain of electrons.
What are redox reactions also called?
Since redox reactions always involve the movement of electrons, they are also called electron transfer reactions. You can see the transfer of electrons by separating a redox reaction into two half equations that show the gain and loss of electrons.
What are oxidising and reducing agents?
- reducing agents give away electrons (they are electron donors)
- oxidising agents accept electrons
Why are oxidation states used?
Oxidation states are used to:
- tell if oxidation or reduction has taken place
- work out what has been oxidised/reduced
- construct half equations and balance redox equations
Oxidation states are also called oxidation numbers.
An oxidation number is the charge an atom would have if electrons in its bonds belonged completely to a more electronegative atom.
What are oxidation states?
Each element in a compound is given an oxidation state. In an ionic compound, the oxidation state simply tells us how many electrons it has lost or gained, compared with the element in its uncombined state. In a molecule, the oxidation state tells us about the distribution of electrons between elements of different electronegativity. The more electronegative element is given the negative oxidation state.
What do the positive and negative numbers mean?
- Every element in its uncombined state has an oxidation state of zero.
- A positive number shows that the element has lost electrons and has therefore been oxidised.
- A negative number shows that the element has gained electrons and has therefore been reduced.
- The more positive the number, the more the element has been oxidised. The more negative the number, the more it has been reduced.
- The numbers always have a + or - sign unless they are zero.
What are the rules for oxidation states?
- Uncombined elements have oxidation state of 0.
- Some elements always have the same oxidation state in all their compounds. Others usually have the same oxidation state.
- hydrogen: +1 except in metal hydrides, where it is -1
- group 1: always +1
- group 2: always +2
- aluminium: always +3
- oxygen: -2 except in peroxides where it is -1, and oxygen fluoride, where it is +2
- fluorine: always -1
- chlorine: -1 except in compounds with F and O, where it has positive values
- The sum of all the oxidation states in a compound = 0, since all compounds are electrically neutral.
- The sum of the oxidation states of a complex ion, such as NH4 + or SO4 2-, equals the charge on the ion.
- In a compound, the most electronegative element always has a negative oxidation state.
What are ionic equations?
- Only show species in a reaction that are chemically altered (have a change in oxidation state).
- Identify which species in this equation are chemically altered and which are spectator ions (not chemically altered).
- Write reduction and oxidation equations for these.
- Combine the equations.
What are oxidising and reducing agents?
- An oxidising agent makes oxidation happen. It is a species that removes electrons from another. It itself is reduced as it gains electrons.
- An reducing agent makes reduction happen. It is a species that gives electrons to another. It itself is oxidised as it loses electrons.
What is disproportionation?
A reaction where a substance undergoes both oxidation and reduction simultaneously is called a disproportionation reaction.
How do you write half equations?
- Identify the atom being oxidised or reduced, and make sure there is the same number of that atom on both sides by balancing.
- Calculate oxidation states on each side of the equation (before and after the change).
- Add electrons to one side of the equation so that the oxidation states balance. If the oxidation states become more negative, then it gains electrons. If the oxidation state becomes more positive, then electrons are lost.
- Often reactions take place in aqueous conditions, so a number of spectator species are also present, and although neither is oxidised nor reduced, they still exist and need to be balanced.
- Sort out oxygens. For every oxygen gained/lost, add/remove one H2O molecule.
- Sort out hydrogens. For every hydrogen gained/lost, add/remove one H+ ion.
- Check - if the total electric charge on the left equals that on the right, then it is probably correct. If if it not then you know you have gone wrong.
How do you combine half equations?
- Write out the two half equations.
- Multiply the equations so that the number of electrons in each is the same.
- Add the two equations and cancel out the electrons on either side.
- If necessary, cancel any other species which appear on both sides.
