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.
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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!!
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