Chemical Reactions Chapter 11

What Is a Reaction?  It is only a reaction if a new substance has been produced.  Four clues a reaction has happened: 1.Formation of a gas (bubbles) 2.Change of color 3.Heat change (hotter or colder) 4.Precipitate (solid formed when two liquids meet)

Reaction Equations  Reactants (before rxn) → Products(after rxn)  CH 4 + O 2 → CO 2 + H 2 O  Methane reacts with oxygen to produce carbon dioxide and water  N 2 + H 2 → NH 3

Reaction Equations  If a catalyst was used in the reaction, the equation will show that: N 2 + H 2 → NH 3  If heat was required to make the reaction happen, the equation will show that:  N 2 + H 2 → NH 3

Phases in Equations  Solids-denoted by a subscript (s)  Liquids-denoted by a subscript (l)  Gases-denoted by a subscript (g)  Substance in solution-denoted by subscript (aq)  Example:  Zn (s) + HCl (aq) → ZnCl (aq) + H 2(g)  (aq) is necessary because ionic compounds IONIZE in water. Explanation:

Diatomic Molecules  All elements exist in nature as single atoms, except seven:  bromine, iodine, nitrogen, chlorine, hydrogen, oxygen, and fluorine.  These exist as diatomic molecules, or two- atom molecules (Br 2, H 2, O 2, I 2, Cl 2, N 2, F 2 )  How can I possibly remember them???  BrINClHOF

Balancing Equations  The Law of Conservation of Mass forces chemical equations to be balanced. You can not have atoms disappear, and you can’t form new substances out of thin air.  The total mass of reactants MUST equal the total mass of products, even though they are not the same substances.

 N 2 + H 2 → NH 3

Types of Reactions  Some kinds of reactions happen often enough that we have noticed categories.

Decomposition  AB → A + B  H 2 O 2(aq) →  C 6 H 12 O 6(s) →

Single Replacement  A + BX → AX + B  Al (s) + CuCl 2(aq) →  Cu (s) + AgNO 3(aq) →

Double Replacement  AX + BY → AY + BX  Pb(NO 3 ) 2(aq) + KI (aq) →  AgNO 3(aq) + NaCl (aq) →

Direct Combination/Synthesis  A + B → AB  Mg (s) + O 2(g) →  H 2(g) + O 2(g) →

Combustion  Just a fancy word for burning a substance. If you are burning something, what substance do you need besides the one being burnt?  _____ + O 2(g) → CO 2(g) + H 2 O (g)

How to Write an Eqn. “from scratch”  Magnesium reacts with hydrochloric acid in a single replacement rxn.  1. Write the formulas for your reactants.(ion list)  2. Determine what your products are, and write the formulas for those (ion list again).  3. Balance the equation LAST.

Predicting Products 1.Determine the formulas for your reactants (use ion list, if necessary). 2.Based on the type of reaction, determine the name of your product(s). 3.From those names, determine the formula for your product(s) (use ion list, if necessary). 4.Balance the equation.

Ionic Equations  Reminder: the (aq) designation on a formula says that not only does the substance dissolve, it IONIZES.  NaI (s) → Na + (aq) + I - (aq)  Or: 2NaI (s) → 2Na + (aq) + 2I - (aq)  Or: Pb(NO 3 ) 2(s) → Pb 2+ (aq) + 2NO 3 - (aq)  Or: Al 2 (SO 4 ) 3(s) → 2Al 3+ (aq) + 3SO 4 2- (aq)

2NaI (aq) + Pb(NO 3 ) 2(aq) → 2NaNO 3(aq) + PbI 2(s) Let’s backtrack and put the reactants in water:  2NaI (s) → 2Na + (aq) + 2I - (aq)  Pb(NO 3 ) 2(s) → Pb 2+ (aq) + 2NO 3 - (aq) Then what your reactants actually look like is this: 2Na + (aq) + 2I - (aq) + Pb 2+ (aq) + 2NO 3 - (aq)

 …and what your products actually look like is this:  → 2Na + (aq) + 2NO 3 - (aq) + PbI 2(s)  So, in terms of ions, the reaction actually goes like this: 2Na + (aq) + 2I - (aq) + Pb 2+ (aq) + 2NO 3 - (aq) →2Na + (aq) + 2NO 3 - (aq) + PbI 2(s)  Cancel the items that appear identical on both sides 2Na + (aq) + 2I - (aq) + Pb 2+ (aq) + 2NO 3 - (aq) →2Na + (aq) + 2NO 3 - (aq) + PbI 2(s) So, in actuality, what happens is only this: Pb 2+ (aq) + 2I - (aq) → PbI 2(s) Pb 2+ (aq) + 2I - (aq) → PbI 2(s)

Ionic Equations  Here’s another one:  2NaCl (aq) + Ba(NO 3 ) 2(aq) → 2NaNO 3(aq) + BaCl 2(aq)  Well, in terms of ions, it looks like this:  2Na + (aq) + 2Cl - (aq) + Ba 2+ (aq) + 2NO 3 - (aq) →2Na + (aq) 2NO 3 - (aq) + Ba 2+ (aq) + 2Cl - (aq)  Well, if you cancel what appears identical on both sides, what are you left with?  This reaction has no net ionic equation, because, in effect, nothing has happened.  The items that are your reactants have remained unchanged after the reaction, so no reaction has actually occurred.