Atomic WeightsYou must be able to…·     Describe the mole as the SI unit for amount of substance

 

·     Relate amount of substance to relative atomic mass

 

·     Describe relationship between the mole and Avogadro’s number

 

·     Conceptualise the magnitude of Avogadro’s number

 

·     Describe the relationship between molar mass and relative molecular mass

 

·     Calculate the molar mass of a substance given its formula

 

Chemical Change - ReactionsObjectives: At the end of this section

you should be able to:-• Distinguish between synthesis and decomposition

reactions• Discuss how energy released by chemical reactions is

essential for life• Describe how metals react with oxygen• Explain the reverse process in which metals are

separated from their oxides• Describe the reactions of non-metals with oxygen• Write balanced equations for reactions involving non-

metals.

Physical and chemical change

Nitrogen separated from air by fractional distillation

Lightning causes nitrogen to react with air to form nitrogen

Consider the following analysis of different samples of table salt, what do you notice?

Hint??

Sample no.

Sample mass

Mass of Na

Mass of Cl

1 117g 46g 71g

2 468g 184g 284g

3 59g 23g 36g

The Law states:

A certain chemical compound always contains the ……………………. chemically combined in the ………… …………………….. by ……………………..

NaCl CaCl2 KMnO4 K2Cr2O7

Ratios: …….……….. ………. …………….

These ratios ………………. change.

NaCl is always 1:1 and 23g:35,5g – irrespective of whether the NaCl comes from the UK, USA or SA.

Molecules

O2

a3CH4

b CO2

c

H2S

CH4 + 2O2 ---> CO2 + 2H2O

Methane + oxygen --> water + carbon dioxide

Conservation of Mass, Atoms & Molecules• Show that mass, atoms are conserved but molecules are

not

H2 + O2 H2O

REACTIONSHydrogen and oxygen react to produce water.

H2 + O2 H2O

Ar:

Mr:

Balanced Eqn:

H2 + O2 H2O

LHS:

RHS:

LHS = RHS

MASS is CONSERVED!!

This is true for ALL REACTIONS.

This is the reaction used to power the space shuttle.

REACTIONS

1. Butane reacts with oxygen 2. Carbon dioxide and water are formed

Butane + Oxygen ---> Carbon dioxide + water

C4H10 + O2 ---> CO2 + H2OWhat mass of carbon dioxide is produced from each

butane molecule?

1 2

Reacting Masses

1. Na + Cl2 --> NaCl

2. C + Cl2 --> CCl4

3. ZnS + O2 --> ZnO + SO2

4. FeS + HCl --> H2S + FeCl2

5. SO2 + H2S --> S + H2O

Calculate the mass of each underlined compound either produced or required. (Balance the reactions first)

Volume - Volume Calculations

H2 + N2 --> NH3

If 3.00 dm3 of nitrogen are reacted to produce ammonia, what volume of hydrogen will be required? (At STP)

Balancing Reactions1. Na + Cl2 --> NaCl

2. C + Cl2 --> CCl4

3. ZnS + O2 --> ZnO + SO2

4. FeS + HCl --> H2S + FeCl2

5. SO2 + H2S --> S + H2O

Balancing More Reactions

1. Na + H2O --> NaOH + H2

2. H2 + O2 --> H2O

3. CaCO3 --> CaO + CO2

4. CaCl2 + Na2SO4 --> CaSO4 + NaCl

5. Al(NO3)3 + K2CO3 --> Al2(CO3)3 + KNO3

6. Na3PO4 + MgI2 --> Mg3(PO4)2 + NaI

THE NEUTRAL ATOM• The atom consists of a ……………. containing protons

and neutrons surrounded by a cloud of ………………….• Atomic Number (Z)

Number of ……………. in the Nucleus (= number of electrons in a neutral atom.)

• Mass number (A) = Number of protons + neutrons.

Notation

Z

AX………….. Number

(smaller) ………….

…………. Number (bigger) ………….

symbol

Neutrons = ………………………

Relative Mass Atomic • Certain products, such as paper for example, are sold by the ream. A

ream is 500 sheets. Since it is impractical to actually count out 500 sheets, the weight (mass) of 500 sheets is determined; then each ream is packaged according to this mass.

• Atoms are even smaller than paper, so it is not possible to actually count them. However, it is possible to know the mass of an atom in respect to the mass of another atom.

• The Relative mass of an object is expressed by comparing it mathematically to the mass of another object. So the relative mass of an orange in relation to a grapefruit is .6. The relative mass of the grapefruit in relation to a grapefruit is 1.0.

• Atoms are compared to the lightest atom (hydrogen) which is 12 times lighter (1/12 of the mass of) one carbon atom.

• THE RELATIVE ATOMIC MASS IS THE NUMBER OF TIMES AN ATOM IS HEAVIER THAN 1/12 OF A C12 ATOM.

The MoleThe mole is defined as, “the amount of ………….. with the

same number of ……………………… particles as ….. grams of carbon 12”. (n used as symbol for moles)

602 300 000 000 000 000 000 000Six hundred and two thousand, three hundred, billion billion !

6.023x1023 particles

12.00 g

CSymbol (….)

Number of particles = no of moles x no. particles in a mole

Particles = ……………..

The Carbon Standard• Carbon-12 is the standard upon which the relative mass of other atoms

is determined. It wasn’t always this way. • At first hydrogen was used and it was assigned the atomic mass of

one. If you have equal numbers of nitrogen atoms and hydrogen atoms, the nitrogen atoms are 14 times heavier than the hydrogen atoms. Therefore, nitrogen was assigned the atomic number of 14.

• Later oxygen was used as the standard with an atomic mass of 16. • However, carbon-12 proved to be more convenient to capture and

measure in pure form, so it became the standard. • However, now even carbon-12 is slowly losing its position as standard,

as sophisticated equipment makes it possible to give even more accurate measures of atomic mass. For this reason you will notice that on the periodic table the AMUs are not expressed as exact relative units to carbon-12.

Dozen & Particles

... ... ... ... ... ... ... ... ... ... ... ...particles

1 doz 1 doz 1 doz dozen

12

x36

12 12 12

?3

Moles & Particles

..........................................................................

..........................................................................

..........................................................................

..............................................

..........................................................................

..........................................................................

..........................................................................

..............................................

..........................................................................

..........................................................................

..........................................................................

..............................................particles

1 mol 1 mol 1 mol moles

(n)

L6.023 x 1023

x18.069 x 1023

6.023 x 1023 6.023 x 1023 6.023 x 1023

?3

The Mole and MassThe mole is defined in such a way that the MASS NUMBER (A) of an element

is equal to the relative atomic mass mass of one mole of the substance. (in grams) - THE MOLAR MASS

• Eg Na = 23g/mol, water(H2O)=18g/mol

Z

AXAtomic Number

(smaller)

Mass Number (bigger)

protons + neutrons

Periodic Table Symbol

Relative atomic mass

or

mass(g) of one mole

Relative Masses

• Relative atomic(Ar) - The mass of the atom relative to 1/12 of the mass of a C12 atom. (Number of times heavier than…)

O - 16 one atom of oxygen is 16 times heavier than 1/12 of the mass of a C12 atom, Na - 23 one atom of sodium… , H - 1 etc.

• Formula mass (Mr) - The sum of all the atomic masses of the atoms in a molecule.

Water H2O one molecule of water has a relative mass of (2x(1)+16) = 18 - that is the molecular or formula mass of water.

Mr(H2O) = 18 (Times heavier than…)

Atomic StructureSymbol Notation

Name Protons Neutrons Mass Number

Electrons

Calcium

3115P

14 30

13 6

Mg

ISOTOPESSymbol PROTONS ELECTRONS NEUTRONS

Carbon 1212 6C

Carbon 1313 6C

Boron 1010

5B

Boron 1111

5B

Hydrogen 1

Hydrogen 2

Chlorine 35

Chlorine 37

Relative Masses - examples

Calculate the Relative Atomic Mass of:• O2 (oxygen gas) • Cl2 (chlorine gas)• NaCl (sodium chloride - table salt) • H2SO4 Sulphuric acid• CaCO3 (calcium carbonate)• (NH4)2Cr2O7 (ammonium dichromate)

Isotopes• Isotopes - Atoms of the same element which have different

numbers of neutrons. Eg: 613C & 6

12C

Relative atomic mass is (actually) the average mass (of all the isotopes in a random sample) of the atoms of an element relative to 1/12 of the mass of a carbon-twelve atom.

613C

• 6 protons

• 6 electrons

• 13-6 = 7 neutrons

612C

• 6 protons

• 6 electrons

• 12-6 = 6 neutrons

IsotopesChlorine has two isotopes 37

17Cl & 3517Cl

Cl(35) has 35-17=18neutrons Cl(37) has 20 neutrons!• 37Cl (25%) & 35Cl (75%) - exist in the ratio 1:3

Calculate the average mass of a Cl atom. (Two methods)

Relative Atomic Mass

Z

AXAtomic Number

(smaller)

Mass Number (bigger)

protons + neutrons

Relative atomic mass

or

mass(g) of one mole

Periodic Table Symbol

Calculate: The mass in grams -1. of one mole of copper chloride (CuCl2)2. one mole of carbon dioxide (CO2)3. One and a half moles of oxygen (O2)4. TWO moles of methane (CH4)5. Four moles of water.

m = n x Mr

mass of substance = number of moles x mass of 1 mole

Relative Atomic MassCalculate: The mass in grams –

1. of one mole of copper chloride (CuCl2)

2. one mole of carbon dioxide (CO2)

3. 1.5 mole of oxygen (O2)

4. TWO moles of methane (CH4)

5. Four moles of water.

The Mole - moles --> Massm = n x Mr

Calculate the mass of • 2 moles of copper oxide (CuO)

• 0.5 moles of copper (II) sulphate (CuSO4)

• 0.01 moles of calcium carbonate

• 5 moles of ammonium carbonate

mass = moles x relative mass

The Mole - Mass --> Moles n = m/Mr

• Eg calculate the number of moles of water that would have a mass of 100g.

• How many moles of iron (II) chloride are in 50 g of iron (II) chloride?

• Calculate the number of moles needed to get1kg of calcium carbonate.

• How many moles of CuSO4.5H2O would give you 0.1g of water?

The Mole & Mass --> Relative Mass n = m/Mr = 5.56 mol

• Eg Calculate the relative mass of a compound for which 0.001 moles have a mass of 0,0056 g.

• What is the relative mass of a compound for which 0.01 mols has a mass of 0.18g

• Identify the element for which 0.05 moles has a mass of 0.16 g ?

Mr (X) = m/n= 0.0056/0.001 = 5.6 g/mol

The Mole - ReactionsSodium reacts with water to form hydrogen and sodium hydroxide

according to the equation.

Na + H2O H2 + NaOHIf 46g of sodium are reacted with excess water what mass of hydrogen

would be formed?1. Balance the reaction

2. Work out moles of reactant GIVEN.

3. Go through the equation to find out the number of moles reacting and being formed. (Molar ratio).

4. Work out quantity asked for.

The Mole - mass calculationsC + O2 CO2

Carbon reacts with oxygen to form carbon dioxide as shown.If 0.12g of carbon are reacted with excess oxygen what mass of

carbon dioxide would be formed?1. Balance the reaction2. Work out moles of reactant(mass given).

3. Go through the equation to find out the number of moles being formed

4. 4. Work out quantity asked for.

Mole examples - B & J p119 21 & p120 22

1. Na + Cl2 NaClCalculate the mass of salt formed if 2.3g of

sodium is reacted with XS chlorine.2. Zn + HCl ZnCl2 + H2

What mass of HCl is needed to produce 100g of hydrogen?

3. KClO3 KCl + O2

What mass of oxygen is produced from 1kg of potassium chlorate?

4. Fe2O3 + H2 Fe + H2OWhat mass of iron is produced if 3g of rust (Fe2O3)

is reacted with XS(100g )of hydrogen?

Reactions – Limiting reagentThe reagent that runs out first and stops the

reaction is known as the LIMITING REAGENT.

If 46g of sodium are reacted with excess water what mass of hydrogen would be formed?

Na + H2O H2 + 2NaOH46g 2 moles XS

Na will run out first

Na is LIMITING REAGENT

What is the minimum amount of water needed to react completely with 46g of sodium??

Percentage CompositionAnalysis of a compound by mass makes it

possible to work out the % mass of each element.

eg Table salt: NaCl mass analysis:One mole of NaCl would have a mass of

23 + 35.5 = 58.5g• The % composition can be found using the formula:

Mass element X x100 Total Mass Compound

• %Na = […../ (…..) ]x100 = …………..% (by mass)

• %Cl = (…../ (…….) )x100 = …………%

% Mass Element X =

Percentage Composition from mass.Eg2 Calculate the % of oxygen in water.

Empirical and Molecular Formula.A compound consists of carbon, hydrogen and oxygen only. The % by mass are Carbon 40.0% and 6.7% hydrogen. Calculate the empirical and molecular formula of the compound if Mr = 60g·mol-1

%(O) = 100 – (40+6.7) = 53.3

C H O

In 100g: …….g ……..g ….…g

n=m/Mr: …/… 6.7/….

53.3/……

…… …… ……..……. …… …….

Simplest: … …… ….

Empirical Formulae: ……. (12+2+16 = …..)

Molecular Formula: 2(CH2O) ……… (Mr = …. X 30)

Empirical and Molecular Formula.

OH

O

CH3

C

MOLECULAR FORMULA: CH3COOH or C2H4O2

Actual formula

Mr: 2(12)+4(1)+2(16)=60g.mol-1

%C: (24/60)x100 = 40.0%

%H: (4/60)x100 = 6.7%

%O: (32/60)x100 = 53.3%

EMPIRICAL FORMULA: CH2O

(Simplest whole number ratio)

Empirical and Molecular Formulae.

Eg3. If a compound consisting of nitrogen and oxygen only - contains 30.4% by mass of nitrogen. What is the molecular formula of the compound? >>

Concentration - MolarityThe concentration of a solution is defined as the ………………. of

……………………… per ………………. (dm3) of ………………….

solute

solute

Final volume of ……………..

500cm3

=+

Concentration =Amount of ……… (……….)

Volume of ………………

30g of NaCl

C = n

v

Volume Conversions

1 dm = ….. cm1 dm3 = ………… cm3

1 m3 = …………….. dm3 = ………………….cm3 (10….)

1cm3

1 dm3 (1 litre)

10 cm3

10 cm3

10 cm3

Volume Conversions

1 dm = 10 cm1 dm3 = 1000 cm3

1 m3 = 1000 dm3 = 1000 000cm3 (106)

1cm3

1 dm3 (1 litre)

10 cm3

10 cm3

10 cm3

Decimal Conversions

King Henry Died a miserable death called measles

Kilo Hecta Decca m(unit) deci centi milli

Conversions 1 cm = …………. dm

1 cm2 = …………dm2

1 cm2 = ………. dm2

1 cm3 = …………….dm3

1 cm3 = …………….. dm3

1 dm3= …………….. cm3

25 cm3 = …………. dm3

Molar VolumesOne mole of an ideal (ANY) gas occupies a volume of ………….3 at ………………………… temperature and pressure. (STP)

STP: T= ….ºC, ……K P =1 atmosphere (……...kPa)

Fe2O3 + 3H2 2Fe + 3H2OWhat volume of hydrogen reacts with 50g of Fe2O3

Fe2O3 : H2

… : …..

n(H2) =…..n(Fe2O3) = ……………………

v(H2) = …………………………… dm3

n(Fe2O3) = m/Mr = ………………….= ………………mol

moles = volume/molar volume ==> n = …./…..

Molar Volumes

One mole of an ideal (ANY) gas occupies a volume of 22,4dm3 at standard temperature and pressure. (STP)

ASKEDGIVEN

Mole Calculations

MOLES MOLES

MASS MASS

VOLUME VOLUMEMOLARRATIO

Concentration – Mass Eg

Concentration CalculationsHydrochloric acid reacts with calcium carbonate according to the following

equation:HCl + CaCO3 CaCl2 + H2O + CO2

What mass of calcium carbonate would be needed to react completely with 25 cm3 of a 0.1 mol.dm-3 hydrochloric acid solution?

Volume - Volume Calculations1. Balance the equation2. Calculate the moles of the substance given.3. Work through the molar ratio to find out the moles of the

substance asked.4. Calculate the quantity asked for. (Volume V = n x Mv)

Mv = 22.4dm3 At STP

EG: H2 + N2 --> NH3If 3.00 dm3 of nitrogen are reacted to produce ammonia, what

volume of hydrogen will be required? (At STP)

H2 + N2 --> NH3

Volume - Volume CalculationsH2 + N2 --> NH3

If 3.00 dm3 of nitrogen are reacted to produce ammonia, what volume of hydrogen will be required? (At STP)

H2 + N2 --> NH3

Mass Volume Calculations1. KClO3 KCl + O2

What volume of oxygen is produced by the decomposition of 1kg of potassium chlorate?

2. H2 + N2 --> NH3

How much nitrogen (in dm3) would be needed to produce 46dm3 of ammonia?

3. S + O2 --> SO2

What volume of sulphur dioxide could be produced from 20.0dm3 of oxygen?

4. Zn + 2HCl ZnCl2 + H2

What mass of zinc is needed to produce 100dm3 of hydrogen?

5. Fe2O3 + H2 Fe + H2OIf 3.00kg of iron oxide is reacted with 0.256dm3 of

hydrogen, what mass of water would be produced?

Volume - Mass Limiting Reagent

1. Balance the equation - Fe2O3 + 3H2 2Fe + 3H2O

2. Calculate the moles of the substance given.

3. Work through the molar ratio to find out the moles of the

substance asked.

4. Calculate the quantity asked for. (mass m = n x Mr)

5. Fe2O3 + H2 Fe + H2OIf 3.00kg of iron oxide is reacted with 0.256dm3 of hydrogen,

what mass of water would be produced?

Volume - Mass Limiting Reagent

1. Balance the equation - Fe2O3 + 3H2 2Fe + 3H2O

2. Calculate the moles of the substance given.

n(Fe2O3) = m/Mr = 3000/(2(56)+3(16)) = 18.75 mol XS

n(H2) = v/Mv = 0.256/22.4 = 0.0114 mol LIMITING REAGENT

3. Work through the molar ratio to find out the moles of the substance asked.

H2 : H2O as 1 : 1

n(H2O) = n(H2) = 0.0114 mol

4. Calculate the quantity asked for. (mass m = n x Mr)

m(H2O) = n x Mr = 0.0114(2+16) = 0.206g

5. Fe2O3 + H2 Fe + H2OIf 3.00kg of iron oxide is reacted with 0.256dm3 of hydrogen,

what mass of water would be produced?

Standard Solution

A standard solution is one for which the concentration is precisely known.

Since

c = n(solute)/v(solvent)

= m/Mr V

• The number of moles of solute (Mass)

• The volume of solution.These values must be

accurately determined.

2.45g

Mass is determined accurately using an electronic balance.

• Possible accuracies of 0.1 - 0.0001g

KMnO4

250 cm3

Volume is measured using a volumetric flask.

• 250 cm3

• 100 cm3, 200 cm3,

Weighing TechniqueProcedure - Weighing by difference.

1. Zero scales and clean the pan.

2. Weigh the weighing container.

3. Add (approximately) the required amount of salt. Take care not to drop any salt onto the pan.

4. Transfer the salt to a clean beaker.

5. Reweigh the weighing container.

6. Subtract the final mass of the container from the mass of salt and container to give the mass of salt transferred to the beaker.

Mass is determined accurately using an balance (electronic or triple beam).

• Possible accuracies of 0.1 - 0.0001g

2.45g

KMnO4

Results: Mass salt + container: ………… Final Mass container: …………

Mass salt transferred:

Volumetric FlaskMaking a standard solution.1. Rinse a clean & dry 100 cm3 beaker with

a little …………………water.2. Transfer the correctly weighed amount

of salt to the beaker. Ensure NO SALT IS LOST.

3. Add 50 - 80 cm3 water the salt and stir gently with a glass rod until all salt is dissolved. DO NOT REMOVE THE ROD FROM THE SOLUTION NOR ALLOW ……………. OF SOLUTION TO ESCAPE.

4. Add ALL the solution to volumetric flask via funnel. Ensure glass rod and beaker are thoroughly rinsed. (……………………rinsings.)

5. Add enough solvent to bring the level up to the ……………………...

250 cm3