Chapter 21: Chemical Reactions Page 632 Section 1: Chemical Change/Conservation of mass Section 2:...

Chapter 21: Chemical ReactionsPage 632

Section 1: Chemical Change/Conservation of mass

Section 2: Chemical Equations

Section 3: Classifying Chemical Reactions

Section 4: Reactions and Energy

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

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

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.

Physical or Chemical Change?

Some indicators of chemical changes include:

• color changes, • energy (such as heat or light)

production or loss,• production of bubbles (which

indicates the production of a gas), • precipitation of a new solid when

two solutions are mixed.

Evidence of Chemical Reaction (p462)

Chemical ChangesChemical Changes

The burning of charcoal (carbon) in oxygen is a chemical reaction and can be expressed as the chemical equation

carbon + oxygen carbon dioxide

reactants product

Burning of gasoline, wood, or charcoal are examples of chemical changes.

A chemical change is referred to as a chemical reaction.

• 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).

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.

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

Using Words

• Reactants are on the left side of an arrow, separated from each other by plus signs and products are on the right side of an arrow, separated from each other by plus signs..

Chemical ChangesChemical Changes

chemical equations may beexpressed in words, but are mostcommonly expressed using chemical formulas.

carbon + oxygen carbon dioxide

C + O2 CO2

reactants product

Symbols used in equations (P635)Symbol Meaning

Produces or forms

+ Plus

(s) Solid

(l) Liquid

(g) Gas

(aq) Aqueous, substance dissolved in water

Heat ∆

The reactants are heated

Light

The reactants are exposed to light

Elec

The reactants are exposed to electric current

• 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.

Conservation of Mass (P632)

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

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

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

• Chemical equations are similar to math equations, the mass must be equal on the two sides.

Conservation of Mass

Example (P633)

• 10 grams of solid Mercury (II) Oxide when heated produced 9.3 grams of liquid mercury and 0.7 grams of oxygen gas.

• HgO Hg + O2

• 10.0 gm 9.3 gm + 0.7 gm

• Mass Balanced Chemical Equation

Law of Conservation of MassPage 463

• When a chemical reaction takes place, the total mass of reactants equals the total mass of products.

• If 18g of hydrogen react completely with 633 g of chlorine, how many grams of HCl are formed? The equation for the reaction is

• H2 + Cl2 2 HCl

18g + 633g ______ g

Assignment (p463:1,2 and 637:6)• In the following reaction, 24g of CH4 (methane) react completely with 96g of O2

(oxygen gas) to form 66g of CO2. How many grams of H2O are formed?

CH4 + 2O2 CO2 + 2 H2O

24g + 96 g 66g + ?• In the following reaction, 54 g of Al react completely with 409.2 g of ZnCl2 to form

196.2 g of Zn metal. How many grams of AlCl3 are formed?

2 Al + 3 ZnCl2 3 Zn + 2 AlCl354g + 409.2g 196.2g + ?

• In making soap, 890 g of fat react completely with 120 g of sodium hydroxide to produce 92 g of glycerin and soap. How much soap is produced?

fat + sodium hydroxide glycerin + soap

• Not only must the mass be equal on both sides but the type and number of atoms of each type must be equal.

Conservation of Matter

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

Balancing Chemical Equations

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

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

Balancing Chemical Equations

Balancing Equations

•HgO Hg + O2

• Not all chemical equations are balanced so easily. Trial and error

Balancing Chemical Equations

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

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

Count the Atoms

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

• Changing these subscripts change the identity of the compound.

How to Balance an Equation (p639)• Magnesium burns with such a brilliant white light that it is

often used in emergency flares. Burning magnesium leaves a white powder called magnesium oxide. To write a balanced equation for this follow these 4 steps.

• STEP 1 Write formula and symbols to describe the reaction. Remember that oxygen is one of the diatomic gases. BrINClHOF

• Solid Magnesium metal + oxygen Magnesium oxide

• Mg(s) + O2(g) MgO(s)

• STEP 2 Count the atoms in reactants and products.• Mg• O

Balancing Continued

• Step 3 Choose coefficients that balance the equation.

• NEVER CHANGE SUBSCRIPTS

• ____ Mg(s) + ____ O2(g) ____ MgO

• STEP 4 Recheck the number of each type of atom on each side of the equation.

Practice (page 640)• When lithium metal is treated with water, hydrogen gas

and lithium hydroxide are produced.• STEP 1 Write the formulas and symbols

• STEP 2 Count the atoms in reactants and products

• STEP 3 Choose coefficients that balance the equation. Begin with atoms that occur only once, then polyatomic ions, then O and H

• STEP 4 Recheck

You Practice• Iron metal reacts with Chlorine gas to form

solid Iron(III)Chloride.

• Write the formula for each reactant and product.

• Iron Metal

• Chlorine gas

• Iron(III)Chloride

• Write the equation

• Balance the equation

Chapter 21 Assignments• Practice Problems (test extra credit)• Conservation of Matter Problems• Writing equations worksheets• Note taking worksheet• Chapter Review page 656: 11-19, 29, 30• Thursday 4/28 Types of reactions• Friday 4/29 Energy and Chemical Reactions• Monday 5/2 Chapter Review• Tuesday 5/3 Lab – John Adams Hall• Wednesday 5/4 Test

Chapter 21 Section 3Classifying Chemical

Reactions

Page 641

• Synthesis (composition)– Combustion

• Decomposition– Electrolysis

• Single Displacement• Double Displacement

• You can’t learn these by going over them in class once! You must study them!

Section 3: Classifying Chemical Reactions(4 or 5 classes)

• In a synthesis reaction two or more substances combine to form a new compound.

• Synthesis reactions are also known as composition reactions.

• Synthesis reactions may be represented by the general equation

Substance-1 + substance-2 new substanceA + B AB

Synthesis ReactionsSynthesis Reactions

• Metals react with oxygen to form metal oxides.

2 Mg(s) + O2(g) 2 MgO(s)

Synthesis ReactionsSynthesis Reactions

04m15vd1

carbon + oxygen carbon dioxide + heat

C(s) + O2(g) CO2(g) + heat

Combustion ReactionsCombustion ReactionsPage 641 (subclass)Page 641 (subclass)

•In a combustion reaction a substance combines with oxygen, releasing a large amount of energy in the forms of heat and light.

Combustion (cont)

• The burning of hydrocarbons such as methane, propane, and gasoline are examples of combustion.

• Combustion of a hydrocarbon always produces carbon dioxide and water.

• Propane burns to form carbon dioxide and water.

• C3H8 + O2 CO2 + H2O

Hydrocarbons

• Methane CH4

• Ethane C2H4

• Propane C3H8

• Butane C4H10

• In a decomposition reaction a single compound reacts to form two or more simpler substances.

• It is the opposite of a synthesis reaction.• Usually requires energy (heat, light, electricity)

• Decomposition reactions may be represented by the general equation

Substance-1 substance-2 + substance-3 AB A + B

Decomposition Decomposition ReactionsReactions

• There are many types of decomposition reactions.

• The decomposition of a substance by an electric current is called electrolysis.

• elec

2 H2O(l) 2 H2(g) + O2(g)

Decomposition Decomposition ReactionsReactions

• Metal carbonates decompose with heat to yield a metal oxide and carbon dioxide gas.

heat

CaCO3(s) CaO(s) + CO2(g)

Decomposition Decomposition ReactionsReactions

• Displacement reactions are also called replacement reactions.

• In a single-displacement reaction one element replaces a similar element in a compound.

• Single-replacements reactions may be represented by the general equations

metal + compoundnew compound + different metal or

Y + XB YB + X

Single-Displacement ReactionsSingle-Displacement Reactions

• Hydrogen in water may be displaced by a metal to produce hydrogen and a basic solution.

Single-Displacement ReactionsSingle-Displacement Reactions

2Na(s) + 2 H2O(aq) 2 NaOH(aq) + H2(g)

02m17vd1

What Replaces Activity Series (P643)• An element will replace any element below it on the

activity series.• Lithium Li• Potassium K• Calcium Ca• Sodium Na• Aluminum Al• Zinc Zn• Iron Fe• Tin Sn• Lead Pb• Hydrogen H• Copper Cu• Silver Ag• Gold Au

• Displacement reactions are also called replacement

reactions.

• In a double-displacement reaction the ions of two compounds exchange places in an aqueous solution to form two new compounds.

• Double-replacements reactions may be represented by the general equation

AB(aq) + CD(aq) AD(s)+ CB(aq)

Double-Displacement Double-Displacement ReactionsReactions

The “driving force” is the formation of an insoluble compound — a precipitate.

Lead nitrate + potassium iodide potassium nitrate + lead iodide

Pb(NO3)2(aq) + 2 K I(aq) 2 KNO3(aq) +PbI2(s)

Double-Displacement ReactionsDouble-Displacement Reactions

Double Displacement• Ba(NO3)2(aq) + K2 SO4(aq) BaSO4(s) +2 KNO3(aq)

• This type reaction takes place only when one of the products formed is insoluable in water.

Reaction Types (P641-643)

Combustion

A +O2 CO2 + ? + heat

Synthesis or composition (2 or more to 1)

A + B AB

Decomposition

AB A + B

Single Displacement

A + BC AC + B

Double Displacement

AB + CD AD + CB

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

Energy in Chemical Reactions Section 4 page 646

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

• Where does this energy come from?

Chemical Energy Released

• 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 forms—light, sound, and heat.

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

Energy Absorbed

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

Energy Absorbed

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

• Electrolysis

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

Energy Absorbed/Released

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

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

Heat Released

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

Heat Absorbed

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

• The two substances are separated by a plastic divider.

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

• The dissolving process absorbs heat energy.

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

Energy in the Equation

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

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

• Energy = heat, light, electricity, sound

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.

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

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.

• Rust inhibitors. Fruit fresh (lemon juice)• Some inhibitors completely stop reactions.

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

Speeding Up Reactions• A catalyst is a substance that speeds up a

chemical reaction.

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.

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

Chapter 21 Schedule• Monday --4-12

– Balancing Chemical Equations

• Tuesday – 4-13– Reaction Types

• Wednesday -- 4-14– Chemical Reactions and Energy

• Thursday – 4/15– Lab-Evidence for Chemical Reactions– Chapter Review P656-657:11-19,23,25,26,29,30

• Friday – 4/16– Notetaking worksheets

• Monday 4/19– Chapter review

• Tuesday 4/20– Chapter 21 Test

• Sometimes energy is released rapidly.

Rapid Release

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

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

Slow Release

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

How Fast?

• Fireworks explode in rapid succession on a summer night.

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

How Fast?

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

• Others seem to start on their own.

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

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.

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

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

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.

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.

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.

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

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

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.

• To start a campfire use wood cut into fine sticks called kindling wood because it is has more surface area but less weight.