Stoichiometry: Chemical Calculations

Chemistry 120

Stoichiometry: Chemical Calculations

Chemistry is concerned with the properties and the

interchange of matter by reaction i.e. structure and

change.

In order to do this, we need to be able to talk about

numbers of atoms.

The key concept is the mole and the relationship

between the mole and the mass of the atom.

Chemistry 120


Each element has a distinct atomic mass – based on

the natural abundances of the various isotopes

present.

Atoms combine to form molecules in fixed

proportions which are usually small integers for

simple molecular or ionic compounds

Chemistry 120


The molecular mass of a compound is the mass of

each atom multiplied by the number of times it

appears in the formula unit

Chemistry 120





CH4

SF6

NaCl

Na2S2O3

Chemistry 120





CH4 12.0115 + 4 x 1.0079

SF6

NaCl

Na2S2O3

Chemistry 120





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl

Na2S2O3

Chemistry 120





CH4 12.0115 + 4 x 1.0079

SF6 32.066 + 6 x 18.9984

NaCl 22.9898 + 35.453

Na2S2O3 2 x 22.9898 + 2 x 32.066 + 3 x 15.9994

Chemistry 120


The mole and atomic mass

The mole is defined as

the number of elementary entities as are present in 12.00 g of 12C.

Numerically, this is equal to Avogadro’s Number

6.022 x 1023

Therefore, in 12.00 g of 12C there are 6.022 x 1023 ‘elementary entities’, in this case atoms.

Chemistry 120


The mole and atomic mass

Atomic masses, in atomic units, u, are defined relative to 12C.

Therefore,

The formula mass of an element or compound contains 1 mole, 6.022 x 1023, of particles

Chemistry 120


Examples

How many particles are there in 5 g of Na?

Chemistry 120


Examples


The particles are atoms – how many atoms are there in 5 g of Na?

Chemistry 120


Examples


The particles are atoms – how many atoms are there in 5 g of Na?

Atomic mass of Na = 22.9898 u

Chemistry 120


Examples


The particles are atoms – how many atoms are there in 5 g of Na

Atomic mass of Na = 22.9898 uAs

1 u = 1/12 x mass (12C) And

1 mole = 6.022 x 1023 particles = number of particles in 12 g 12C

Chemistry 120


Examples


The particles are atoms – how many atoms are there in 5 g of Na

Atomic mass of Na = 22.9898 u

Mass of 1 mole of Na = 22.9898 g

Chemistry 120


Examples

How many particles are there in 5.0000 g of Na?

22.9898 g Na = 1 mole Na

Then 1 g Na = 1 mol Na

22.9898

5 x 1 g Na = 5 x 1 mol Na

22.9898

5 g Na = 0.2175 mol Na

5 g Na = 0.2175 x (6.022 x 1023) particles Na

Chemistry 120


Examples

How many particles are there in 5.0000 g of Na?

1.310 x 1023 atoms

Chemistry 120


Examples

What is the mass of 0.23 mol of butane?

Chemistry 120


Examples


Molecular formula of butane: C4H10

Chemistry 120


Examples



Atomic mass of C = 12.011g

Atomic mass of H = 1.0079g

Chemistry 120


Examples





Molecular mass of C4H10 = (4x12.011)+(10x1.0079)u

= 58.123 u

Chemistry 120


Examples





Molecular mass of C4H10 = (4x12.011)+(10x1.0079)u

= 58.123 u

Relative Molecular Mass of Butane = 58.123 g

Chemistry 120


Examples



1 mole of butane = 58.123 g

0.23 x 1 mole of butane = 0.23 x 58.123 g

Chemistry 120


Examples



1 mole of butane = 58.123 g

0.23 x 1 mole of butane = 0.23 x 58.123 g

0.23 mole of butane = 13.368 g

Chemistry 120


Chemical Equations

These are formulæ which show the chemical change taking place in a reaction.

Sr(s) + Cl2(g) SrCl2(s)

Chemistry 120


Chemical Equations


Physical state


Chemistry 120


Chemical Equations


Physical state


Reactants Product

Chemistry 120


Chemical Equations

As matter cannot be created or destroyed in a chemical reaction, the total number of atoms on one side must be equal to the total number of atoms on the other.

Chemistry 120


Chemical Equations

Example

Cyclohexane burns in oxygen to give carbon dioxide and water

Chemistry 120


Chemical Equations

Example


Reactants: Cyclohexane, C6H12

Oxygen, O2

Chemistry 120


Chemical Equations

Example


Reactants: Cyclohexane, C6H12

Oxygen, O2

Products: Carbon Dioxide, CO2

Water, H2O

Chemistry 120


Chemical Equations

Example

Initially, we can write the reaction as

C6H12 + O2 CO2 + H2O

Chemistry 120


Chemical Equations

Example


C6H12 + O2 CO2 + H2O

This is NOT a correct equation – there are unequal numbers of atoms on both sides

Chemistry 120


Chemical Equations

Example


C6H12 + O2 CO2 + H2O

This is NOT a correct equation – there are unequal numbers of atoms on both sides

Reactants: 6 C, 12 H, 2 O

Products: 1 C, 2 H, 3 O

Chemistry 120


Balancing the equation

C6H12 + O2 CO2 + H2O

Chemistry 120

Stoichiometry: Chemical CalculationsBalancing the equation

6 C, 12 H, 2 O 1 C, 2 H, 3 O6 C on LHS means there must be 6 C on the RHS

C6H12 + O2 CO2 + H2O

C6H12 + O2 6CO2 + H2O

6 C, 12 H, 2 O 6 C, 2 H, 13 O

13 O on RHS means there must be 13 O on LHSC6H12 + 13/2 O2 6CO2 + H2O

6 C, 12 H, 13 O 6 C, 2 H, 13 O

Chemistry 120

Stoichiometry: Chemical CalculationsBalancing the equation

C6H12 + 13/2 O2 6CO2 + H2O

6 C, 12 H, 13 O 6 C, 2 H, 13 O

12 H on RHS means there must be 12 H on LHS

C6H12 + 13/2 O2 6CO2 + 6H2O

6 C, 12 H, 13 O 6 C, 12 H, 18 O

18 O on RHS means there must be 18 H on LHS

C6H12 +9O2 6CO2 + 6H2O

6 C, 12 H, 18 O 6 C, 12 H, 18 O

Chemistry 120

Stoichiometry: Chemical CalculationsThe final balanced equation is

and the coefficients are known as the

stoichiometric coefficients.

These coefficients give the molar ratios for reactants and products

This is a stoichiometric reaction – one where exactly the correct number of atoms is present in the reaction

C6H12 +9O2 6CO2 + 6H2O

Chemistry 120


If cyclohexane were burnt in an excess of oxygen,

the quantity of oxygen used would be the same

although O2 would be left over.

Chemistry 120

Solutions and concentration

If cyclohexane were burnt in an excess of oxygen,

the quantity of oxygen used would be the same

although O2 would be left over.

Chemistry 120

Solutions

A solution is a homogenous mixture which is composed of two or more components

the solvent

- the majority component

and

one or more solutes

- the minority components

Chemistry 120

Solutions

Most common solutions are liquids where a solid, liquid or gas (the solute) is dissolved in the liquid solvent.

Some are solids where both the solvent and the solute are solids. Brass is an example

Chemistry 120

Solutions

Most common solutions are liquids where a solid, liquid or gas (the solute) is dissolved in the liquid solvent.

Some are solids where both the solvent and the solute are solids. Brass is an example

Cu

ZnHere copper is the solvent, zinc the solute.

NaCl(s) melts

Chemistry 120

Solutions

Gas-Solid solution: Hydrogen in palladium

Steel

Chemistry 120

Solutions

Common laboratory solvents are usually organic liquids such as acetone, hexane, benzene or ether or water.

Solutions in water are termed aqueous solutions and species are written as E(aq).

Water is the most important solvent. The oceans cover ~ ¾ of the surface of the planet and every cell is mainly composed of water.

Chemistry 120

Solutions

Aqueous Solutions

Water is one of the best solvents as it can dissolve many molecular and ionic substances.

The properties of solutions which contain molecular and ionic solutes are very different and give insight into the nature of these substances and solutions.

Chemistry 120

Solutions

Ionic Solutions

An ionic substance, such as NaClO4, contain ions – in this case Na+ and ClO4

-.

The solid is held together through electrostatic forces between the ions.

In water, the solid dissolves and the particles move away from each other and diffuse through the solvent. This process is termed

Ionic Dissociation

Chemistry 120

Solutions

Ionic Solutions

In an ionic solution, there are therefore charged particles – the ions – and as the compound is electrically neutral, then the solution is neutral.

When a voltage is applied to the solution, the ions can move and a current flows through the solution.

The ions are called charge carriers and whenever electricity is conducted, charge carriers are present.

Chemistry 120

Solutions

Molecular Solutions

A molecular solution does not conduct electricity as there are no charge carriers present.

The bonding in a molecule is covalent and involves the sharing of atoms and there is no charge separation.

Chemistry 120

Solutions

Electrolytes

A solute that, when dissolved, produces a solution that conducts is termed an electrolyte, which may be strong or weak.

A strong electrolyte is one which is fully dissociated in solution into ions

A weak electrolyte is one which is only partially dissociated.

Chemistry 120

Solutions

Moles and solutions

When a substance is dissolved in a solvent, we relate the quantity of material dissolved to the volume of the solution through the concentration of the solution.

The concentration is simply the number of moles of the material per unit volume:

C = n V

n = number of moles; V = volume of solvent

Chemistry 120

Solutions

Moles and solutions

The units of concentration are:

C = n = moles V L3

and we define a molar solution as one which has 1 mole per liter.

Alternatively,

Concentration = Molarity = number of molesvolume of solution

Chemistry 120

Solutions

Example

4 g of Na2SO4(s) is dissolved in 500 ml of water.

What is the concentration of the solution?

Chemistry 120

Solutions

Example



Formula mass of Na2SO4(s):

Molar Atomic Mass of Na: 22.9898 gmol-1

Molar Atomic Mass of S: 32.064 gmol-1

Molar Atomic Mass of O: 15.9994 gmol-1

Chemistry 120

Solutions

Example



Formula mass of Na2SO4(s):

(2 x 22.9898)+ 32.064+(4x15.9994)=142.041gmol-1

1 mole of Na2SO4(s) = 142.041g

1/142.041 mole of Na2SO4(s) = 1 g

Chemistry 120

Solutions

Example



1/142.041 mole of Na2SO4(s) = 1 g

Therefore 4 g of Na2SO4(s) = 4/142.041 mole

= 2.82 x 10-2 mole

Chemistry 120

Solutions

Example



2.82 x 10-2 mole is therefore dissolved in 500 ml of

water;

So in 1 L, there are 2 x 2.82 x 10-2 mole

Molarity of solution = 5.64 x 10-2 molL-1

Chemistry 120

Solutions

Example

The equation for the dissolution of Na2SO4(s) is

So if we have 5.64 x 10-2 molL-1 Na2SO4(s), we must

have 1.13 x 10-1 moles Na+(aq)

and 5.64 x 10-2 mol SO42-

(aq) as there are 2 Na

cations for every sulfate ion

Na2SO4(s)H2O

2Na+(aq) + SO4

2-(aq)

Chemistry 120

Solutions

If we change the volume of the solution then we change the concentration:

If the Na2SO4 solution is diluted with 500ml of water, the concentration or molarity would be halved:

2.82 x 10-2 mole is therefore dissolved in 1000 ml of water

Molarity = 2.82 x 10-2 molL-1

Chemistry 120

Solutions

Dissolution on an atomic level.

Solids are held together by very strong forces.

NaCl(s) melts at 801oC and

boils at 1465 oC but it

dissolves in water at room

temperature.

Chemistry 120

Solutions


When we dissolve NaCl(s) in water we break the bonds between ions but make bonds between the ions and the water

Chemistry 120

Solutions


When we dissolve NaCl(s) in water we break the bonds between ions but make bonds between the ions and the water

The ions are hydrated or solvated in solution and these bonds between solvent and solute make the dissolution energetically possible

If something does not dissolve then the energetics are wrong for it do do so.

Chemistry 120

Solutions

Solubility rules

All ammonium and Group I salts are soluble.

All Halides are soluble except those of silver, lead and mercury (I)

All Sulfates are soluble except those of barium and lead.

All nitrates are soluble.

Everything else is insoluble

Chemistry 120

The Exam

Chemistry 120

Solutions

• Solutions are homogenous mixtures in which the

majority component is the solvent

and the

minority component is the solute

• Solutions are normally liquid but solutions of gases in solids and solids in solids are known.

• Ionic compounds dissolve in water to give conducting solutions – they are electrolytes

Chemistry 120

Solutions

• Electrolytes are either strong or weak depending on the degree of dissociation in solution

• Molecular solutions do not conduct as molecules do not dissociate in solution

• The concentration or molarity of a solution is defined by

C = n = moles V L3

and the units are molL-1 or moldm-3

Chemistry 120

Solutions

• When ionic substances dissolve,

bonds between particles in the solid break

and

bonds between the solvent and the ions are made

• There are general rules for the solubilities of ionic compounds

Chemistry 120

Reactions in Solution

Reactions in solution include

• Acid – base reactions

• Precipitation reactions

• Oxidation- reduction reactions

Chemistry 120

Reactions in Solution

Reactions and equilibria

Reactions are often written as proceeding in one

direction only – with an arrow to show the direction

of the chemical change, reactants to products.

Not all reactions behave in this manner and not all

reactions proceed to completion.

Even those that do are dynamic.

Chemistry 120

Reactions in Solution: Acid - Base

NaI*(s)

NaI(aq)

A saturated solution of NaI is placed in contact

with Na131I(s), which is radioactive.

Chemistry 120


NaI*(s)

NaI(aq)

A saturated solution of NaI is placed in contact

with Na131I(s), which is radioactive.

After time, the activity

in the solution is

measured and ..........

Chemistry 120

I-

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+

I-

I-

I-

I-

I-

I-

I-

I-I-

I-

I-

I-I-I-

I-

I- I-

Na+

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+Na+


Radioactivity is found in the solution, even though

the concentration of I-(aq) has not changed.

Chemistry 120


The equilibrium here is composed of two reactions:

So we write

Na131I(s)H2O

Na+(aq) + 131I-

(aq)

H2ONa+

(aq) + I-(aq) NaI(s)

H2ONa+

(aq) + I-(aq)NaI(s)

Chemistry 120


Such reactions are termed equilibria and all chemical reactions are equilibria.

The symbol for an equilibrium is a double-headed arrow

Chemistry 120

+ =

Forward reaction

Reverse reaction


Such reactions are termed equilibria and all chemical reactions are equilibria.

The symbol for an equilibrium is a double-headed arrow

Chemistry 120


Equilibria are important in the chemistry of acids and bases

Strong acids and bases are completely ionized

But.....

Weak acids and bases are not.

Chemistry 120


The Arrhenius definition of acid and bases are:

an acid is a compound which dissolves

in water or reacts with water to give

hydronium ions, H3O+(aq)

a base is a compound which dissolves

in water or reacts with water to give

hydroxide ions, OH- (aq)

Svante Arrhenius

(1859 – 1927)

Chemistry 120


A strong acid is a compound which dissolves and dissociates completely in water or reacts with water to give hydronium ions, H3O+

(aq)

- the double arrow implies that the reaction can go both ways – it is an equilibrium.

As a strong acid, the reaction is completely on the RHS:

HCl(g)H2O

H3O+(aq) + Cl-(aq)

HCl(g)H2O

H3O+(aq) + Cl-(aq)

Chemistry 120

Reactions in Solution: Acid - BaseA strong base is a compound which dissolves and dissociates completely in water or reacts with water to give hydroxide ions, OH- (aq)

Again, we could write this reaction as an equilibrium with a double headed arrow, but the base is strong and the reaction is completely over to the right hand side.

NaOH(s)H2O

Na+(aq) + OH-

(aq)

Chemistry 120

Reactions in Solution: Acid - BaseIn a reaction such as

we write the reaction as going from LHS to RHS.

Chemical reactions run both ways, so in this reaction, there are two reactions present:

Ionization

Recombination

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2HH3O+

(aq) + MeCO2-(aq)

Chemistry 120

Reactions in Solution: Acid - BaseWe write the reaction for acetic acid, MeCO2H, as

an equilibrium to include the ionization and recombination. Ionization

Recombination

As the amount of ionization and recombination are the same, the concentrations of the ions and the molecular form are constant

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

H2OMeCO2HH3O+

(aq) + MeCO2-(aq)

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

Chemistry 120


In solution, weak acids establish an equilibrium between the un-ionized or molecular form and the ionized form:

un-ionizedmolecular form

ionized

H2OMeCO2H H3O+

(aq) + MeCO2-(aq)

Chemistry 120


In solution, strong acids are completely ionized and even though there is an equilibrium, it lies entirely on the RHS and recombination is negligible:

un-ionizedmolecular form

ionized

H2OHBr(g) H3O+

(aq) + Br-(aq)

Chemistry 120


Acids with more than one ionizable hydrogen are termed

Polyprotic

The common polyprotic acids are

H3PO4 Phosphoric acid

H2SO4 Sulfuric acid

Chemistry 120


Polyprotic acids can ionize more than once

H3PO4

Each proton is ionizable and the anions, dihydrogen phosphate (H2PO4

-(aq))

and hydrogen phosphate (HPO42-

(aq)) both act as acids, though H3PO4 is a weak acid.

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

HPO42-

(aq) H3O+(aq)

+ PO42-

(aq)H2O

Chemistry 120


Polyprotic acids can ionize more than once

H3PO4

H2SO4

H3PO4(aq) H3O+(aq)

+ H2PO4-(aq)H2O

H2PO4-(aq) H3O+

(aq) + HPO4

2-(aq)H2O

HPO42-

(aq) H3O+(aq)

+ PO42-

(aq)H2O

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

Chemistry 120


In contrast, H2SO4 is a strong acid and hydrogen

sulfate (HSO4-(aq)) is also a strong acid.

H2SO4(aq) H3O+(aq)

+ HSO4-(aq)H2O

HSO4-(aq) H3O+

(aq) + PO4

2-(aq)H2O

Chemistry 120

Reactions in Solution: Acid - BaseStrong or weak?

All acids can be assumed to be weak except the following:

HCl(aq) hydrochloric acid

HBr(aq) hydrobromic acid

HI(aq) hydriodic acid

HClO4(aq) perchloric acid

HNO3(aq) nitric acid

H2SO4(aq) sulfuric acid

Chemistry 120

Reactions in Solution: Acid - BaseHydrogens attached to carbon are not ionizable in water

Acetic acid, MeCO2H (or CH3CO2H) has the

structure H

H

H

O

O H

Chemistry 120


Only the hydrogen attached to oxygen is ionized in aqueous solution

The methyl hydrogens are NOT ionizable in aqueous solution.

H

H

H

O

O HH2O

H

H

H

O

O

O

HHH+

Chemistry 120


Strong bases are those which ionize in solution of react to generate hydroxide ion. The common strong bases are those which already contain the OH- ion in the solid. 2 Li

3

3 Na11 Mg12

4 K19

Ca20

5 Rb37

Sr38

6 Cs55

Ba56

Strong bases are therefore the hydroxides of the group I and II metals

Chemistry 120


Weak bases are the majority and are usually amines and ammonia. These react with water and deprotonate it, forming hydroxide ion and an ammonium ion:

Trimethylamine

Trimethylammonium

N

H3C CH3

CH3H2O

N

H3C CH3

CH3

H

+ OH-

Chemistry 120

Reactions in Solution: Acid - BaseReactions in Solution: Acid - Base

Neutralization reactions and titrations Hydroxide and hydronium ions will react to form water.

From the stoichiometry of the balanced equation, the hydroxide and hydronium react in a 1:1 ratio.

We can therefore neutralize a known concentration of base or acid with the same quantity of acid or base. This is an example of a titration.

2H2O(l)H3O+(aq) + OH-

(aq)

Chemistry 120

Reactions in Solution: Acid - BaseReactions in Solution: Acid - Base

Neutralization reactions and titrations We use an indicator to determine the acidity or basicity of a solution:

An indicator is a compound which changes color strongly at a certain level of acidity.

Chemistry 120

Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseNeutralization reactions and titrations

We add acid or base – the titrant - to a solution of unknown concentration containing a few drops of the indicator solution.

When the solution is still acid, no color change occurs; when the indicator changes color, we know the equivalence point – the point where the acidity or basicity has been neutralized.

By knowing the concentration and the volume of the titrant, we can calculate the concentration of the of the unknown solution.

Chemistry 120

Reactions in Solution: Acid - BaseReactions in Solution: Acid - BaseDecompostion in acid

A solid base, such as Ca(OH)2(s), will dissolve with

reaction in an acid. The anion, hydroxide, reacts

with the acid to form the calcium salt of the acid:

Chemistry 120





Is this balanced?

Chemistry 120





Is this balanced? No

Chemistry 120





Is this balanced? No

Ca(OH)2(s) + H2SO4(aq) CaSO4(s) + H2O(l)

2Ca(OH)2(s) + H2SO4(aq) CaSO4(s) + 2H2O(l)

Chemistry 120


Some anions also decompose in acid. These are usually anions which are derived from gases which are not soluble in water:

CO32-

(aq) carbonate CO2(g)

HCO3-(aq) hydrogen carbonate CO2(g)

S2-(aq) sulfide H2S(g)

HS-(aq) hydrogen sulfide H2S(g)

SO32-

(aq) sulfite SO2(g)

HSO3-(aq) hydrogen sulfite SO2(g)

Chemistry 120

Solution Reactions: Oxidation-Reduction

Reactions between elements and between

compounds often involves the exchange of electrons.

Mg(s) + Cl2(g) MgCl2(s)

• Magnesium and chlorine are in their elemental

states and react together so that magnesium forms

Mg2+ and chlorine forms Cl-

• The overall product is neutral - MgCl2(s) has no

net charge (even though it is ionic).

Chemistry 120


In this reaction, Mg has lost two electrons:


Mg2+ + 2e-Mg

½Cl2 has gained an electron:

1/2Cl2 + e-Cl-

Chemistry 120


The oxidation state of magnesium has changed from

zero to +2

The oxidation state of ½Cl2 (Cl) has changed from

zero to -1

Mg Mg2+ + 2e-

1/2Cl2 + e- Cl-

Chemistry 120



zero to +2 Mg Mg2+ + 2e-

Oxidation state zero Oxidation state two

Chemistry 120



zero to +2

The oxidation state of ½Cl2 (Cl) has changed from

zero to -1

Mg Mg2+ + 2e-


1/2Cl2 + e- Cl-


Chemistry 120



In this reaction, Mg has been oxidized

- the oxidation state of Mg has increased -

and Cl has been reduced

- the oxidation state of Cl has decreased -

Chemistry 120







Chlorine is an oxidant or an oxidizing agent

Chemistry 120







Chlorine is an oxidant or an oxidizing agent

Magnesium is a reductant or a reducing agent

Chemistry 120


Oxidation and reduction reactions are extremely

common and involves the formal interchange of

electrons between atoms.

The Oxidation State and Oxidation Number are key

concepts in the discussion of these reactions

Chemistry 120


In an oxidation-reduction reaction, or redox reaction,

there MUST be and oxidation part and a reduction

part.

Chemistry 120


In an oxidation-reduction reaction, or redox reaction,

there MUST be and oxidation part and a reduction

part.

WHY?

Chemistry 120


If the reduction and oxidation portions of the

reaction do not balance then

• Charges will not balance overall

Chemistry 120





• the Mass Balance - the number of atoms on

both sides - will not balance

Chemistry 120





• the Mass Balance - the number of atoms on

both sides - will not balance

• Mass and energy will therefore be created or

destroyed

Chemistry 120


Initially,

Oxidation implied reaction with oxygen

Reduction implied the liberation of a metal from it’s

ore - usually by reaction with carbon or air

Chemistry 120


The modern definition is based on the changes in

oxidation numbers and the actual charges or the

formal charges on atoms and ions (including

polyatomic ions)

(In organic chemistry, oxidation is still based on

reaction with oxygen and reduction in the addition of

hydrogen)

Chemistry 120


Some rules for redox reactions

• All elements have an oxidation state of zero

• The oxidation state of simple mono-atomic

cations is the charge on the ion:Element Ox.

StateGroup IA: Li+, Na+, K+, Rb+, Cs+ +1Group IIA: Be2+, Mg2+, Ca2+, Sr2+, Ba2+ +2Group IIIA: B3+, Al3+, Ga3+ +3

Chemistry 120


The mono-atomic anions can be treated in the same

way:Element Ox.

StateGroup VIIA: F-, Cl-, Br-, I- -1Group VIA: O2-, S2-, -2 Group VA: N3-, P3-, -3

Chemistry 120


For an ion the oxidation state is the charge -

assignment of the charge requires some thought

Q: What is the oxidation state in MnO4-?

Q: What is the oxidation state in SO4-?

To answer these questions , we use some basic rules

which count the valence electrons in a species

Chemistry 120


All Group IA cations have an oxidation state of +1

Hydrogen has an oxidation state of +1, rarely -1

Fluorine always has an oxidation state of -1

The sum of the oxidation states must always equal

the charge on the species

Chemistry 120


Example 1:

Sodium fluoride, NaF

Chemistry 120


Example 1:


The formula unit is neutral, so the oxidation numbers

must sum to zero.

Chemistry 120


Example 1:



must sum to zero.

All group 1A cations have an oxidation number (or

are in oxidation state) +1

Chemistry 120


Example 1:



must sum to zero.

All group 1A cations have an oxidation number (or

are in oxidation state) +1

Fluoride must have an oxidation state of -1

Chemistry 120


Example 2:

What is the oxidation state of P in PO43-?

Chemistry 120


Example 2:


The charge on this ion is 3-

Chemistry 120


Example 2:



Oxygen has an oxidation number of -2

Chemistry 120


Example 2:




With four oxygens present, the total oxidation

number of the oxygens is 4 x -2 = -8

Chemistry 120


Example 2:



Oxygen has an oxidation number of -2With four oxygens present, the oxidation number of all the oxygens is 4 x -2 = -8 The balance of the oxidation states must equal -3

Chemistry 120


Example 2:




With four oxygens present, the oxidation number of

all the oxygens is 4 x -2 = -8

The balance of the oxidation states must equal -3

So, P has an oxidation state of +5 as (+5) + (-8) = -3

Chemistry 120


Example 2:


So, P has an oxidation state of +5 as (+5) + (-8) = -3

Remember that this does NOT imply that PO43- is

ionic, just that the oxidation state of P is +5

Chemistry 120


Example 3:

What is the oxidation state of Fe in Fe3O4?

Chemistry 120


Example 3:


The formula unit, Fe3O4, is neutral so all the

oxidation numbers must sum to zero

Chemistry 120


Example 3:




O has an oxidation state of -2, -2 x4 = 8

Chemistry 120


Example 3:




O has an oxidation state of -2, -2 x4 = 8

The oxidation states of Fe must balance this number,

so the oxidation state of Fe is 8/3 - a fractional

oxidation state.

Chemistry 120


Example 3:


The oxidation state of Fe is 8/3

This is the average over all Fe in the solid and does

not represent the charges on the ions.

Fe3O4 is actually Fe2O3.FeO, where the oxidation

states are + 3 and +2. The average is 1/3[(2 x 3) + 2]

Chemistry 120


Redox reactions

This reaction is the reduction of Copper (II) Oxide

with hydrogen gas to give copper metal and water.

CuO(s) + H2(g) Cu(s) + H2O(l)

Chemistry 120


Redox reactions

This reaction is the reduction of Copper (II) Oxide

with hydrogen gas to give copper metal and water.

What are the oxidation states of the reactants and

products?


Chemistry 120


Cu2+: oxidation state +2 O2-: oxidation state -2


H2 is the elemental form, Oxidation state = 0

Chemistry 120


H: oxidation state +1O: oxidation state -2


Cu is the elemental form, Oxidation state = 0

Chemistry 120



Redox reactions

The oxidation state of Cu has changed from +2 to 0

- it has been reduced

The oxidation state of H2 has changed from 0 to +1

- it has been oxidized

Stoichiometry: Chemical Calculations

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