Types of Chemical Reactions

Chemical Reactions

Effects of chemical reactions:Chemical reactions rearrange atoms in the reactants to form new products.The identities and properties of the products are completely different from that of the reactants.Production of gases and color changes are signs of chemical reactions.

Energy and ReactionsEnergy must be ADDED to BREAK bonds.

Energy is RELEASED when bonds are FORMED.

Chemical energy is CONSERVED in chemical reactions.

Exo- vs. Endo-EXOTHERMIC REACTIONS: release energy (More energy is released as the products form bonds than is absorbed to break the bonds in the reactants.)

ENDOTHERMIC REACTIONS: absorb energyChemical EquationsChemical equations are used to represent or describe chemical reactions. For example when hydrogen H2 burns, it reacts with oxygen, O2, in the air to form water. We write the chemical equation for this reaction as follows:2H2 + O2 > 2H2OChemical EquationsAn equation shows Formulas of reactantsFormulas of productsMolar ratios of all compounds in the reaction.Chemical EquationsWe read the (+) sign as reacts with and the arrow (>) as produces or yields. 2H2 + O2 > 2H2OReactantsProductsTo show physical states of each substance:(s) or - solid (l) - liquid(g) or - gas(aq) - aqueous aqueous means dissolved in waterTo show physical states of each substance:Consider the reaction of iron with oxygen to form iron (III) oxide, or rust.

Fe(s) + O2(g) Fe2O3(s)(unbalanced)Coefficients & SubscriptsCOEFFICIENTS: numbers in front of compound that represents the number of molecules/moles of that compoundSUBSCRIPTS: small numbers that help define the compound. 2H2SO4

CoefficientSubscriptH2OOne molecule of water

2H2OTwo molecules of water

H2O2One molecule of Hydrogen PeroxideDuring a chem. rxn.; atoms are rearranged (NOT created or destroyed!)Chemical equations must be balanced to show the relative amounts of all substances.Balanced means: each side of the equations has the same # of atoms of each element.

CH4 + O2 > H2O + CO2UnbalancedCH4 + 2O2 > 2H2O + CO2Balanced

In order to balanceWrite correct formulas for all reactants and productsReactants ProductsCount the number of atoms of each element in reactants & products.Balance one at a time using coefficients.Check for balanceAre the coefficients in the lowest possible ratio?

Balancing EquationsNOTE: When balancing equations, you may change coefficients as much as you need to, but you may never change subscripts because you cant change what substances are involved.Fe(s) + O2(g) Fe2O3(s)(unbalanced)

4Fe(s) + 3O2(g) 2Fe2O3(s)(balanced)Sample Problem 1Water is decomposed (broken down) to form the gaseous products hydrogen, H2, and oxygen, O2. Write the balanced equation for this reaction.H2O H2 + O22H + 1O 2H + 2O O is not balanced2H2O 2H2 + O24H + 2O 4H + 2O The equation is balanced!Sample Problem 2Chlorine gas, Cl2, reacts with potassium bromide, KBr, to form potassium chloride and bromine, Br2. Write the balanced equation for this reaction,Cl2 + KBr KCl + Br22Cl + 1K + 1Br 1Cl + 1K +2Br Cl and Br are not balancedCl2 + 2KBr 2KCl + Br22Cl + 2K + 2Br 2Cl + 2K +2Br The equation is balanced!Balancing equations involves a great deal of trial and error at first, but there are some tricksFor example..Sodium metal reacts with water to produce sodium hydroxide and hydrogen gas.

Na + H2O > NaOH + H2Note that on the product side (right side) there are an odd number of hydrogens. On the reactant side (left side) there is an even number. This implies there must be an even coefficient in front of the NaOH. Lets start with 2_Na + _H2O > 2NaOH + _H2

Now lets balance sodium; we need a 2 in front of the Na2Na + _H2O > 2NaOH + _H2

Now consider hydrogen2Na + 2H2O > 2NaOH + H22Na + 2H2O > 2NaOH + H2

Check to see if it balances

2 Na on the left2 Na on the right4 hydrogen 2 + 2 = 4 hydrogen2 oxygen2 oxygenthe equation is balanced.

ExamplesCuCl2(aq) + Al(s) Cu(s) +AlCl3(aq)

3CuCl2(aq) + 2Al(s) 3Cu(s) +2AlCl3(aq)

(3:2:3:2)

ExamplesPropane, C3H8, burns in oxygen, O2, to form carbon dioxide and water.

C3H8 + O2 CO2 + H2O

Balance C then H then O

C3H8 + 5O2 3CO2 + 4H2O

(1:5:3:4)

ExamplesPentane, C5H12, burns in oxygen, O2, to form carbon dioxide and water.

C5H12 + O2 CO2 + H2O

Balance C then H then O

C5H12 + 8O2 5CO2 + 6H2O

(1:8:5:6)

ExamplesSilver nitrate reacts with copper to produce silver and copper (II) nitrate.

AgNO3 + Cu Ag + Cu(NO3)2

2AgNO3 + Cu 2Ag + Cu(NO3)2

(2:1:2:1)

ExamplesPhosphorus reacts with oxygen gas to produce diphosphorus pentoxide.

P + O2 P2O5

4P + 5O2 2P2O5

(4:5:2)

ExamplesC7H14 + O2 CO2 + H2O

Balance C then H then O

C7H14 + 10O2 7CO2 + 7H2O

2C7H14 + 21O2 14CO2 + 14H2O

(2:21:14:14)

Types of ReactionsTypes of Chemical ReactionsSynthesis / Combination DecompositionSingle ReplacementDouble ReplacementCombustionSynthesis / Combination ReactionsDefinition: Reaction where two or more substances react to form a single substance.A + B ABExamples:2K(s) + Cl2(g) 2KCl(s)SO2(g) + H2O(l) H2SO3(aq)

Decomposition ReactionsDefinition: Reaction where a single compound is broken down into two or more products.AB A + BExamples:2H2O(l) 2H2(g) + O2(g) CaCO3 CaO + CO2

Single-Replacement ReactionsDefinition: Reaction where atoms of one element replace atoms of a second element in a compound.XA + B BA + XNote: A reactive metal will replace any metal listed below it in the activity series. Generally, nonmetal replacement is limited to the halogens. The activity of the halogens decreases as you go down Group 7A of the periodic table. See handout.Examples:2AgNO3 + Mg Mg(NO3)2+2AgMg+LiNO3 no reaction

LiKCaNaMgAlZnFePb(H)*CuHgAg

Increasing ActivityAny element will replace any element below it.Activity Series*Metals from Li to Na will replace H from acids and water; from Mg to Pb they will replace H from acids onlyFor ExampleCa + MgO CaO + MgThe Ca will replace the Mg because Ca is more active than Mg. That is to sayCa is above Mg on the activity list.34ForDouble-Replacement ReactionsDefinition: Reaction that involves an exchange of positive ions between two compounds.XA + BY BA + XYNote: These reactions generally take place between two ionic compounds in aqueous solution, and are often characterized by one of the products coming out of solution in some way. Examples:2NaCN(aq)+H2SO4(aq) 2HCN(g)+Na2SO4(aq)Na2S(aq)+Cd(NO3)2(aq) CdS(s)+2NaNO3(aq)Combustion ReactionsDefinition: Reaction where an element or a compound reacts with oxygen, often producing energy in the form of heat and light.Examples:CH4+2O2 CO2+2H2O + heat + light2Mg(s)+O2(g) 2MgO(s)

Combustion of HydrocarbonsIf the reactant is a hydrocarbon, the products are always carbon dioxide and water.

CH4 + 2O2 CO2 + 2H2OIonic EquationsWhen a soluble substance is dissolved in water, the substance often breaks into ions. This solution is said to be an aqueous solution.

Pb(NO3)2(aq) Pb2+ + 2NO3-NaI(aq) Na+ + I-

Ionic EquationsConsider the reactionPb(NO3)2(aq) + NaI(aq) PbI2(s) + NaNO3(aq)

What is really going on isPb2+ + NO3- + Na+ + I- PbI2(s) + Na+ + NO3-

Note that the Na+ ion and the NO3- ion are not reacting. They are said to be spectator ions.

Net Ionic EquationsIt is often useful to write an equation showing only the species that are actually reacting. This is called a net ionic equation. It does not show the spectator ions.Pb2+ + NO3- + Na+ + 2I- PbI2(s) + Na+ + NO3-becomes.Pb2+ + 2I- PbI2(s)

From the types of rxns wkst