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Chapter Seventeen: Chemical Change
17.1 Chemical Reactions
17.2 Balancing Equations
17.3 Classifying Reactions
17.1 Chemical ReactionsA chemical reaction is
the process of breaking of chemical bonds in one or more substances, and the reforming of new bonds to create new substances.
When you make pizza, which changes are physical and which are chemical changes?
17.1 Evidence of chemical change
Four indicators of chemical change are:
1. Formation of new gas
2. Formation of new solid
3. Release of energy (heat or light)
4. Color change
17.1 Products and reactants In chemical reactions, you start
with reactants that are combined to make products.
The reactants are the starting substances.
The products are the new substances which result from the chemical reaction.
17.1 Reactants and productsIn the reaction, methane (a natural gas)
is burned or combusted.
Some energy is added to get the reaction started.
17.2 Balancing Equations
Antoine Laurent Lavoisier, established an important principal based on his experiments with chemical reactions.
He stated that the total mass of the products of a reaction is equal to the total mass of the reactants.
The law of conservation of mass holds true for even a burning mass of wood.
17.2 Balancing Equations
The combined mass of the burning wood and oxygen is converted into carbon dioxide and water.
17.2 Conservation of massLavoisier 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 and after the reaction is the same.
17.2 Formula mass
The sum of the atomic mass values of the atoms in a chemical formula is called the formula mass.
17.2 Avogadro’s NumberThe Avogadro number was named in
honor of Amedeo Avogadro who discovered that a mole of any gas under the same conditions has the same number of molecules.
Johann Josef Loschmidt, a German physicist, named and discovered the Avogadro number.
Loschmidt realized that a mole of any substance—be it a gas, liquid, or solid—contains 6.02 x 1023 atoms or molecules.
17.2 Molar Mass
The mass (in grams) of one mole of a compound is called its molar mass.
What is the molar mass of one mole of CaCO3?
1. Looking for: … molar mass of CaCO3
2. Given … chemical formula
3. Relationships: no. amu in formula = molar mass in
grams
Solving Problems
4. Solution
Solving Problems
Formula mass CaC03 = 100.19 g
1 mole CaC03 = 100.19 g CaCO3
17.2 Chemical EquationsWhen a chemical reaction is written using chemical formulas and symbols, it is called a chemical equation.
17.2 Chemical equations
An arrow is always included between reactants and products.
It means “to produce” or “to yield.”
Reactants Products
to produce
“Methane combines with oxygen gas to produce carbon dioxide gas and water vapor.”
17.2 Numbers in equations
17.2 Balancing equationsThe law conservation of mass is applied
by balancing the number and type of atoms on either side of the equation.
17.2 Balancing equations Counting atoms is necessary to
balance an equation.
How many hydrogen atoms?
How many oxygen atoms?
How many carbon atoms?
17.2 Steps for balancing
1. If not provided, write the word form of the equation.
Calcium carbonate reacts with hydrochloric acid to produce calcium chloride, carbon dioxide and water.
17.2 Steps for balancing
2. If not provided, write the chemical equation from the word form.
17.2 Steps for balancing3. Count the number of each type of
atom on both sides.
17.2 Steps for balancing4. Add coefficients to balance the
equation.
In this reaction, chalcocite (a mineral) reacts with oxygen in the presence of heat. The products are a type of copper oxide and sulfur dioxide. Balance this equation:
Cu2S + O2 → Cu2O + SO2
Solving Problems
1. Looking for: …the coefficients for each molecule
2. Given … chemical formulas which show types
and no. of atoms
Solving Problems
3. Relationships Coefficients can be added in front of any
chemical formula in a chemical equation. When a coefficient is added in front of a
chemical formula, all atoms in that formula are multiplied by that number.
Use common denominators to help choose coefficients to try.
Solving Problems
4. Solution- Trial and error
Solving Problems
17.3 Addition reactionsThe process of creating large molecules from small ones is called polymerization.
17.3 Addition reactionsIn an addition reaction, two or more substances combine to form a new compound.
17.3 Decomposition reactionsA chemical reaction in which a
single compound is broken down to produce two or more smaller compounds is called a decomposition reaction.
17.3 Single Displacement
In a single-displacement reaction, one element replaces a similar element in a compound.
17.3 Double DisplacementIn a double-displacement reaction, ions
from two compounds in solution exchange places to produce two new compounds.
One of the compounds formed is usually a precipitate that settles out of the solution, a gas that bubbles out of the solution, or a molecular compound such as water.
17.3 Precipitation reactions
A precipitate is a new solid product that comes out of solution in a chemical reaction.
The formation of a cloudy precipitate is evidence that a double-displacement reaction has occurred.
17.3 PrecipitationThe limewater test for carbon dioxide is a precipitation reaction.
17.3 Combustion reactionsA combustion reaction, also called burning, occurs when a substance such as wood, natural gas, or propane combines with oxygen and releases a large amount of energy in the form of light and heat.
17.3 Combustion reactions
What do reactants like wood, natural gas, and propane have in common?
Chapter 18.1 Learning Goals
Contrast endothermic and exothermic reactions.
Explain why activation energy is needed to begin chemical reactions.
Describe what happens when ionic compounds are dissolved in water.
18.1 Energy and Reactions
Energy is involved in chemical reactions in two ways:
1. to break some (or all) bonds between atoms in the reactants so the atoms can form new bonds or
2. when the atoms or products form new bonds to make new products.
18.1 Two Types of Reactions
We classify chemical reactions based on how the energy of the reactants compares to the energy of the products.
18.1 Exothermic reactions
If forming new bonds releases more energy than it takes to break the old bonds, the reaction is exothermic.
18.1 Exothermic reactions
A good example is the reaction of hydrogen with oxygen.
Once started, exothermic reactions tend to keep going as each reaction releases more energy to fuel neighboring molecules.
18.1 Endothermic reactions
If forming new bonds in the products releases less energy than it took to break the original bonds, the reaction is endothermic.
18.1 Endothermic reactions
An important endothermic reaction is photosynthesis.
Plants need energy from sunlight to make glucose and oxygen from carbon dioxide and water.
18.1 Activation Energy
Activation energy is the energy needed to begin a reaction and break chemical bonds in the reactants.
18.1 Activation Energy
This is why a flammable material like gasoline does not burn without a spark or flame.
18.1 Carbon Reactions
Carbon dioxide and other atmospheric gases are called “greenhouse gases.”
Scientists believe the rise in amount of greenhouse gases will result in rises in sea level and changes in weather.
18.1 Examples of Endothermic Reactions
Most of the reactions used in industry to produce useful materials require more energy than they produce.
One process that uses endothermic reactions is the refining of ores to produce useful metals.
18.1 Examples of Endothermic ReactionsMost of the reactions
used in industry to produce useful materials require more energy than they produce.
The reaction taking place inside an instant cold pack is endothermic.
18.1 Examples of Endothermic Reactions
When you squeeze the plastic bag the water reacts with the ammonium nitrate crystals, and the reaction dissolves the ionic bonds in the ammonium nitrate.
18.1 Examples of Endothermic Reactions
The reaction is also a dissolution reaction.
Dissolution occurs when an ionic compound (like ammonium nitrate) dissolves in water to make an ionic solution.
