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