Chapter 23 Chemical Reactions

23.1 Forming New Substances Chemical Reaction (Rxn): process in which one or more substances change to make one or more new substances Chemical and physical properties of the new substance differ from those of the original The combustion of fuel is an example of a chemical reaction

What are some signs of a chemical rxn? Gas Formation

What are some signs of a chemical rxn? Formation of a precipitate, or a solid in a solution

What are some signs of a chemical rxn? Energy is released

What are some signs of a chemical rxn? Color Change

Chemical Bonds Force that holds two atoms together in a molecule. For a chemical rxn to take place, bonds must be broken and rearranged. If molecules bump into each other with enough energy, bonds will break and atoms will be rearranged.

23.2 Chemical Formulas and Equations Chemical formulas use element symbols and subscripts to represent the number of atoms in a substance. Chemical equations use formulas to describe a chemical rxn Reactants = starting material Products = ending material

Structure of an Equation Reactant(s)  Product (s)  means “yields”

Law of Conservation of Mass Antoine Lavoisier - 1774 Matter is not created or destroyed during a chemical reaction Nice hair!

Balancing Chemical Equations The number of atoms on both sides of the chemical equation must be the same. We must balance a chemical reaction to represent the law of conservation of mass.

Steps for balancing equations Write the equation Count the atoms Use Coefficients to balance atoms (do not use the number 1 – if there is no coefficient the “1” is implied) Note: Because of the Law of Definite Proportions, we cannot change subscripts without changing the type of chemical compound. Check your work. Make sure the LCF’s are used as coefficients.

Practice Balancing Equations 1)____ AlBr3 + ____ K  ____ KBr + ___ Al 2)___ FeO + ___ PdF2  ___ FeF2 + __ PdO 3)____ P4 + ____ Br2  ____ PBr3   4)__ LiCl + ___ Br2  __ LiBr + ____ Cl2  5)__ PbBr2 + __ HCl  __ HBr +_ PbCl2 6) __ CoBr3 + ___ CaSO4  __ CaBr2 + __ Co2(SO4)3 7) ____ Na3P + ____ CaF2  ____ NaF + ____ Ca3P2  8) ____ Mn + ____ HI  ____ H2 + ____ MnI3 9) ____ Li3PO4 + ____ NaBr  ____ Na3PO4 + ____ LiBr 10)____ CaF2 + ____ Li2SO4  ____ CaSO4 + ____ LiF

Answers 1) 1 AlBr3 + 3 K  3 KBr + 1 Al 2) 1 FeO + 1 PdF2  1 FeF2 + 1 PdO 3) 1 P4 + 6 Br2  4 PBr3 4) 2 LiCl + 1 Br2  2 LiBr + 1 Cl2 5) 1 PbBr2 + 2 HCl  2 HBr + 1 PbCl2   6) 2 CoBr3 + 3 CaSO4  3 CaBr2 + 1 Co2(SO4)3 7) 2 Na3P + 3 CaF2  6 NaF + 1 Ca3P2 8) 2 Mn + 6 HI  3 H2 + 2 MnI3 9) 1 Li3PO4 + 3 NaBr  1 Na3PO4 + 3 LiBr 10) 1 CaF2 + 1 Li2SO4  1 CaSO4 + 2 LiF

23.3 Types of Chemical Rxns Synthesis: when 2 or more elements combine to form a more complex substance A + B  AB

Types of Chemical Rxns Decomposition: breaks down compounds into simpler substances AB  A + B

Types of Chemical Rxns Single Displacement (or single-replacement): When one element replaces another element in a compound A + BC  AC + B

Reactivity of Elements In a single-displacement rxn, a more reactive element can displace a less-reactive element in a compound.

Types of Chemical Rxns Double Displacement (double replacement): When elements in one compound “trade places” with elements in another compound AB + CD  AD + CB

Classify the following: MgCO3 + 2HCl  MgCl2 + H2CO3 2H2O  2H2 + O2 4Fe + 3O2  2Fe2O3 2Na + 2H2O  2NaOH + H2 2KCl + Pb(NO3)2  2KNO3 + PbCl2 H2CO3  H2O + CO2 2Na + Cl2  2NaCl Zn + 2HCl  ZnCl2 + H2

Answers Double Replacement Decomposition Synthesis Single Replacement

23.4 Energy and Rates of Chemical Rxns

Energy and Reactions

Energy and Reactions

Energy and Reactions

Energy and Reactions All chemical reactions need energy to get started. The activation energy (Ea) is the minimum energy needed. Sources of Ea: Friction Spark Light Heat

Energy and Reactions Endothermic Rxns Need Ea to start Absorb E E of products > reactants Exothermic Rxns Need Ea to start Release E E of products < reactants

Endothermic Rxn

Exothermic Reaction

Rates of Chemical Rxns Factors that can alter the rate (speed) of a reaction: Surface Area of Reactants Temperature Concentration of Reactants Catalysts and Inhibitors

If you dice the potatoes up smaller, they cook faster! Surface Area If you dice the potatoes up smaller, they cook faster! If the reactants are more exposed to each other, the reaction happens faster!

Temperature An increase in temperature causes an increase in kinetic energy, leading to more interaction of reactants and therefore a faster rate.

Concentration of Reactants

Catalysts and Inhibitors Catalysts speed up reactions A biological catalyst is called an enzyme Inhibitors slow down reactions