Topic 1.3 Chemical reactions and related calculations

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Topic 1.3: Chemical reactions and related calculations

When elements react, their atoms join with other atoms to form compounds

04/13/2023

The reaction of Iron and Sulfur

Fe + S FeS

Relative formula mass (Mr)

The relative formula mass is the sum of the masses of the atoms in the numbers shown in the formula.

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

The decimal points are really important, if it is 1.0 on the

periodic table then that is the number you use, and you must quote your answer to the same

number of decimal places!

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 =

Calculate the relative formula mass (Mr) of:

A) Cl2

B) Ne

C) NH3

D) CH4

E) MgBr2

F) S8

G) Ca(OH)2

H) K2SO4

I) NH4NO3

J) Ca(NO3)2

K) Al2(SO4)3

L) H2C2O4.2H2O

Calculate the relative formula mass (Mr) of:

A) Cl2 71.0

B) Ne 20.2

C) NH3 15.0

D) CH4 16.0

E) MgBr2 184.1

F) S8 256.8

G) Ca(OH)2

74.1

H) K2SO4

174.3

I) NH4NO3

80

J) Ca(NO3)2

164.1

K) Al2(SO4)3

342.3

L) H2C2O4.2H2O 126

The reaction of Magnesium and Oxygen

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!

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!

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.

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

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.

Some common anion names:

F Fluoride

Cl Chloride

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.

Make a copy of this table in your book!

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

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

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

Homework

Complete worksheet:

Equations and Molar Ratios

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%

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

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%

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

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.

Mole calculations

1. Calculate the number of moles that you have using the mass you are given in the question and the Mr

Number of moles of ______ = _____ / _____ = ____

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

Mass

Moles Mr

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)

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

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

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

Revision materials

Key words Clear explanations

Colourful and attractive to look at

Include test questions

Include model answers

Use diagrams