Ch. 7: Chemical Reactions Dr. Namphol Sinkaset Chem 152: Introduction to General Chemistry

I. Chapter Outline Introduction Evidence of a Chemical Reaction Balanced Chemical Equations Aqueous Solutions Precipitation Reactions Acid/Base and Gas Forming Rxns Oxidation-Reduction Reactions Classifying Reactions

I. Chemical Reactions Everything around us is powered by chemical reactions. Cars, laundry, processes in our body. Chemical reactions produce changes in how atoms are arranged in matter.

I. So Many Reactions There are countless reactions, but only a few categories of reactions. Two ways to classify: (1) based on chemistry that occurs or (2) based on what atoms do. With experience, it becomes easier to identify what will happen in a reaction.

II. Evidence of a Reaction If we could see individual atoms, it would be easy to tell a reaction occurred. We can only see what happens macroscopically and use that as an indication that a reaction might have occurred. Note that only chemical analysis can conclusively prove a reaction happened.

II. Examples of Evidence

II. Examples of Evidence

II. Summary of Types of Evidence

III. Representing Reactions We use chemical equations to describe what happened in a reaction. We place reactants (chemicals we started with) on the left and products (chemical we end up with) on the right. The physical state of each substance is indicated with a symbol.

III. Indicating Physical State

III. Writing an Equation Example reaction: Methane (CH4) reacts with molecular oxygen on a gas stove to form carbon dioxide and water. Translated to formulas: CH4(g) + O2(g)  CO2(g) + H2O(g). Something’s not right…

III. We Can’t Create Atoms!

III. We Can’t Destroy Atoms!

III. Balanced Equations A balanced equation has the same number of each type of atom on both sides. Equations must balance to comply with the Law of Mass Conservation. When balancing, only coefficients can be added/changed, not subscripts!

III. Balanced Methane Reaction

III. Checking Your Work

III. Balancing Equations Guidelines for balancing: Translate into formulas, if necessary. Reactants on left, products on right. Balance atoms one at a time. Start w/ atoms that are in only one compound on both sides of the equation. If more than one, balance metals before nonmetals. Balance single elements at the end. Can ONLY change coefficients, not subscripts!! Adjust to smallest whole numbers, if necessary. Check work! Specify states.

III. Sample Problem Balance the following. Solid calcium carbonate reacts with nitric acid to form carbon dioxide gas, liquid water, and aqueous calcium nitrate. PCl3 + HF  PF3 + HCl C4H10(g) + O2(g)  CO2(g) + H2O(g)

IV. Aqueous Solutions Being the most widely available liquid, much of reaction chemistry is water based. An aqueous solution is a homogeneous mixture of a substance with water. When ionic compounds like NaCl are added to water, they dissociate into their constituent ions.

IV. NaCl in Water

IV. AgNO3 in Water

IV. Electrolytes Compounds that dissociate completely in water and lead to electrical conductivity are called strong electrolytes. Electrical conductivity is the result of freely-moving charged particles. In a strong electrolyte solution, the ions are free to move through solution. Electrons don’t swim!

IV. Solution Conductivity

V. Insolubility Some ionic compounds do not dissociate when in water. These compounds are described as insoluble. Soluble ionics dissociate in water.

V. Precipitation Reactions The formation of a precipitate (ppt) is a strong driving force for a reaction. Precipitate is a fancy word for solid. Ppt formation is predicted using solubility rules.

V. Solubility Rules Na+, Li+, K+, NH4+ salts are soluble. NO3-, CH3COO-, ClO4- salts are soluble. Ag+, Pb2+, Hg22+ salts are insoluble. Cl-, Br-, I- salts are soluble. CO32-, S2-, O2-, OH- salts are insoluble. SO42- salts are soluble except for CaSO4,SrSO4, and BaSO4. If none of these apply, it’s insoluble.

V. Predicting Precipitates Does a precipitate form when aqueous solutions of KI and Pb(NO3)2 are combined?

V. Predicting Precipitates When the solutions are initially combined, the ions move around independently. But they run into each other and could form a ppt.

V. Predicting Precipitates You consider new combinations of cation/anion then use the solubility rules to see if any would be insoluble (which means it would form a precipitate).

V. Predicting Precipitates In this example, Rules 1 and 3 apply and predict lead(II) iodide precipitates. NOTE: if both new combinations are soluble, then there’s NO REACTION.

V. Sample Problem Predict the precipitates in the following aqueous reactions. sodium hydroxide + cadmium(II) nitrate magnesium bromide + potassium acetate ammonium sulfate + barium chloride sodium iodide + lead(II) nitrate

V. Writing Reactions No spectator ions

V. Sample Problem Write balanced molecular, total ionic, and net ionic equations for the reaction between magnesium sulfate and calcium sulfide.

VI. Acids and Bases Acids are ionic compounds that have H+ as the cation. Acids have a sour taste and dissolve some metals. Bases are ionic compounds that have OH- as the anion. Bases have a bitter taste and a slippery feel.

VI. Common Acids/Bases

VI. Neutralization Reactions In an acid/base reaction, the H+ from the acid combines with the OH- from the base to form water. The general form of the molecular equation is: Acid + Base  Water + Salt. Salt is a general term for “ionic compound.”

VI. Sample Problem Write balanced molecular, total ionic, and net ionic equations for the reaction between nitric acid and calcium hydroxide.

VI. Gas Evolution Reactions In some reactions, a new combo of a cation and an anion leads to gas formation.

VI. Decomposition Into Gases Some gas formation reactions involve the decomposition of an intermediate.

VI. Common Gas Evolution Products

VII. Reactions w/ e- in Motion Reactions involving transfer of electrons are called oxidation-reduction or redox reactions. Redox reactions occur if one of the following applies: A substance reacts with molecular oxygen. A metal reacts with a nonmetal. One substance transfers electrons to another substance.

VII. Combustion Reactions A combustion reaction is a type of redox reaction. In these reactions, a substance reacts with O2 to form one or more oxygen-containing compounds. For compounds containing C and H or C, H, and O, the products are carbond dioxide and water.

VIII. Classifying Reactions We have been classifying reactions based on what happens chemically. Another way to classify is based on what atoms or groups of atoms do in a reaction. There are 4 classes: (1) synthesis or combination, (2) decomposition, (3) single displacement, (4) double displacement.

VIII. Synthesis or Combination Simpler substances combine to form a more complex substance. General equation: A + B  AB.

VIII. Decomposition Complex substance breaks down into simpler substances. General equation: AB  A + B.

VIII. Single Displacement One element takes the place of another element in a compound. General equation: A + BC  AC + B. Example: Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g).

VIII. Double Displacement Also known as metathesis. Two elements or groups of elements in two different compounds exchange places to form new compounds. General equation: AB + CD  AD + CB. Example: 2NaCl(aq) + Pb(NO3)2(aq)  PbCl2(s) + 2NaNO3(aq)