Transcript chemical reaction

Chapter 14
Chemical Reactions
Little book chapter 2 (page 26-54)
Section 1 Objectives
• Describe how chemical reactions produce new substances
that have different chemical and physical properties.
• Identify four signs that indicate that a chemical reaction might
be taking place.
• Explain what happens to chemical bonds during a chemical
reaction.
Chemical Reactions
• A chemical reaction is a process in
which one or more substances
change to make one or more new
substances.
• The chemical and physical
properties of the substance will
change.
Chemical Reactions
Signs of a chemical reaction:
- Precipitate – solid substance that is
formed in a solution.
- Light
- Heat change
- Electricity
- Color change
- Gas formation
Signs of chemical reactions
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The dark red precipitate is
called silver chromate. It is
formed when potassium
chromate is added to a silver
nitrate solution.
Nitrogen dioxide gas is formed when
copper is placed into nitric acid.
Energy is released during some
chemical reactions.
Bleach reacts with the dye on the
fabric and causes the color of the
material to change.
Section 2 Objectives
• Interpret and write simple chemical formulas.
• Write and balance simple chemical equations.
• Explain how a balanced equation shows the law of
conservation of mass.
Chemical Formulas
• The names of many covalent compounds use prefixes
to express the number of atoms of each element.
mono-
1
Hexa-
6
di-
2
hepta-
7
tri-
3
octa-
8
tetra-
4
nona-
9
penta-
5
deca-
10
Chemical Formulas 2
• In an ionic compound the name of metallic
element is written first and –ide is added to the
nonmetal.
Chemical Equations
• A chemical equation is a representation of a
chemical reaction in which the reactants and
products are expressed as formulas.
• Reactant + Reactant → Product
• Reactants are the substances that undergo
the change.
• Products are the new substances formed.
C + O2 → CO2
Reactants → Product
2+3=5
• Antoine Lavoiser determined that the mass of the
products is always equal to the mass of the
reactants.
• The law of conservation of mass states that mass
is neither created nor destroyed in a chemical
reaction.
• In order to show mass is
conserved an chemical
equation must be balanced.
Balanced Equations
• Equations need to be balanced.
• Matter can not be created nor
destroyed. This means there needs to
be the same number of atoms on each
side of the equation.
• The equation below is balanced
• S8 + 12 O2→8 SO3
Balanced Equations
• Subscript
• small numbers to the lower right of
chemical symbols
• represent the number of atoms of each
element in the molecule
• Coefficients
• large numbers in front of chemical
formulas
• represent the number of molecules of
the substance in the reaction
How Many Atoms?
• Using coefficients and subscripts to count
atoms in equations:
• number of atoms = coefficient X
subscript
• Example: How many of each type of atom
are represented by: 2Al2(SO4)3
• # of Al atoms = 2 X 2 = 4
• # of S atoms = 2 X 1 X 3 = 6
• # of O atoms = 2 X 4 X 3 = 24
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Section 3 Objectives
• Describe four types of chemical reactions.
• Classify a chemical equation as one of four types of chemical
reactions.
Types of Reactions
• A synthesis reaction is a reaction in
which two or more substances
form a single substance. (Simple →
Complex)
• A + B → AB
• A decomposition reaction is when a
compound breaks down into two or
more simpler substances. (Complex
→ Simple)
• AB → A + B
Types of Reactions
• Single Displacement reaction in which one
element takes the place of another element
in a compound.
• (One person breaks up a couple and goes
out with one of them.
• A + BC→ AC + B
Types of Reactions
• Double Displacement reaction is one
in which two different compounds
exchange positive ions and form two
new compounds.
• AB + CD → AD + CB
Energy Changes
• Chemical energy
• the energy stored in the chemical bonds
of a substance.
• Breaking a chemical bond requires
energy.
• During a chemical reaction, energy is
either
• Gained
• released
Section 4 Objectives
• Compare exothermic and endothermic reactions.
• Explain activation energy.
• Interpret an energy diagram.
• Describe five factors that affect the rate of a reaction.
Conservation of Energy Review
• The law of conservation of energy
states
• neither mass or energy can be
created or destroyed in a chemical
reaction.
• Energy can be
• transferred from one object to
another.
IN/OUT
• EXOTHERMIC Reaction
• the energy released as the products form is
greater than the energy required to break the
bonds.
• Reaction in which energy is
• RELEASED
• Exo = EXITS
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Examples:
Fire
Digestion
Hand warmers
IN/OUT
• ENDOTHERMIC Reaction
• MORE energy is required to break the bonds in the
reactants than is released by the formation of the
products.
• Reaction in which energy is taken in
• Endo = ENTERS
• Examples:
• Alka-seltzer
• Ice packets
• Camera
Reaction Rate
•Reaction rates
•tell you how fast a reaction is
going.
Factors That Affect Reaction
Rates
• Temperature
• – Increasing temperature increases
the reaction rate.
• Stirring
• – increases the exposure of the
reactant increases the reaction rate.
Factors That Affect Reaction
Rates
• Surface Area
• Increasing surface area increases
the reaction rate
• The larger the surface area, the faster
the reaction rate
• Concentration
• increasing concentration increases
the reaction rate.
Factors That Affect Reaction
Rates
•Catalyst
• a substance that increases the
reaction rate without being used up
in the reaction.
• A substance that speeds up a
reaction without being permanently
changed. NOT A REACTANT!!
• Ex: Enzymes speed up reactions in
your body.
Get Started
• Many reactions
• occur at room temperature, but some
need a “little help” to get started.
• You may need to add
• heat (Lighting a candle) or
• pressure (Diesel engine)
• Start a reaction
Activation Energy
• ACTIVATION ENERGY
• Energy needed to START a reaction.
• This is represented by an activation
curve on a graph.
Reaction Rate & Catalyst
• You can speed up a
reaction (rxn) rate by:
•
•
•
•
Increasing Temperature
Increase Surface Area
Increase Concentration
Using a catalyst
• A catalyst is a substance
that speeds up a reaction
without being part of the
reaction.
• A catalyst works by
• decreasing the
amount of activation
energy required for
the reaction to occur.
• Remember, reactions
require
• activation energy
• catalyst simply lowers
the amount of energy.
Catalyst Continued
• A catalyst is neither
• a reactant or
• a product
• When you finish the
reaction all the catalyst
is still there – none was
used or changed in the
reaction.
Inhibit
• An inhibitor is a
substance that slows
down or stopping a
reaction may
sometimes be useful.
• Examples:
• Penicillin
• Preservatives
• Fungicide
Section 1 Review
• 1. Use the following terms in the same sentence: chemical
reaction and precipitate.
• 2. Most chemical reactions:
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A. have starting substances that collide with each other.
B. do not break bonds.
C. do not rearrange atoms.
D. cannot be seen.
• 3. If the chemical properties of a substance have not changed,
has a chemical reaction occurred?
• 4. Steam is escaping from a teapot. Is this a chemical reaction?
Explain.
Section 2 Review
• 1. Make the following statement TRUE: A chemical formula
describes a chemical reaction
• 2. Make the following statement TRUE: The substances
formed from a chemical reaction are reactants.
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A. have starting substances that collide with each other.
B. do not break bonds.
C. do not rearrange atoms.
D. cannot be seen.
• 3. The correct chemical formula for carbon tetrachloride is:
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A. CCl3
B. C3Cl
C. CCl
D. CCl4
Section 3 Review
• 1. In your own words, write a definition for each of the
following terms: synthesis reaction and decomposition
reaction.
• 2. What type of reaction does the following equation
represent? FeS + 2HCl -> FeCl2 +H2S
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A. synthesis reaction
B. double-displacement reaction
C. single-replacement reaction
D. decomposition reaction
• 3. Describe the difference between single- and doubledisplacement reactions.
• 4. Write the balanced equation in which potassium iodide, KI,
reacts with chlorine to form potassium chloride, KCl, and
iodine.
Section 4 Review
• 1. Make the following TRUE: An exothermic reaction absorbs
energy.
• 2. Make the following TRUE: The rate of a reaction can be
increased by adding an inhibitor.
• 3. Which of the following will not increase the rate of a
reaction?
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A. adding a catalyst
B. increasing the temperature of the reaction
C. decreasing the concentration of reactants
D. grinding a solid into powder
• 4. How does the concentration of a solution affect the rate of
the reaction?