1. Topic 1.3 Chemical reactions and related
calculations
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2. Topic 1.3: Chemical reactions and related
calculations
 When elements react, their atoms join with other
atoms to form compounds

3. The reaction of Iron and Sulfur
 Fe + S  FeS
11/8/2011

4. Relative formula mass (Mr)
 The relative formula mass is the sum of the masses
The decimal points are really
important, if it is of the atoms in
1.0 on the the numbers shown in the formula.
periodic table then that is the
number you use, and you must
quote your answer to the same
number of decimal places!
 E.g.
 The relative atomic mass (Ar) of Hydrogen is 1.0
 There are two hydrogen atoms in a molecule of H2
so the Mr of H2 is 1.0 + 1.0 = 2.0

5. From yesterday’s practical
 Iron reacts with sulfur to form iron sulfide with a
formula of FeS
 Fe has an Ar of 55.8
 S has an Ar of 32.1
 Therefore the Mr of FeS is 55.8 + 32.1 =

6. Calculate the relative formula mass (Mr) of:
 A) Cl2  G) Ca(OH)2
 B) Ne  H) K2SO4
 C) NH3  I) NH4NO3
 D) CH4  J) Ca(NO3)2
 E) MgBr2  K) Al2(SO4)3
 F) S8  L) H2C2O4.2H2O

7. Calculate the relative formula mass (Mr) of:
 A) Cl2 71.0  G) Ca(OH)2
74.1
 B) Ne 20.2
 H) K2SO4 174.3
 C) NH3 15.0
 I) NH4NO3
 D) CH4 16.0 80
 E) MgBr2 184.1  J) Ca(NO3)2
164.1
 F) S8 256.8
 K) Al2(SO4)3
342.3
 L) H2C2O4.2H2O 126

8. The reaction of Magnesium and Oxygen

10. Avogadro’s number & the mole
 A mole is the amount of a substance in grams which has the
same number of particles as there are atoms in 12 g of 12C.
 Italian physicist Amedeo Avogadro was investigating the
number of molecules in different volumes of gases, and found
that, under the same conditions, one mole of a gas always
contained the same number of particles. So, one mole of CO2
has the same number of particles as one mole of helium or
one mole of methane, or any other gas. The symbol for the
Avogadro constant is L after the Austrian Loschmidt, who
originally calculated the value for the constant, which is 6.023
x 1023, (602,300,000,000,000,000,000,000,000).
You do not need to learn this!

11.  The Avogadro constant is given the symbol L.
 L = 6.023 x 1023 particles mol-1
 The mole is a quantity of particles.
 1 mol is 6.023 x 1023 particles
 The mass of a 12C atom is 1.992 x 10-23 g. If a mole contains 6.023 x
1023 of these atoms, then a mole of 12C has a mass of:
 6.023 x 1023 x 1.992 x 10-23 = 11.998 g
 Avogadro’s number is the number of atoms in 12 g of carbon
You do not need to learn this!

12.  The relative formula mass of a substance, in grams,
is known as one mole of that substance
 Candidates are expected to use the relative formula
mass of a substance to calculate the number of
moles in a given mass of that substance and vice
versa.

13. Moles and masses:
 To convert the mass of a substance to moles use:
 Number of moles of an element = mass
Ar
 Number of moles of a compound = mass
Mr

14. Balanced equations
 Chemical reactions can be represented by word
equations or symbol equations.
 Candidates should be able to write word and
balanced symbol equations for reactions in the
specification.

15. Some common anion names:
F Fluoride
Make a copy of this Cl Chloride
table in your book! I Iodide
Br Bromide
O Oxide
OH Hydroxide
CO3 Carbonate
SO4 Sulfate
NO3/NO2 Nitrate
PO4 Phosphate
The –ate at the end of some anion names means that they contain oxygen, eg
sulfate contains sulfur and oxygen, sulfide is just sulfur on its own.

16. Balancing equations - 1
 From the worksheet, first try to balance the
equation, then try to write a word equation for the
reaction
 E.g. H2 + Cl2  HCl
 H2 + Cl2  2HCl Balanced
 Hydrogen + Chlorine  Hydrogen chloride

17. States: (s), (l), (g), (aq)

18.  In a chemical equation the states are shown for the
elements of compound at room temperature.
 E.g.
Sodium + water  sodium hydroxide + hydrogen
2Na(s) + 2H2O(l)  2NaOH(aq) + H2(g)
It tells us that solid sodium reacts with liquid water to
produce a solution of sodium hydroxide and hydrogen
gas

19. Homework
 Complete worksheet:
 Equations and Molar Ratios

20. Percentage mass
 The percentage mass of an element in a formula
can be calculated by dividing the the relative mass
of the compound by the relative atomic mass of the
element in the formula:
 E.g. The percentage mass of H in CH4
 There are 4 x 1.0 = 4.0
 4.0/16.0 = 0.25 or 25%

21. Calculate the percentage mass in the
following
a) C in CH4 e) N in Ca(NO3)2
b) Br in MgBr2 f) O in Ca(NO3)2
c) S in K2SO4 g) O in Ca(OH)2
d) N in NH4NO3 h) O in Fe(NO3)3

22. Calculate the percentage mass in the
following
a) C in CH4 e) N in Ca(NO3)2
75.0% 17.1%
b) Br in MgBr2 f) O in Ca(NO3)2
86.8% 58.5`%
c) S in K2SO4 g) O in Ca(OH)2
18.4% 43.2%
d) N in NH4NO3 h) O in Fe(NO3)3
35.0% 59.6%

23. Find the % of C in CO2 27.3%
Find the % of H in H2O 11.1%
Find the % of Zn in ZnCO3 52.2%
Find the % of Mg in MgCl2 25.3%
Find the % of Pb in PbO2 86.6%
Find the % of Cl in MgCl2 74.7%
Find the % of Pb in Pb2O3 89.6%

24. Calculating reacting masses
 The masses of reactants and products can be
calculated from balanced symbol equations
 Candidates should be able to calculate the mass of
a reactant or product from information about the
mass of the other reactants and products in the
reaction and the balanced symbol equation

25. The reaction of a iodine with zinc
 Intention – to carry out a reaction that will allow us
to apply all the skills we have been learning this
week (balancing equations, calculating Mr values
and working out numbers of moles)
 Take photos of each stage, you will need them to
produce a flow diagram of the investigation.

26. Mole calculations
 1. Calculate the number of moles that you have using
the mass you are given in the question and the Mr
Mass  Number of moles of ______ = _____ / _____ = ____
Moles Mr  Find the ratio (stoichiometry) of the reaction equation
 If I have ____ moles of _____, then I must have ____
moles of _____.
 Find the mass of the unknown:
 Mass of _____ = moles x Mr = _____ x _____ = ______g

27. Finding the value of X in CuSO4.XH2O
 Intention – to carry out a reaction that will allow us
to further develop all the skills we have been
learning (balancing equations, calculating Mr values
and working out numbers of moles)

30. Irreversible reactions
 The combustion of methane (CH4) to make CO2 and
H2O is an irreversible reaction:
 CH4 + 2O2  CO2 + 2H2O
 What this means is that we cannot react CO2 and
H2O together to make CH4 and O2
 CO2 + 2H2O  CH4 + 2O2

31. Reversible reactions
 A reversible reaction is a chemical change in which
the products can be converted back to the
reactants under suitable conditions.
 A reversible reaction can be shown by the sign

32. Revision materials
Task, in pairs, produce revision material(s) on one of the
following topics:
 Balancing equations
 The mole
 Relative molecular mass
 Conservation of mass in a reaction
 Yield
 Reversible reactions

33. Key words Clear explanations
Colourful and
Use diagrams Revision materials attractive to look at
Include model Include test
answers questions