Presentation on theme: "Ch. 7: Chemical Reactions"— Presentation transcript:
1 Ch. 7: Chemical Reactions Dr. Namphol SinkasetChem 152: Introduction to General Chemistry
2 I. Chapter Outline Introduction Evidence of a Chemical Reaction Balanced Chemical EquationsAqueous SolutionsPrecipitation ReactionsAcid/Base and Gas Forming RxnsOxidation-Reduction ReactionsClassifying Reactions
3 I. Chemical ReactionsEverything around us is powered by chemical reactions.Cars, laundry, processes in our body.Chemical reactions produce changes in how atoms are arranged in matter.
4 I. So Many ReactionsThere 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.
5 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.
9 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.
11 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…
14 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!
17 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.
18 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 + HClC4H10(g) + O2(g) CO2(g) + H2O(g)
19 IV. Aqueous SolutionsBeing 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.
22 IV. ElectrolytesCompounds 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!
24 V. Insolubility Some ionic compounds do not dissociate when in water. These compounds are described as insoluble.Soluble ionics dissociate in water.
25 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.
26 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.
27 V. Predicting Precipitates Does a precipitate form when aqueous solutions of KI and Pb(NO3)2 are combined?
28 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.
29 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).
30 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.
31 V. Sample ProblemPredict the precipitates in the following aqueous reactions.sodium hydroxide + cadmium(II) nitratemagnesium bromide + potassium acetateammonium sulfate + barium chloridesodium iodide + lead(II) nitrate
33 V. Sample ProblemWrite balanced molecular, total ionic, and net ionic equations for the reaction between magnesium sulfate and calcium sulfide.
34 VI. Acids and BasesAcids 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.
36 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.”
37 VI. Sample ProblemWrite balanced molecular, total ionic, and net ionic equations for the reaction between nitric acid and calcium hydroxide.
38 VI. Gas Evolution Reactions In some reactions, a new combo of a cation and an anion leads to gas formation.
39 VI. Decomposition Into Gases Some gas formation reactions involve the decomposition of an intermediate.
41 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.
42 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.
43 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.
44 VIII. Synthesis or Combination Simpler substances combine to form a more complex substance.General equation: A + B AB.
45 VIII. DecompositionComplex substance breaks down into simpler substances.General equation: AB A + B.
46 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).
47 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)