3.1.7- Oxidation, reduction and redox equations (PAPER 1) Flashcards
What is oxidation? (o,e,h)
The gain of oxygen, the loss of electrons, the loss of hydrogen.
What is reduction? (o,e,h)
The loss of oxygen, the gain of electrons, the gain of hydrogen.
How to remember oxidation and reduction?
OILRIG
oxidation is loss of e- and H, reduction is gain of e- H
What is a redox reaction?
When one chemical gains electrons/ is reduced and another loses electrons/ is oxidised.
What is an oxidation state?
The number of electrons something needs to lose or gain to become neutral.
What is the oxidation state of uncombined elements/ diatomic molecules?
0 as they are already neutral.
Eg- Na, Fe, Br2, Cl2
What is the oxidation state of group 1 elements?
+1
What is the oxidation state of a monatomic ion? -ion made up of one single atom with charge.
Eg- K+, Al3+, Cl-
The same as the charge on the atom/ion.
Eg- K+ has an oxidation state of +1
Eg- Al3+ needs to gain 3 electrons to become neutral so oxidation state is +3
Eg- Cl- needs to lose 1 electron so its oxidation state is -1
What is the oxidation state of group 2 elements?
+2
What is the typical oxidation state of group 3 elements?
+3
What is the typical oxidation state for group 5 elements?
-3
What is the typical oxidation state for group 6 elements?
-2
What is the typical oxidation state for group 7 elements?
-1
How to know transition metal oxidation states? Eg- iron(iii) oxide
The roman numeral next to the metal
Eg- iron (iii) oxide
iron = +3 oxidation state
What is the oxidation state for hydrogen+ anomaly?
+1 Eg- HF
UNLESS in hydrides, then it is -1 Eg- NaH
What is the oxidation state for chlorine + anomaly?
-1 Eg- KCl
UNLESS in a compound with F and O, it would be + Eg ClF3 Cl= =3
What is the oxidation state for fluorine + anomaly?
-1 Eg- KF
What is the oxidation state for oxygen + anomaly?
-2 Eg- Li2O
UNLESS in peroxides Eg- H202 O= -1
UNLESS in F20 O= +2
How to work out oxidation state of a compound?
All oxidation states must add to give overall charge of compound= eg 0, -1, -2
What is the oxidation state of N in NH4 + ?
H= +1 x 4 = +4
how to get from +4 to +1?
N= -3
What is the oxidation state of Cr in Cr2O7 2- ?
O= -2 x 7 = -14
how to get from -14 to -2?
Cr2 = +12 so Cr = +6
Difference between oxidation and reduction half equation?
In reduction half equations, the electrons appear on the left as they are being gained.
In oxidation half equations, the electrons appear on the right as they are being lost.
What are the 5 steps to writing half equations?
1) Write what you know as an equation.
2) Balance for atoms (not O or H)
3) Balance for O by adding H2O
4) Balance for H by adding H+ ions
5) Balance charge by adding e- ON THE SAME SIDE AS THE + CHARGE
Write the half equation for I- reacting to make I2
1) I- —-> I2
2) 2I- —–> I2
5) 2I- ——> I2 + 2e-
Iodide has been oxidised as electrons are on the right so have been lost.
ITS OXIDATION NUMBER INCREASES.
Write the half equation for MnO4 - to Mn 2+
1) MnO4 - ——-> Mn2+
3) MnO4 - ——–> Mn2+ 4H20
4) MnO4- + 8H+ ———> Mn2+ + 4H2O (4H2 = 8H)
5) RHS is +2, LHS is +7
MnO4 - + 8H+ + 5e- ——–> Mn 2+ +4H2O
What is the method for combining half equations?
1) Write out the 2 half equations
2) Multiply the equations so they both have the same number of e-
3) Add the 2 equations, cancel out the same e- on each side
4) Cancel out other species which appear on both sides of final equation
Combine these 2 half equations:
Fe 3+ + 3e- —-> Fe
2 I- —–> I2 + 2e-
Fe 3+ + 3e- —-> Fe X2
2Fe 3+ + 6e- ——> 2Fe
2 I- —–> I2 + 2e- X3
6I- ——> 3I2 + 6e-
add the new equations=
2Fe 3+ + 6e- + 6I- —–> 2Fe + 3I2 + 6e-
cancel out
2Fe 3+ + 6I- ——-> 2Fe + 3I2
How to recognise which is reducing and oxidising agent from 2 half equations?
Eg-
Fe—–> Fe2+ + 2e-
-electrons on right side= reactant is losing electrons, oxidation
MnO4- + 5e- + 8H+ —–> Mn 2+ + 4H2O
-electrons on left side= reactant is gaining electrons, reduction
Red——-ox
REDUCTION ON LEFT, OXIDATION ON RIGHT
How to recognise which is reducing and oxidising agent from full equation?
Eg- 5Fe + 2MnO4- —–> 5Fe 2+ + 2Mn 2+ + 8H20
work out oxidation states
O state 5Fe= 0
O state 5Fe 2+ = +2
Fe OXIDISED
O state 2MnO4- Mn= +7
O state 2Mn 2+ Mn= +2
Mn REDUCED
What is a reducing agent?
Causes something else to be reduced by donating electrons.
Is oxidised itself
What is an oxidising agent?
Causes something else to be oxidised by accepting electrons.
Is reduced itself.
