Chemical change
ROBYN BASSON
DOC SCIENTIA
GRADE 10
CAPS
Separating particles
When substances react with one another, there can either be a:
Physical change Chemical changeTB pg. 131
1. Physical Change
• This occurs without any change in composition of the substance.
• Small amounts of energy is required to
• Break/weaken intermolecular forces/electrostatic forces
• Mainly reversible reactions
REMEMBERReversible means: Under certain conditions, new compounds formed in a reaction can react with each other again to form original
compounds
TB pg. 131
1. Physical Change
So what does change?State
Shape Colour• Examples:
• Ice → water
• Water → water vapour
• Water → ice
• Water vapour → water
• Dry ice → smoke
meltingevaporating
freezingcondensing
sublimationTB pg. 132
1. Physical Change
So what does change?
State• No kinetic energy change occurs during the state change. This means that the amount of energy in the system is constant.
• During a state change, temperature and average kinetic energy of the particles remains
constant.• Heat that is transferred can either cause a change in temperature or a state change.
• During a state change, the energy that is added is used to break the forces between particles and not increase the kinetic energy of the particles; hence, temperature is constant.
REMEMBERHEAT = ENERGY = SPEED OF PARTICLES =
TEMPERATURE
TB pg. 134
1. Physical Change
So what does change?
Shape
• The chemical composition remains the same
• Only the shape changes
• Examples:
• Steel rod flattened into a flat sheet
• Clay that is moulded
• Coarse salt grounded into fine salt
• Sugar grounded → icing sugar
TB pg. 135
1. Physical Change
So what does change?
Colour• The chemical composition remains the same
• Only the colour changes temporarily
• Examples:• Iron rod heated → turns red → cools: red colour
disappears
• Stove plate heated → turns red → cools: red colour
disappears• Mercury (II) oxide → orange →bright red →black
when heated →cools down →orange
TB pg. 136
Types of physical changes
State change
Shape change
Colour change
Quick Summary
During a state change, the following remains constant:
Size & mass of particles
Shape of particles (not substance)
Number or particles
Temperature & average kinetic energy
2. Chemical Change
• This results in a new substance being formed, which differs from the original substance in composition and physical properties
• Large amounts of energy is required to
• Break very strong intramolecular bonds/electrostatic forces
• Reactions may be, but are not always reversible reactions
• Types of reactions
• Synthesis reactions
• Decomposition reactions
Two or more substances combine to form fewer substances
A single compound is broken into simpler substancesTB pg. 138
2. Chemical Change
So what does change?
• The chemical composition.• Bonds are broken.• Atoms and ions are rearranged.• Number of atoms remains the same but not always. the number of molecules.• New chemical bonds are formed.• New substances are formed.
example
TB pg. 138
2. Chemical Change
REMEMBERWhen bonds are broken – energy is absorbed
When new bonds are formed – energy is released
TB pg. 139
2. Chemical Change
So what does change?
The size of the molecules will change, but not the size of the individual atoms.
The shape of the molecules will change, but not the shape of the individual atoms.
The number of molecules will change, but not the number of the individual atoms.
TB pg. 140
HomeworkEXERCISE 11 PG. 136
EXERCISE 12 PG. 143-144
Laws
Chemical equation:
𝐻2 + 𝑂2 → 𝐻2𝑂What is mainly wrong with this equation?
Left hand side?
How many hydrogen atoms are there?
How many oxygen atoms are there?
Right hand side?
Physical change or chemical change?
How many molecules are there on the left hand side?
How many molecules are there on the right hand side?
What type of reaction is this?
How do I balance this chemical equation?
2
TB pg. 144
Laws
Mass or energy cannot be created or destroyed.
A balanced chemical equation illustrates this.
Everything that was represented before the arrow, must be there after the arrow.
The reactants (left) consists of just the same amount of matter as the products (right)
TB pg. 144
Law of constant composition
A specific chemical compound always consists of the same elements in exactly the same ratio
This law is what we use to “construct” the formula of the compound.
A molecule with 1C atom for every 2O atoms is carbon dioxide
A molecule with 1N atom for every 3H atoms is ammonia
A molecule with 2N atoms for every 4O atoms is dinitrogen tetrooxide
𝐂𝐎𝟐
𝐍𝐇𝟑
𝑵𝟐𝑶𝟒
TB pg. 144
EXAMPLES
𝑺𝟎𝟐
𝑯𝟐𝟎𝟐water Hydrogen peroxide
RATIO??
𝑯𝟐𝟎
𝑯𝟑𝑷𝑶𝟒 𝑯𝑪𝒍1S : 2O 3H : 1P : 4O 1H : 1Cl
TB pg. 145
Applying the Law
1. Calculate the percentage (%) composition
𝐶𝐻4
Calculate the percentage composition of
carbon and hydrogen in methane
Example 1
Applying the Law
𝑁𝐻3
Calculate the percentage of nitrogen and
hydrogen in ammonia
Example 2
“
”Complete the worksheet
Part 1
Law of constant composition
Law of conservation of mass
Total mass before a reaction and the total mass after the reaction remains constant.
Type of atoms in reactants = Type of atoms in products
Number of atoms in reactants = Number of atoms in products
Mass before = Mass after
Number of ? in reactants ≠Number of ? in products
TB pg. 146
Practice
Practical Activity page 147
Schematic diagram
Mass calculation
TB pg. 147
𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
Applying the Law
Show by means of a calculation that the law of
conservation of mass is valid for the reaction
below
Example 1
𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
Applying the Law
Write the necessary equation and calculate the
mass of magnesium oxide that forms when 15g
of Mg burns in oxygen
Example 2
𝑤ℎ𝑎𝑡? ? ? ? 𝑛𝑜 𝑒𝑞𝑢𝑎𝑡𝑖𝑜𝑛? ? eish
Applying the Law
Calculate the mass of table salt that can be
prepared from 20g of sodium hydroxide
Example 3
𝑁𝑎𝑂𝐻 + 𝐻𝐶𝑙 → 𝑁𝑎𝐶𝑙 + 𝐻2𝑂
“
”Complete the worksheet
Part 2
Law of conservation of mass
Law of volume relationships in
gas reactions
A specific number of particles of any gas occupies the same volume at a fixed temperature and pressure
In simple words, this means that when gases combine with one
another (at a fixed temperature and pressure), the ratio of their
volumes are simple whole number.
𝟐𝑆𝑂2 + 𝟏𝑂2 → 𝟐𝑆𝑂2 TB pg. 146
Applying the Law
Nitrogen reacts with oxygen to form nitrogen
dioxide.
Calculate the amount of nitrogen dioxide that will
form from 2.5𝑑𝑚3of 𝑁2when it reacts with a sufficient
amount of oxygen.Example 1
𝑁2 + 2𝑂2 → 2𝑁𝑂2
Applying the Law
Butane (𝐶4𝐻10) burns in oxygen to form carbon
dioxide and oxygen.
What volume of carbon dioxide will be formed from
7.85𝑐𝑚3 of butane?
Example 2
2𝐶4𝐻10 + 13𝑂2 → 8𝐶𝑂2 + 10𝐻2𝑂
“
”Complete the worksheet
Part 3
Law volumes in gas relationships
Using the laws – Exercise 13
Give the formula
and draw the
diagrams of the
following
compounds
a) Aluminium and chlorine
b) Lithium and oxygen
c) Iron (III) and chlorine
Use the law of
conservation of
mass to determine if
this equation is
balanced or not. If
not, balance it.
𝑵𝒂 + 𝑪𝒍𝟐 → 𝑵𝒂𝑪𝒍
Balance the
following equation.
𝟔𝒅𝒎𝟑𝑵𝑯𝟑(𝒈) → 𝑯𝟐 𝒈 + 𝑵𝟐(𝒈)
HomeworkEXERCISE 13 PG. 150-151
SUMMARY PG. 153-154
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