Honors Chemistry Chapter 9 Chemical Equations

9.1 Representing Chemical Changes

� Chemical Reaction – the process by which 1 or more substs are changes into 1 or more diff substs

� Correct equation shows: 1.  What changes take place 2.  Relative amts of various elems & comps that

take part in these changes

9.1 Representing Chemical Changes � Reactants – starting substs ina chemical

rxn � Products – substs formed by the chemical

rxn � 2C2H2(g) + 5O2(g) 4CO2(g) + 2H2O(l) ◦ Reactants yield products

� Letters in ( ) indicate physical state of ea subst involved ◦  (g) – gas, (l) – liquid, (s) or (cr) – solid

(crystalline), (aq) – aqueous or water soln

9.2 Balancing Equations �  To write a correct chemical eqn:

1.  Determine reactants & products 2.  Write reactants on left side connected w/ a +

an write products on right side �  Connect 2 sides w/ an arrow showing direction of rxn

�  SYMBOLS & FORMULAS MUST BE WRITTEN CORRECTLY 3. Balance eqn - show an = # of atoms of ea elem on

both sides of the eqn

9.2 Balancing Equations

3. Balance eqn ◦  show an = #of atoms of ea elem on both

sides of the eqn

�  In a chem rxn, mass is not lost or gained ◦  Same amt of matter before & after rxn ◦  Same # & kind of atoms must be present on

both sides of an eqn

9.2 Balancing Equations

� Example: ◦ C3H8 + O2 CO2 + H2O ◦ C3H8 + 5 O2 3CO2 + 4H2O

� Balance by changing only coefficients, never subscripts!

9.3 Classifying Chemical Changes

� 6 General Types of Rxns �  Single Displacement Rxn ◦  1 elem displaces another in a compound ◦  Ex) Cl2 + 2KBr 2KCl + Br2 ◦  Ex) 2Al + Fe2O3 Al2O3 + 2Fe ◦ General Form: �  elem + comp elem + comp �  A + BX B + AX �  X + AY Y + AX

9.3 Classifying Chemical Changes

� Double Displacement Rxn (Ionic Rxn) ◦ The (+) and (-) ions of 2 comps are

interchanged ◦  Ex) PbCl2 + Li2SO4 2LiCl + PbSO4

◦  Ex) ZnBr2 + 2AgNO3 Zn(NO3)2 + 2AgBr: ◦  General Form: �  Comp + comp comp + comp Or �  AX + BY AY + BX

9.3 Classifying Chemical Changes

� Neutralization ◦ A special type of double displ rxn ◦ Acid + Base Salt (ionic comp) + water �  Strong acid start w/ H, bases end in OH �  Ex) HBr + NaOH NaBr + HOH �  Ex) Ba(OH)2 + 2HNO3 Ba(NO3)2 + 2HOH

9.3 Classifying Chemical Changes

� Decomposition ◦  Subst breaks up into simpler substs when

energy is supplied �  Ex) CdCO3 CdO + CO2 �  Ex) Pb(OH)2 PbO + H2O

◦ General Form: �  Comp 2 or more elems or comps �  AB A + B

◦  Energy may be heat, light, electricity, or mechanical shock

9.3 Classifying Chemical Changes

�  Synthesis ◦  2 or more substs combine to form 1 new

subst (sometimes called composition rxn) �  Ex) NH3 + HCl NH4Cl �  Ex) 2H2 + O2 2H2O

◦ General Form: �  Elem or comp. + elem or comp comp �  A + B AB

9.3 Classifying Chemical Changes

� Combustion (of hydrocarbons) ◦ Not usually considered one of the major

types of rxns �  Most organic comps will burn in air

� Many are very flammable

�  When a comp burns in air, it is really reacting w/ oxygen in air � Combustion rxns are referred to as oxidation rxns

�  Products of the combustion of a hydrocarbon under normal conditions are CO2(g) & H2O(g)

9.3 Classifying Chemical Changes

� Combustion (cont) ◦  Ex) Methane (CH4) burns in air �  CH4 + 2O2 CO2 + 2H2O

◦  Ex) Butane (C4H10) burns in air �  2C4H10 + 13O2 8CO2 + 10H2O

◦ General Form: �  Hydrocarbon + oxygen CO2 + H2O

9.4 Mass-Mass Relationships

� Balanced eqns represent the relationship betw the # of particles that react & the # of particles produced ◦ There is also a relationship betw reacting

masses & masses of products

� The coefficients of a balanced eqn give the relative amts in moles of reactants & products.

Mass-Mass Relationships

� To solve mass-mass problems: ◦  1. Write a balanced eqn �  Show rxn that is taking place

◦  2. Start w/ what your given & change to moles of given subst ◦  3. Using ratio of moles in the eqn, go to mole

of required subst (what your asked for) ◦  4. Change from moles of required subst to

grams ◦ Work out prob (checking units)

9.5 Energy & Chemical Change

� Chemical changes are always accompanied by a change in energy ◦  Endothermic – a rxn in which energy is

absorbed �  Products are higher in energy than reactants �  ΔHr = +

◦  Exothermic – a rxn in which energy is released �  Products are lower in energy than reactants �  ΔHr = -

9.5 Energy & Chemical Change

� All rxns require a certain minimum amt of energy to get started ◦  - Activation Energy