Chapter 20 Chemical Reactions. 20.1 Chemical Changes When you bite into your food, you chew it into smaller pieces. Chewing does not alter the chemical.

Chapter 20Chapter 20

Chemical Reactions Chemical Reactions

20.1 Chemical Changes20.1 Chemical Changes

When you bite into your food, you chew it into smaller pieces.

Chewing does not alter the chemical composition of your food.

Chewing causes a physical change in your food.

A physical change is a change that affects only the physical properties of a substance (size, shape, and state).

When you bite into your food, you chew it into smaller pieces.

Chewing does not alter the chemical composition of your food.

Chewing causes a physical change in your food.

A physical change is a change that affects only the physical properties of a substance (size, shape, and state).

Physical ChangesPhysical Changes

The physical properties of ice can be changed by crushing it or by letting it melt.

The only difference between solid water (ice), liquid water, & water vapor is the amount of energy involved in each state.

Water vapor is the most energetic form of water.

The physical properties of ice can be changed by crushing it or by letting it melt.

The only difference between solid water (ice), liquid water, & water vapor is the amount of energy involved in each state.

Water vapor is the most energetic form of water.

Chemical Changes Chemical Changes

A chemical change is a change in a substance that involves breaking and reforming of chemical bonds to make one or more different substances.

So… when you chew your food, glands in your mouth produce saliva. This immediately breaks down complex molecules by breaking bonds.

A chemical change is a change in a substance that involves breaking and reforming of chemical bonds to make one or more different substances.

So… when you chew your food, glands in your mouth produce saliva. This immediately breaks down complex molecules by breaking bonds.

Chemical Changes cont…Chemical Changes cont…

Chemical changes are the result of chemical reactions.

They are also the breaking of bonds in one or more substances, & reforming of new bonds to create new substances.

Chemical changes are the result of chemical reactions.

They are also the breaking of bonds in one or more substances, & reforming of new bonds to create new substances.

Chemical Changes cont…Chemical Changes cont…

How do you know when a chemical change has occurred?

When one or more starting substances are mixed & you get products that appear to be different from those staring substances.

This involves ENERGY!

How do you know when a chemical change has occurred?

When one or more starting substances are mixed & you get products that appear to be different from those staring substances.

This involves ENERGY!

A Chemical Change A Chemical Change

Making a cake: You need flour, water, sugar,eggs….. Mix the ingredients together and bake it

in a oven. When the cake is done, a chemical

change has occurred. But how do you know?

Because none of the ingredients can go back to their original substance!

Making a cake: You need flour, water, sugar,eggs….. Mix the ingredients together and bake it

in a oven. When the cake is done, a chemical

change has occurred. But how do you know?

Because none of the ingredients can go back to their original substance!

Chemical Reactions Chemical Reactions

A chemical reaction involves breaking the chemical bonds in one or more reactants and reforming chemical bonds into one or more products.

Reactants Products Reactants are substances that

change. Products are substances that are

formed.

A chemical reaction involves breaking the chemical bonds in one or more reactants and reforming chemical bonds into one or more products.

Reactants Products Reactants are substances that

change. Products are substances that are

formed.

20.2 Chemical Equations 20.2 Chemical Equations

Evidence that chemical changes are occurring:

1. Bubbling2. Turns cloudy 3. Temperature change4. Color change

Evidence that chemical changes are occurring:

1. Bubbling2. Turns cloudy 3. Temperature change4. Color change

Examples of Chemical Changes

Examples of Chemical Changes

How Are Chemical Reactions Written?How Are Chemical Reactions Written?

Use chemical formulas that correspond to the elements & compounds in the reaction.

When chemical formulas and symbols are used to represent a reaction instead of using words, it is called a Chemical Equation.

Use chemical formulas that correspond to the elements & compounds in the reaction.

When chemical formulas and symbols are used to represent a reaction instead of using words, it is called a Chemical Equation.

How Chemical Reactions Are Written

How Chemical Reactions Are Written

Words:Methane gas reacts with oxygen gas

to produce carbon dioxide and water.

Chemical Equation:CH4 + O2 CO2 + H20

Words:Methane gas reacts with oxygen gas

to produce carbon dioxide and water.

Chemical Equation:CH4 + O2 CO2 + H20

How Chemical Reactions Are Written cont…

How Chemical Reactions Are Written cont…

A few things to keep in mind: Most elemental gases do not exist

as single atoms (with the exception of most gases).

A few things to keep in mind: Most elemental gases do not exist

as single atoms (with the exception of most gases).

Hydrogen, H2

Nitrogen, N2 Oxygen, O2

Fluorine, F2 Chlorine, Cl2 Bromine, Br2

How Are Chemical Reactions Written cont…

How Are Chemical Reactions Written cont…

Observe this chemical equation: CH4 + O2 CO2 + H2O This is technically not correct! It does not

agree with the Conservation of Atoms. This principle says that the number

of each type of atom on the reactants side must equal the number of each type of atom on the products side of the equation.

Observe this chemical equation: CH4 + O2 CO2 + H2O This is technically not correct! It does not

agree with the Conservation of Atoms. This principle says that the number

of each type of atom on the reactants side must equal the number of each type of atom on the products side of the equation.

Chemical Equation & Conservation of AtomsChemical Equation &

Conservation of Atoms It is necessary to Balance the

number and type of atoms on the reactants and products sides of the equation.

Observe: CH4 + O2 CO2 + H2O

It is necessary to Balance the number and type of atoms on the reactants and products sides of the equation.

Observe: CH4 + O2 CO2 + H2OType of Atom

Total on Reactants

Total on Products

Balanced ?

C 1 1 yes

H 4 2 no

O 2 3 no

Chemical Equations & Conservation of Atoms

cont…

Chemical Equations & Conservation of Atoms

cont… The chemical equation is not

balanced because the number of hydrogen atoms and oxygen atoms are different on both sides of the equation.

To balance the equation, you must figure out what number to multiply each compound by in order to make the numbers add up!

The chemical equation is not balanced because the number of hydrogen atoms and oxygen atoms are different on both sides of the equation.

To balance the equation, you must figure out what number to multiply each compound by in order to make the numbers add up!

Chemical Equations & Conservation of Atoms

cont…

Chemical Equations & Conservation of Atoms

cont… NOTE: You CANNOT change the

number of individual atoms in a compound. You can only change the number of molecules of that compound.

NOTE: You CANNOT change the number of individual atoms in a compound. You can only change the number of molecules of that compound.

Chemical Equations cont…Chemical Equations cont…

To change the number of molecules of a compound, you can write a whole number “Coefficient” in front of the chemical formula.

When this happens, all of the types of atoms in that formula are multiplied by that number.

2CH4 gives you…. 2 x 1 C = 2 Carbon 2 x 4 H = 8 Hydrogen

To change the number of molecules of a compound, you can write a whole number “Coefficient” in front of the chemical formula.

When this happens, all of the types of atoms in that formula are multiplied by that number.

2CH4 gives you…. 2 x 1 C = 2 Carbon 2 x 4 H = 8 Hydrogen

Chemical Equations cont…Chemical Equations cont…

Look at this balanced equation: CH4 + 2O2 CO2 + 2H2O

Look at this balanced equation: CH4 + 2O2 CO2 + 2H2O

Atom Total on Reactants side

Total of Products side

C 1 1

H 4 2 x 2 = 4

O 2 x 2 = 4 2 + (2 x 1) = 4

Chemical Equations cont…Chemical Equations cont…

The balanced equation reads as follows

CH4 + 2O2 CO2 + 2H2O One molecule of methane reacts

with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

The balanced equation reads as follows

CH4 + 2O2 CO2 + 2H2O One molecule of methane reacts

with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water.

Let’s Practice Let’s Practice

20.3 Conservation of Mass20.3 Conservation of Mass

Chemical reactions were a big mystery to people in the eighteenth century.

A scientist named Antoine Laurent Lavoisier (1743-94) stated that the total mass of the products of a reaction is equal to the total mass of the reactants.

This is known as the Law of Conservation of Mass.

Chemical reactions were a big mystery to people in the eighteenth century.

A scientist named Antoine Laurent Lavoisier (1743-94) stated that the total mass of the products of a reaction is equal to the total mass of the reactants.

This is known as the Law of Conservation of Mass.

Conservation of MassConservation of Mass

Lavoisier showed that a “closed” system must be used when studying chemical reactions.

When chemicals are reacted in a closed container, you can show that the mass before & after the reaction is the same.

Lavoisier showed that a “closed” system must be used when studying chemical reactions.

When chemicals are reacted in a closed container, you can show that the mass before & after the reaction is the same.

Antoine LavoisierAntoine Lavoisier

He placed 10.0 grams of mercury (II) oxide into a sealed container. He heated the container so that the mercury (II) oxide reacted to produce oxygen and mercury.

He observed the reaction, and noticed that the mercury (II) oxide bubbled and turned into a smaller amount of a silvery liquid.

In the reaction, 10.0 grams of mercury (II) oxide produced 0.7g of oxygen gas & 9.3g of mercury.

He placed 10.0 grams of mercury (II) oxide into a sealed container. He heated the container so that the mercury (II) oxide reacted to produce oxygen and mercury.

He observed the reaction, and noticed that the mercury (II) oxide bubbled and turned into a smaller amount of a silvery liquid.

In the reaction, 10.0 grams of mercury (II) oxide produced 0.7g of oxygen gas & 9.3g of mercury.

THE END!!THE END!!