Chapter: Chemical Reactions

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Table of Contents
Chapter Chemical Reactions
Section 1 Chemical Formulas and Equations
Section 2 Rates of Chemical Reactions
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Chemical Formulas and Equations
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Physical or Chemical Change?

• Matter can undergo two kinds of changesphysical
  and chemical.

• Physical changes in a substance affect only
  physical properties, such as its size and shape,
  or whether it is a solid, liquid or gas.

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Chemical Formulas and Equations
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Physical or Chemical Change?

• Chemical changes produce new substances that have
  properties different from those of the original
  substances.

• A process that produces chemical change is a
  chemical reaction.

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Chemical Formulas and Equations
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Chemical Equations

• To describe a chemical reaction, you must know
  which substances react and which substances are
  formed in the reaction.

• The substances that react are called reactants
  (ree AK tunts).

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Chemical Formulas and Equations
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Chemical Equations

• Reactants are the substances that exist before
  the reaction begins.

• The substances that form as a result of the
  reaction are called the products.

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Chemical Formulas and Equations
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Describing What Happens

• What goes on in the chemical reaction can be more
  than what you see with you eyes.

• Chemists try to find out which reactants are used
  and which products are formed in a chemical
  reaction.

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Chemical Formulas and Equations
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Describing What Happens

• They can write it in a shorthand form called a
  chemical equation.

• A chemical equation tells chemists at a glance
  the reactants, products, physical state, and the
  proportions of each substance present.

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Chemical Formulas and Equations
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Using Words

• One way you can describe a chemical reaction is
  with an equation that uses words to name the
  reactants and products.

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Chemical Formulas and Equations
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Using Words

• The reactants are listed on the left side of an
  arrow, separated from each other by plus signs.

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Chemical Formulas and Equations
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Using Words

• The products are placed on the right side of the
  arrow, also separated by plus signs.

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Chemical Formulas and Equations
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Using Words

• The arrow between the reactants and products
  represent the changes that occur during the
  chemical reaction.

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Chemical Formulas and Equations
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Using Chemical Names

• Chemical names are usually used in word equations
  instead of common names.

• In a baking soda and vinegar reaction, the
  chemical names of the reactants are sodium
  hydrogen carbonate and acetic acid.

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Chemical Formulas and Equations
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Using Chemical Names

• The names of the products are sodium acetate,
  water, and carbon dioxide.

• The word equation for the reaction is as follows

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Chemical Formulas and Equations
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Using Formulas

• The word equation for the reaction of baking soda
  and vinegar is long.

• You can convert a word equation into a chemical
  equation by substituting chemical formulas for
  the chemical names.

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Chemical Formulas and Equations
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Using Formulas

• The chemical equation for the reaction between
  baking soda and vinegar can be written as
  follows.

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Chemical Formulas and Equations
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Subscripts

• When you look at chemical formulas, notice the
  small numbers written to the right of the atoms.

• These numbers called subscripts, tell you the
  number of atoms of each element in that compound.

• If an atom has no subscript, it means that only
  one atom of that element is in the compound.

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Chemical Formulas and Equations
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Conservation of Mass

• According to the law of conservation of mass, the
  mass of the products must be the same as the mass
  of the reactants in that chemical reaction.

• This principle was first stated by the French
  chemist Antoine Lavoisier (1743-1794).

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Chemical Formulas and Equations
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Conservation of Mass

• He showed that chemical reactions are much like
  mathematical equations.

• In math equations, the right and left sides of
  the questions are numerically equal.

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Chemical Formulas and Equations
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Conservation of Mass

• Chemical equations are similar, but it is the
  number and kind of atoms that are equal on the
  two sides.

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Chemical Formulas and Equations
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Balancing Chemical Equations

• When you write the chemical equation for a
  reaction, you must observe the law of
  conservation of mass.

• When you count the number of carbon, hydrogen,
  oxygen, and sodium atoms of each side of the
  arrow in the equation, you find equal numbers of
  each kind of atom.

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Chemical Formulas and Equations
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Balancing Chemical Equations

• This means the equation is balanced and the law
  of conservation of mass is observed.

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Chemical Formulas and Equations
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Balancing Chemical Equations

• Not all chemical equations are balanced so
  easily.

• The following unbalanced equation shows what
  happens when silver tarnishes.

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Chemical Formulas and Equations
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Count the Atoms

• Count the number of atoms of each type in the
  reactants and in the products.

• One silver atom is on the reactant side and two
  silver atoms are on the product side. This
  cannot be true.

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Chemical Formulas and Equations
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Count the Atoms

• Place a 2 in front of the reactant Ag and check
  to see if the equation is balanced. Recount the
  number of atoms of each type.

• The equation is now balanced.

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Chemical Formulas and Equations
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Count the Atoms

• When balancing chemical equations, numbers are
  placed before the formulas as you did for Ag.

• These are called coefficients. However, never
  change the subscripts written to the right of the
  atoms in a formula.

• Changing these numbers change the identity of the
  compound.

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Chemical Formulas and Equations
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Energy in Chemical Reactions

• Often, energy is released or absorbed during a
  chemical reaction.

• For example, energy of a welding torch is
  released when hydrogen and oxygen combine to form
  water.

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Chemical Formulas and Equations
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Energy Released

• Where does this energy come from?

• In reactions that release energy, the products
  are more stable, and their bonds have less energy
  than those of the reactants.

• The extra energy is released in various
  formslight, sound, and heat.

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Chemical Formulas and Equations
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Energy Absorbed

• In reactions that absorb energy, the reactants
  are more stable, and their bonds have less energy
  than those of the products.

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Chemical Formulas and Equations
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Energy Absorbed

• As you have seen, reactions can release or absorb
  energy of several kinds, including electricity,
  light, sound, and heat.

• Electrical energy is needed to break water into
  its components.

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Chemical Formulas and Equations
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Energy Absorbed

• Endothermic (en doh THUR mihk) reactions absorb
  heat energy.

• Exothermic (ek soh THUR nihk) reactions release
  heat energy.

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Chemical Formulas and Equations
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Heat Released

• Burning is an exothermic chemical reaction in
  which a substance combines with oxygen to produce
  heat along with light, carbon dioxide, and water.

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Chemical Formulas and Equations
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Rapid Release

• Sometimes energy is released rapidly.

• For example, charcoal lighter fluid combines with
  oxygen in the air and produces enough heat to
  ignite a charcoal fire within a few minutes.

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Chemical Formulas and Equations
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Slow Release

• Other materials also combine with oxygen but
  release heat so slowly that you cannot see or
  feel it happen.

• This is the case when iron combines with oxygen
  in the air to form rust.

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Chemical Formulas and Equations
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Heat Absorbed

• An example of an endothermic physical process
  that absorbs heat energy is the cold pack shown.

• The heavy plastic cold pack holds ammonium
  nitrate and water.

• The two substances are separated by a plastic
  divider.

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Chemical Formulas and Equations
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Heat Absorbed

• When you squeeze the bag, you break the divider
  so that the ammonium nitrate dissolves in the
  water.

• The dissolving process absorbs heat energy.

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Chemical Formulas and Equations
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Energy in the Equation

• The word energy often is written in equations as
  either a reactant or a product.

• Energy written as a reactant helps you think of
  energy as a necessary ingredient for the reaction
  to take place.

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Chemical Formulas and Equations
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Energy in the Equation

• Similarly, in the equation for an exothermic
  reaction, the word energy often is written along
  with the products.

• This tells you that energy is released.

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Section Check
Section Check
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Question 1
In a chemical reaction, the substances that react
are called the _______.
Answer
The substances that react are called reactants.
The substances that form as a result of the
reaction are known as the products.
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Section Check
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Question 2
What kind of expression is Acetic acid Sodium
hydrogen carbonate ? Sodium acetate Water
Carbon dioxide?
Answer
This is a word equation. It spells out the actual
words that are used to describe the reactants
rather than writing them as chemical formulas.
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Section Check
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Question 3
What kind of expression is CH3COOH NaHCO3 ?
CH3COONa H2O CO2?
Answer
This is an example of a chemical equation. It
uses chemical formulas instead of word names.
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Rates of Chemical Reactions
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How Fast?

• Fireworks explode in rapid succession on a summer
  night.

• Old copper pennies darken slowly while they lie
  forgotten in a drawer.

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Rates of Chemical Reactions
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How Fast?

• Not all chemical reactions take place at the same
  rate.

• Some reactions, such as fireworks or lighting a
  campfire, need help to get going.

• Others seem to start on their own.

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Rates of Chemical Reactions
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Activation Energy Starting a Reaction

• Before a reaction can start, molecules of the
  reactants have to bump into each other, or
  collide.

• The collision must be strong enough.

• This means the reactants must smash into each
  other with a certain amount of energy.

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Rates of Chemical Reactions
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Activation Energy Starting a Reaction

• To start any chemical reaction, a minimum amount
  of energy is needed.

• This energy is called the activation energy of
  the reaction.

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Rates of Chemical Reactions
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Activation Energy Starting a Reaction

• Most fuels need energy to ignite.

• The Olympic Torch provided the activation energy
  required to light the fuel in the cauldron.

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Rates of Chemical Reactions
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Reaction Rate

• A rate tells you how much something changes over
  a given period of time.

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Rates of Chemical Reactions
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Reaction Rate

• Chemical reactions have rates, too.

• The rate of reaction tells how fast a reaction
  occurs after it has started.

• To find the rate of a reaction, you can measure
  either how quickly one of the reactions is
  consumed or how quickly one of the products is
  created.

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Rates of Chemical Reactions
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Temperature Changes Rate

• You can keep the food you buy at the store from
  spoiling so quickly by putting it in refrigerator
  or freezer.

• Food spoiling is a chemical reaction.

• Lowering the temperature of the food slows the
  rate of this reaction.

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Rates of Chemical Reactions
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Temperature Changes Rate

• Meat and fish decompose faster at higher
  temperatures, producing toxins that can make you
  sick.

• Keeping these foods chilled slows the
  decomposition process.

• Bacteria grow faster at higher temperatures, too,
  so they reach dangerous levels sooner.

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Rates of Chemical Reactions
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Temperature Affects Rate

• Most chemical reactions speed up when temperature
  increases.

• Molecules collide more frequently at higher
  temperatures that at lower temperatures.

• This means they are more likely to react.

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Rates of Chemical Reactions
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Temperature Affects Rate

• This works the other way too.

• Lowering the temperatures slows down most
  reactions.

• If you set the oven temperature too low, your
  cake will not bake properly.

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Rates of Chemical Reactions
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Concentration Affects Rate

• The closer reactant atoms and molecules are to
  each other, the greater the chance of collisions
  between them and the faster the reaction rate.

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Rates of Chemical Reactions
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Concentration Affects Rate

• The amount of substance present in a certain
  volume is called the concentration of that
  substance.

• If you increase the concentration, you increase
  the number of particles of a substance per unit
  of volume.

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Rates of Chemical Reactions
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Surface Area Affects Rate

• The exposed surface area of reactant particles
  also affects how fast the reaction can occur.

• Only the atoms or molecules in the outer layer of
  the reactant material can touch the other
  reactants and react.

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Rates of Chemical Reactions
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Slowing Down Reaction

• A substance that slows down a chemical reaction
  is called an inhibitor.

• And inhibitor makes the formation of a certain
  amount of product take longer.

• Some inhibitors completely stop reactions.

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Rates of Chemical Reactions
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Speeding Up Reactions

• A catalyst is a substance that speeds up a
  chemical reaction.

• Catalysts do not appear in chemical equations,
  because they are not changed permanently or used
  up.

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Rates of Chemical Reactions
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Speeding Up Reactions

• Many catalysts speed up reaction rates by
  providing a surface for the reaction to take
  place.

• Other catalysts reduce the activation energy
  needed to start the reaction.

• When the activation energy is reduced, the
  reaction rate increases.

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Rates of Chemical Reactions
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Catalytic Converters

• Catalysts are used in the exhaust systems of cars
  and trucks to aid fuel combustion.

• The exhaust passes through the catalyst, often in
  the form of beads coated with metals such as
  platinum or rhodium.

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Rates of Chemical Reactions
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Catalytic Converters

• Catalysts speed the reactions that change
  incompletely burned substances that are harmful,
  such as carbon monoxide, into less harmful
  substances, such as carbon dioxide.

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Rates of Chemical Reactions
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Enzymes Are Specialists

• Enzymes are large protein molecules that speed up
  reactions needed for your cells to work properly.

• They help your body convert food to fuel, build
  bone and muscle tissue, convert extra energy to
  fat, and even produce other enzymes.

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Rates of Chemical Reactions
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Enzymes Are Specialists

• Like other catalysts, enzymes function by
  positioning the reacting molecules so that their
  structures fit together properly.

Click image to view movie.
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Rates of Chemical Reactions
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Other Uses

• One class of enzymes, called proteases (PROH tee
  ay ses), specializes in protein reactions.

• They work within cells to break down large,
  complex molecules called proteins.

• Contact lens cleaning solutions contain proteases
  that break down proteins from your eyes that can
  collect on your lenses and cloud your view.

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Section Check
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Question 1
The minimum amount of energy required to start a
reaction is called the _______.
Answer
Activation energy is the minimum amount of energy
required to start a reaction.
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Section Check
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Question 2
Most chemical reactions _______ when temperature
increases.
A. abate B. remain the same C. slow down D. speed
up
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Section Check
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Answer
The answer is D. Temperature has a direct effect
on the reaction rate.
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Section Check
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Question 3
Most chemical reactions _______ when
concentration increases.
A. remain the same B. slow down C. speed up D.
stop
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Section Check
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Answer
The answer is C. Higher concentration means a
greater likelihood of collisions between the
atoms in the reactants.
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