Chapter: Chemical Reactions

Table of ContentsChapter: Chemical ReactionsSection 1: Chemical ChangesSection 2: Chemical EquationsSection 3: Classifying ChemicalReactionsSection 4: Chemical Reactions andEnergy

Chemical Changes1Describing Chemical Reactions Chemical reactions are taking place allaround you and even within you. A chemicalreaction is achange in whichone or moresubstances areconverted intonew substances.A chemical reaction occurs whenyou bake a cake.

Chemical Changes1Describing Chemical Reactions The substances that react are calledreactants. The new substances produced are calledproducts. This relationship can be written as follows:

Chemical Changes1Conservation of Mass The French chemist Antoine Lavoisierestablished that the total mass of the productsalways equals the total mass of the reactants. For example, the mass ofthe candles and oxygenbefore burning is exactlyequal to the mass of theremaining candle andgaseous products.

Chemical Changes1Lavoisier's Contribution One of the questions that motivatedLavoisier was the mystery of exactly whathappened when substances changed form. He began to answer this question byexperimenting with mercury.

Chemical Changes1Lavoisier's Contribution Lavoisier placed a carefully measured massof solid mercury (II) oxide, which he knewas mercury calx, into a sealed container. When he heated this container, he noted adramatic change. The red powder had beentransformed into a silveryliquid that he recognizedas mercury metal, and agas was produced.

Chemical Changes1Lavoisier's Contribution When he determined the mass of the liquidmercury and gas, their combined masseswere exactly the same as the mass of the redpowder he had started with. Lavoisier also established that the gasproduced by heating mercury(II) oxide,which we call oxygen, was a component ofair.

Chemical Changes1The Father of Modern Chemistry Lavoisier is known today as the father ofmodern chemistry for his more accurateexplanation of the conservation of mass andfor describing a common type of chemicalreaction called combustion.

Chemical Changes1Nomenclature Lavoisier developed the system of namingsubstancesbased ontheircomposition thatwe stillusetoday.

Chemical Changes1Writing Equations Scientists have developed a shorthandmethod to describe chemical reactions. A chemical equation is a way to describe achemical reaction using chemical formulasand other symbols.

Chemical Changes1Writing Equations Some of the symbolsused in chemicalequations are listedin the table.

Chemical Changes1Metals and the Atmosphere When iron is exposed to air and moisture, itcorrodes or rusts, forming hydrated iron(III) oxide. Rust can seriouslydamage ironstructures because itcrumbles and exposesmore iron to the air.

Chemical Changes1Metals and the Atmosphere Aluminum also reacts with oxygen in the airto form aluminum oxide. Unlike rust, aluminum oxide adheres to thealuminum surface, forming an extremelythin layer that protects the aluminum fromfurther attack.

Chemical Changes1Metals and the Atmosphere Copper is anothermetal that corrodeswhen it is exposed toair, forming a bluegreen coating called apatina. You can see this typeof corrosion on manypublic monuments.

Section Check1Question 1What is a chemical reaction?AnswerA process is a chemical reaction if one ormore substances change into new substances.

Section Check1Question 2The principle that the total starting mass ofall reactants equals the total final mass of allproducts is known as .AnswerThis is the law of conservation of mass;during a chemical change, matter can neitherbe created nor destroyed.

Section Check1Question 3Describe the products in the followingreaction:NiCl2(aq) 2NaOH(aq)2NaCl(aq)A. aqueousB. liquidNi(OH)2(s) C. solid and liquidD. solid and aqueous

Section Check1AnswerThe answer is D. The products are solidnickel(II) hydroxide and aqueous sodiumchloride.

Chemical Equations2Balanced Equations Lavoisier’s mercury(II) oxide reaction can bewritten as: Notice that the number of mercury atoms isthe same on both sides of the equation butthat the number of oxygen atoms is not thesame.

Chemical Equations2Balanced Equations One oxygen atom appears on the reactantside of the equation and two appear on theproduct side. According to the law of conservation ofmass, one oxygen atom cannot just becometwo. Nor can you simply add the subscript 2and write HgO2 instead of HgO.

Chemical Equations2Balanced Equations The formulas in a chemical equation mustaccurately represent the compounds thatreact. Fixing this equation requires a processcalled balancing. The balancing process involves changingcoefficients in a reaction to achieve abalanced chemical equation, which has thesame number of atoms of each element onboth sides of the equation.

Chemical Equations2Choosing Coefficients Finding out which coefficients to use tobalance an equation is often a trial-and-errorprocess. In the equation for Lavoisier’s experiment,the number of mercury atoms is balanced, butone oxygen atom is on the left and two are onthe right.

Chemical Equations2Choosing Coefficients If you put a coefficient of 2 before the HgOon the left, the oxygen atoms will bebalanced, but the mercury atoms becomeunbalanced. To balance the equation, also put a 2 in frontof mercury on the right. The equation is nowbalanced.

Chemical Equations2Try Your Balancing Act Magnesium burns with such a brilliant whitelight that it is often used in emergency flares. Burning leaves a white powder calledmagnesium oxide. To write a balancedchemical equation forthis and most otherreactions, follow thesefour steps.

Chemical Equations2Try Your Balancing Act Step 1 Write a chemical equation for thereaction using formulas and symbols. Step 2 Count the atoms in reactants andproducts.

Chemical Equations2Try Your Balancing Act Step 3 Choose coefficients that balance theequation. Remember, never change subscripts of acorrect formula to balance an equation.

Chemical Equations2Try Your Balancing Act Step 4 Recheck the numbers of each atomon each side of the equation and adjustcoefficients again if necessary.

Section Check2Question 1In a chemical formula, how is the number ofatoms of an element indicated?AnswerIn a chemical formula, subscripts are usedwith symbols for elements to indicate thenumber of atoms.

Section Check2Question 2What does it mean for a chemical equationto be balanced?A. compounds exist in the same amountsboth before and after a reactionB. coefficients are the same for the reactantsas for the products

Section Check2C. there are the same number of atoms ofeach element on both sides of thechemical reactionD. the rate of formation of products is equalto the rate of formation of reactants

Section Check2AnswerThe answer is C. Balancing an equation doesnot change what happens in the reaction.

Section Check2Question 3What is the correct balanced equation for thereaction of magnesium and oxygen?A.B.C.D.Mg(s) O(g) MgO(s)Mg(s) O2(g) MgO(s)Mg2(s) O2(g) 2MgO(s)2Mg(s) O2(g) 2MgO(s)

Section Check2AnswerThe answer is D. Oxygen is a diatomicmolecule. To balance an equation, changethe coefficients, not the subscripts.

Classifying Chemical Reactions3Types of Reactions There are literally millions of chemicalreactions that occur every day. Chemists have defined five main categoriesof chemical reactions: combustion, synthesis,decomposition, single displacement, anddouble displacement.

Classifying Chemical Reactions3Combustion Reactions If you have ever observed something burning,you have observed a combustion reaction. Our definition statesthat a combustionreaction occurswhen a substancereacts with oxygento produce energy inthe form of heat andlight.

Classifying Chemical Reactions3Combustion Reactions Combustionreactions alsoproduce one ormore productsthat contain theelements in thereactants.

Classifying Chemical Reactions3Synthesis Reactions In a synthesis reaction, two or moresubstances combine to form anothersubstance. The generalized formula for this reaction typeis as follows: A B AB.

Classifying Chemical Reactions3Decomposition Reactions A decomposition reaction is just the reverseof a synthesis. Instead of two substances coming together toform a third, a decomposition reactionoccurs when one substance breaks down, ordecomposes, into two or more substances.

Classifying Chemical Reactions3Decomposition Reactions The general formula for this type of reactioncan be expressed as follows: AB A B. Most decomposition reactions require the useof heat, light, or electricity.

Classifying Chemical Reactions3Single Displacement When one element replaces anotherelement in a compound, it is called asingle-displacement reaction. Single-displacement reactions are describedby the general equation A BC AC B. Here you can see that atom A displacesatom B to produce a new molecule AC,a single displacement reaction.

Classifying Chemical Reactions3The Activity Series We can predict which metal will replaceanother using the diagram shown whichlists metalsaccording tohow reactivethey are. A metal willreplace anyless activemetal.

Classifying Chemical Reactions3Double Displacement In a double-displacement reaction, thepositive ion of one compound replaces thepositive ion of the other to form two newcompounds. A double displacement reaction takesplace if a precipitate, water, or a gas formswhen two ionic compounds in solution arecombined.

Classifying Chemical Reactions3Double Displacement A precipitate is an insoluble compoundthat comes out of solution during this typeof reaction. The generalized formula for this type ofreaction is as follows: AB CD AD CB.

Classifying Chemical Reactions3Oxidation-Reduction Reactions One characteristic that is common tomany chemical reactions is the tendencyof the substances to lose or gain electrons. Chemists use the term oxidation todescribe the loss of electrons and the termreduction to describe the gain ofelectrons.

Classifying Chemical Reactions3Oxidation-Reduction Reactions Chemical reactions involving electron transferof this sort often involve oxygen, which isvery reactive, pulling electrons from metallicelements. Corrosion ofmetal is a visibleresult.

Classifying Chemical Reactions3Oxidation-Reduction Reactions The substance that gains an electron orelectrons obviously becomes morenegative, so we say it is reduced. On the other hand, the substance that losesan electron or electrons then becomesmore positive, and we say it is oxidized.

Classifying Chemical Reactions3Oxidation-Reduction Reactions The electrons that were pulled from oneatom were gained by another atom in achemical reaction called reduction. Reduction is the partner to oxidation; thetwo always work as a pair, which iscommonly referred to as redox.

Section Check3Question 1In a reaction, two or moresubstances combine to form ompositionsynthesis

Section Check3AnswerThe answer is D. The generalized formulafor synthesis reactions is A B AB.

Section Check3Question 2The opposite of a synthesis reaction is areaction.A.B.C.D.combustiondouble displacementdecompositionsingle displacement

Section Check3AnswerThe answer is C. A decomposition reactionoccurs when one substance breaks down intotwo or more substances.

Section Check3Question 3The reaction in which the positive ion of onecompound replaces the positive ion of the otherto form two new compounds is thereaction.A.B.C.D.combustiondouble displacementdecompositionsingle displacement

Section Check3AnswerThe answer is B. A double displacementtakes place if a precipitate, water, or a gasform when two ionic compounds in solutionare combined.

Chemical Reactions and Energy4Chemical Reactions—EnergyExchanges A dynamic explosion isan example of a rapidchemical reaction. Most chemical reactionsproceed more slowly, butall chemical reactionsrelease or absorb energy.

Chemical Reactions and Energy4Chemical Reactions—EnergyExchanges This energy can take many forms, such asheat, light, sound, or electricity. Chemical bonds are the source of this energy.