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: What changes take place 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: Determine reactants & products 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 DHr = + Exothermic – a rxn in which energy is released Products are lower in energy than reactants DHr = -

9.5 Energy & Chemical Change All rxns require a certain minimum amt of energy to get started - Activation Energy