Reactions in Aqueous Solutions Chapter 8

Predicting Whether a Reaction Will Occur  Seem to be several changes that will cause a reaction to occur  Formation of a solid  Formation of water  Transfer of electrons  Formation of a gas

Reactions in Which a Solid Forms  Formation of a solid is called Precipitation  Solid called Precipitate  Reaction called Precipitation Reaction  Formation of solid tells you a chemical reaction took place

What Happens When an Ionic Compound Dissolves in Water?  When a solid containing ions is dissolved in water, the ions separate  Call this Dissociation  Ba(NO 3 ) 2(aq) contains no Ba(NO 3 ) 2(aq) units but instead is Ba 2+ and NO 3 - ions  There are 2 NO 3 - ions for every Ba 2+ ion  Know it separates b/c conducts electricity  When all ions separate called a strong electrolyte

What can we do with this info?  If we know that ionic compounds break up in solution can make predictions of what happens when 2 solutions dissolve  K 2 CrO 4(aq) + Ba(NO 3 ) 2(aq) forms a yellow solid

So what is the product?  Now that we know reactants can guess products  Know a solid compound must have a neutral charge so anions must combine with cations

 Also know (well you do now) that most ionic materials contain only 2 types of ions  Below are some examples

 So with this info there are only a few possible combinations

 Know it won’t be K 2 CrO 4 and Ba(NO 3 ) 2 because they were the reactants so this leaves KNO 3 and BaCrO 4  A chemist knows that KNO 3 is a white solid and CrO 4 is yellow so it must be this  Where are K + and NO 3 - ions?  They are left in solution, so it is KNO 3(aq)

 So our reaction reads K 2 CrO 4(aq) + Ba(NO 3 ) 2(aq) → BaCrO 4(s) + KNO 3(aq) Which looks like this

How were we able to solve this?  We used 2 types of knowledge  Knowledge of facts  Color of compounds  Knowledge of concepts  Solids are always neutral in charge

Using Solubility Rules (or When will something dissolve)  Soluble Solid – easily dissolves in water  Insoluble solid and slightly soluble solid – tiny amount dissolves in water  You will see that the terms “salt” and “ionic compound” mean the same thing  Some info you will need is on following slides

8.3 Describing Reactions in Aqueous Solutions  K 2 CrO 4(aq) + Ba(NO 3 ) 2(aq) → BaCrO 4(s) + KNO 3(aq)  This is a “molecular equation” (shows complete formulas)  Doesn’t give a very clear picture of what happened  The “complete ionic equation” does  2K + (aq) + CrO 4 2- (aq) + Ba 2+ (aq) + 2NO 3 - (aq) → BaCrO 4(s) + 2K + (aq) + 2NO 3 - (aq)  In a complete ionic equation all strong electrolytes are shown as ions  Shows which ions participate in reaction

 Spectator ions like 2K + (aq) and 2NO 3 - (aq) don’t participate  They are there as reactants and products  When only those ions involved are shown it is called a Net Ionic Equation  Ba 2+ (aq) + CrO 4 2- (aq) → BaCrO 4(s)

So there are 3 types of Equations for Reactions in Aqueous Solutions  Molecular Equations  Complete Ionic Equations  Net Ionic Equations

Reactions That Form Water: Acids and Bases  Acids (Definition from Arrhenius)  Produces H+ ions when dissolved in water (a proton)  HCl → H + (aq) + Cl - (aq)  HNO3 → H + (aq) + NO 3 - (aq)  If every molecule dissociates into ions, called a “strong acid”

 Bases (Definition from Arrhenius)  Produces OH- ions when dissolved in water  NaOH (s) → Na + (aq) + OH - (aq)  KOH(s) → K + (aq) + OH - (aq)  If every molecule dissociates into ions, called a “strong base”

Acids + Bases = Neutralization  When acids and bases combine the reaction is called neutralization  HCl → H + (aq) + Cl - (aq) (ACID)  NaOH (s) → Na + (aq) + OH - (aq) (BASE) H + (aq) + Cl - (aq) + Na + (aq) + OH - (aq) →H 2 O (l) + Na + (aq) + Cl - (aq)  So an Acid + Base yields Water and a Salt  The Acid and Base have been neutralized  The last equation is a Complete Ionic Equation  Write a Net Ionic Equation for the above reaction  _________________________________________

8.5 Reactions of Metals with Nonmetals (Oxidation-Reduction)  Remember we said atoms will sometimes gain or lose electrons to become ions  A reaction that involves a transfer of electrons is called an Oxidation-Reduction Reaction (Redox reactions)  2Mg (s) + O 2(g) → 2MgO (s)  Mg gives up 2e- and O 2 accepts 2e-  Mg → Mg e-  O + 2e- → O 2-  LEOGER = lose e- oxidation; gain e- reduction  OILRIG = oxidation is lose; reduction is gain

8.6 Ways to Classify Reactions  Precipitation Reaction  Whenever a precipitate forms  Acid-Base Reaction  Involves H+ ion ending up in H 2 O  Combustion Reactions  Any reaction where something combines with Oxygen  Synthesis Reaction  A + B → AB  Decomposition Reaction  AB → A + B  Single-Displacement Reaction  AB + C → AC + B  Double-Displacement Reaction  AB + CD → AD + CB