Chapter: Chemical Reactions Table of Contents Section 1: Chemical Formulas and Equations Chemical...

Chapter: Chemical ReactionsChapter: Chemical Reactions

Table of ContentsTable of Contents

Section 1: Section 1: Chemical Formulas and Equations

Section 2: Section 2: Rates of Chemical Reactions

• Matter can undergo two kinds of changesMatter can undergo two kinds of changes—physical and chemical.—physical and chemical.

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

Physical or Chemical Change?Physical or Chemical Change?

Chemical Formulas and EquationsChemical Formulas and Equations

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

produces produces chemical chemical change is a change is a chemical chemical reactionreaction..

Physical or Chemical Change?Physical or Chemical Change?


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

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

Chemical EquationsChemical Equations


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

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

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

Describing What HappensDescribing What Happens


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

Using WordsUsing Words


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



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



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

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

Using Chemical NamesUsing Chemical Names


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

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

Using FormulasUsing Formulas


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

Using FormulasUsing Formulas


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

SubscriptsSubscripts


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

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

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

Conservation of MassConservation of Mass


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

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

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

Conservation of MassConservation of Mass


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

Balancing Chemical EquationsBalancing Chemical Equations


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• Not all chemical equations are Not all chemical equations are balanced so easily. balanced so easily.

Balancing Chemical EquationsBalancing Chemical Equations


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

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

Count the AtomsCount the Atoms


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

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



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• The equation is now balanced. The equation is now balanced.

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



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

• Changing these numbers change the Changing these numbers change the identity of the compound. identity of the compound.

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

Energy in Chemical ReactionsEnergy in Chemical Reactions


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

• Where does this energy come from? Where does this energy come from?

Energy ReleasedEnergy Released


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

• The extra energy is released in various The extra energy is released in various forms—light, sound, and heat. forms—light, sound, and heat.

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

Energy AbsorbedEnergy Absorbed


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



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

• EndothermicEndothermic reactionsreactions absorb heat absorb heat energy. energy.



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• ExothermicExothermic reactionsreactions release heat release heat energy. energy.

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

Heat ReleasedHeat Released


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• Sometimes energy is released rapidly. Sometimes energy is released rapidly.

Rapid ReleaseRapid Release


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

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

Slow ReleaseSlow Release


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

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

Heat AbsorbedHeat Absorbed


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

• The two substances are The two substances are separated by a plastic divider.separated by a plastic divider.

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

Heat AbsorbedHeat Absorbed


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• The dissolving process absorbs heat The dissolving process absorbs heat energy. energy.

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

Energy in the EquationEnergy in the Equation


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

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

Energy in the EquationEnergy in the Equation


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• This tells you that energy is released. This tells you that energy is released.

Section 2: Reaction RatesSection 2: Reaction Rates

How Fast?How Fast?

• Fireworks explode in Fireworks explode in rapid succession on a rapid succession on a summer night. summer night.

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

Rates of Chemical ReactionsRates of Chemical Reactions

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How Fast?How Fast?

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

• Others seem to start Others seem to start on their ownon their own. .


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

Activation Energy— Activation Energy— Starting a ReactionStarting a Reaction

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

• The collision must be strong enough. The collision must be strong enough.


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

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


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

Activation Energy— Activation Energy— Starting a ReactionStarting a Reaction

Reaction RateReaction Rate

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


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Reaction RateReaction Rate

• Chemical reactions have rates, too. Chemical reactions have rates, too.


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

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

Temperature Changes RateTemperature Changes Rate• You can keep the food you buy at the store You can keep the food you buy at the store

from spoiling so quickly by putting it in from spoiling so quickly by putting it in refrigerator or freezer. refrigerator or freezer.


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• Food spoiling is Food spoiling is a chemical a chemical reaction. reaction.

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

Temperature Changes RateTemperature Changes Rate

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


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• Keeping these foods chilled slows the Keeping these foods chilled slows the decomposition process. decomposition process.

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

Temperature Affects Temperature Affects RateRate• Most chemical Most chemical

reactions speed reactions speed up when up when temperature temperature increases.increases.


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

• This means they are more likely to react. This means they are more likely to react.

Temperature Affects Temperature Affects RateRate

• This works the other way too. This works the other way too.


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• Lowering the temperatures slows down Lowering the temperatures slows down most reactions. most reactions.

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

Concentration Affects RateConcentration Affects Rate

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


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Concentration Affects RateConcentration Affects Rate

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


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

Surface Area Affects RateSurface Area Affects Rate

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


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

Slowing Down ReactionSlowing Down Reaction

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


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

• Some inhibitors completely stop Some inhibitors completely stop reactions. reactions.

Speeding Up ReactionsSpeeding Up Reactions

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


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

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

Enzymes Are SpecialistsEnzymes Are Specialists

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


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

Enzymes Are SpecialistsEnzymes Are Specialists

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


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