Chemical Reactions Part 1: What are Chemical Reactions?

Types of Change

When a substance changes in appearance or shape, but no new substance is formed.

Physical Change

There are two ways that a substance can change:

When a substance changes into one or

more new substances.

Chemical Change

Types of Change

When a substance changes in appearance or shape, but no new substance is formed.

Physical Change

There are two ways that a substance can change:

When a substance changes into one or

more new substances.

Chemical Change

Bending, ripping, cutting, breaking, mixing and

changing physical states.

Burning, reacting, rusting.

It’s still paper, just in smaller pieces!

The paper changed to smoke and ash.

Ripping Burning

Types of Change

Physical Change Chemical Change

Mixing the dough

Rolling the dough

Baking the dough

Your Turn!

Physical Change

Physical Change

Chemical Change

During a chemical reaction, atoms

rearrange themselves to form new substances.

Chemical Reactions

Chemical changes occur when a substance changes into one or more new substances.

This change is called a chemical reaction.

How can we tell that a chemical reaction has occurred?

4 Signs of a Reaction

A change in color

Formation of a gas

A change in energy

Formation of a solid

Take the reaction where magnesium metal (Mg) burns brightly in oxygen

gas (O)…

Chemical Reactions

Chemical reactions can be simplified.

We can explain what goes on in a chemical reaction in an equation.

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Magnesium Oxygen Magnesium Oxide

Substances that react at the start of a reaction

Substances formed at the end of a reaction

An Example

Reactants Products

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Magnesium Oxygen Magnesium Oxide

An Example

Bonds between the reactants are broken.

Bonds between the products are formed.

Chemical Equations

A chemical equation is a simple way to show how atoms rearrange themselves during a reaction.

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Magnesium Oxygen Magnesium Oxide

2 Mg + O2 à 2 MgO

2 Mg + O2 à 2 MgO

Chemical Equations

Products

Reactants changes into…

2 Mg + O2 à 2 MgO

These cannot be changed!

These can be changed!

Chemical Equations

2 Mg + O2 à 2 MgO

chemical formulas

2 Mg + O2 à 2 MgO

coefficients

Balanced Equations

Chemical equations must be balanced.

“In a chemical reaction, mass is not created or destroyed, it is

simply rearranged.”

This is because they must follow the Law of Conservation of Mass.

Balanced Equations

The numbers of reactants on one side MUST equal the number of products on the other side.

2 Mg + O2 à 2 MgO

2 Mg

2 O 2 MgO

4 atoms 4 atoms

Balance Them!

2. Na + O2 à Na2O

1. SiCl4 à Si + Cl2

3. H2 + N2 à NH3

4. Mg + HCl à H2 + MgCl2

5. H2O2 à H2O + O2

Answers

2. Na + O2 à Na2O 4 2

1. SiCl4 à Si + Cl2 2

3. H2 + N2 à NH3 3 2

4. Mg + HCl à H2 + MgCl2 2

5. H2O2 à H2O + O2 2 2

Any Questions?

Let’s look at each of these individually…

Types of Reactions

1. Synthesis

2. Decomposition

3. Oxidation

There are 4 major types of chemical reactions:

4. Displacement

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Hydrogen (H2)

Oxygen (O2)

Water (H2O)

For example, take the synthesis of water:

1. Synthesis

In a synthesis reaction, simpler substances are combined to form compounds.

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Water (H2O)

Hydrogen (H2)

Oxygen (O2)

For example, the decomposition of water:

Decomposition

In decomposition, a compound is broken down into simpler substances.

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Methane (CH4 )

Oxygen (O2 )

Carbon Dioxide (CO2 )

For example, the oxidation of methane gas:

Water (H2O)

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Oxidation / Combustion

In oxidation, oxygen combines with another substance, usually when burned.

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Iron (Fe )

Copper Sulfate (CuSO4 )

Iron Sulfate (FeSO4)

Copper (Cu)

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For example, iron (Fe) + copper sulfate (CuSO4):

Displacement

In displacement, a free element replaces another element in a compound.

Any Questions?

Chemical Reactions Part 2: Collision Theory

Introduction

Some reactions are quick, others are slow.

The area of chemistry that studies the speed of reactions is called kinetics.

Rusting Combustion

Rate of Reactions

The speed of a reaction is called its rate.

Specifically, the rate of a reaction measures how fast the reactants are

changed in a certain unit of time.

To understand reaction rates, we must first learn about Collision Theory…

Collision Theory

Collision Theory states that for substances to react, they must physically collide.

Reaction

However, not ALL collisions cause a reaction!

1. Reactions Need Energy

Firstly, for two reactants to collide with success, they must have enough energy.

No Reaction

To give the reactants more energy, you must increase the temperature.

1. Reactions Need Energy

Firstly, for two reactants to collide with success, they must have enough energy.

Reaction!

To give the reactants more energy, you must increase the temperature.

2. Reactions Need Orientation

Secondly, for two reactants to collide with success, they must be lined up properly.

No Reaction

If the reactants are orientated correctly, the collision will be successful.

2. Reactions Need Orientation

Secondly, for two reactants to collide with success, they must be lined up properly.

Reaction!

Note: Orientation is by random chance only! It cannot truly be controlled.

Increasing the Rate of Reaction

The rate of reaction can be increased.

Collision Theory states that as the number of collisions (per second)

increases, so will the rate of reaction.

The number of collisions can be increased in 4 ways:

h the temperature h the concentration

h the surface area Use of a catalyst

Effect of Temperature

Increasing the temperature increases the rate.

As temperature increases, the reactants gain more kinetic energy. This increases

the number of successful collisions.

Low Temperature High Temperature

Effect of Concentration

Increasing the concentration increases the rate.

Increasing the number of molecules of one or more reactants makes the

collisions more likely and more frequent.

Low Concentration High Concentration

Effect of Surface Area

Increasing surface area increases the rate.

Breaking a reactant into smaller pieces (or a powder) makes more of it exposed

to collide with another reactant.

Low Surface Area High Surface Area

Catalysts

Using a catalyst increases the rate.

A catalyst is a substance that speeds up the reaction without being consumed. It helps make the reactants collide easier.

Catalysts work by lowering the energy the reactants need to react.

Reaction!

Any Questions?