Bettelheim, Brown, Campbell and Farrell Chapter 4 Chemical Reactions Bettelheim, Brown, Campbell and Farrell Chapter 4

Chemical Reactions In a chemical reaction, one or more reactants is converted to one or more products Formula weight: the sum of the atomic weights in atomic mass units (amu) of all atoms in a compound’s formula Mass Relationships

Compound Mass Molecular Weight: Molecular compounds only Formula Weight: Molecular or ionic compounds

Chemical Equations The following chemical equation tells us that propane gas and oxygen gas react to form carbon dioxide gas and water vapor Gives reactants and products and their physical states. Equation above is NOT balanced Must be balanced to get mass relationships

Balancing Equations Must have same number of atoms of each kind on both sides of equation. Change coefficients until it is balanced.

Balancing Equations Practice problems: balance these equations

Balancing Equations Practice problems: balance these equations

1 mol HCl = 6.022 x 1023 HCl molecules = 36.5 g HCl The Mole Mole (mol)- amount of substance whose mass in grams is numerically equal to its molecular or formula weight Amount of substance that contains as many atoms, molecules, or ions as are in exactly 12 g of carbon-12 Number of formula units in a mole is known as Avogadro’s number Equal to 6.02214199 x 1023 formula units per mole 1 mol HCl = 6.022 x 1023 HCl molecules = 36.5 g HCl

Moles to Molecules A tablet of aspirin, C9H8O4, contains 0.360 mol of aspirin. How many aspirin molecules is this?

Moles to Atoms A tablet of aspirin, C9H8O4, contains 0.360 mol of aspirin. How many atoms of oxygen does this contain?

Molar Mass Molar mass: the formula weight of a substance expressed in grams Glucose, C6H12O6 molecular weight: 180 amu molar mass: 180 g/mol Urea, (NH2)2CO molecular weight 60.0 amu molar mass: 60.0 g/mol

Molar Mass Molar mass can be used to convert from grams to moles, and from moles to grams calculate the number of moles of water in 36.0 g water

Grams to Moles Calculate the number of moles in 5.63 g of sodium sulfate, Na2SO4 (I mol = 142 g)

Grams to Moles Calculate the number of moles of sodium ions, Na+, in 5.63 g of sodium sulfate, Na2SO4

Stoichiometry Stoichiometry: the study of mass relationships in chemical reactions

Stoichiometry Problem: how many moles of nitrogen, N2, are required to produce 3 moles of ammonia, NH3

Stoichiometry Problem: how many grams of nitrogen, N2, are required to produce 51 g of ammonia, NH3

Limiting Reagent Limiting reagent: the reagent that is used up first in a chemical reaction Need to determine which reagent is present with the fewest moles Suppose 12 g of carbon is mixed with 64 g of oxygen and the following reaction takes place Maximum amount of product can only be equal to the amount produced from the limiting reagent

Percent Yield Theoretical yield: the mass of product that should be formed according to the stoichiometry of the balanced chemical equation Actual yield: the mass of product formed in a chemical reaction Percent yield: actual yield divided by theoretical yield times 100

Percent Yield 32.0 g of methanol reacts with excess carbon monoxide to produce 58.7 g of acetic acid

Percent Yield 32.0 g of methanol reacts with excess carbon monoxide to produce 58.7 g of acetic acid

Reactions When an ionic compound dissolves in water, it dissociates to form aqueous ions AgNO3(s) → Ag+(aq) + NO3-(aq) Dissociation Reactions

Reactions When two ionic compounds are dissolved in water, the ions may combine to form new compounds

Reactions Between Ions in Solution Reaction will occur if a precipitate (solid) forms (see solubility table) a compound that is insoluble in water a gas is formed (bubbles or fizzing) an acid neutralizes a base and water is formed one of the ions can oxidize another (Redox reaction)

Molecular Equation Everything shown as molecules AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

Full Ionic Equation: Shows ions as ions and precipitates as compounds

Net Ionic Equation Shows only those species that change in reaction Ag+(aq) + Cl-(aq) AgCl(s) Spectator ions do not participate in reaction and do not change (Na+ and NO3-)

Solubility Rules Soluble: Compounds with: Exceptions Na+, K+ and NH4+ NO3- and CH3CO2- Cl-, Br- and I- Pb2+ , Ag+ and Hg22+ halides are insoluble Insoluble: Compounds with CO32- and PO43- All Na+, K+ and NH4+ are soluble OH- and S2- Na+, K+ and Ca2+ and NH4+ sulfides and hydroxides are soluble

Oxidation-Reduction Reactions Oxidation: the loss of electrons or hydrogen (or gain of oxygen) Reduction: the gain of electrons or hydrogen (or loss of oxygen) Oxidation-reduction (redox) reaction: any reaction in which electrons are transferred from one species to another Must have BOTH oxidation AND reduction

Oxidation-Reduction Reaction Example: zinc metal is placed in a beaker containing a solution of copper(II) sulfate some of the zinc metal dissolves some of the copper ions deposit on the zinc metal the blue color of Cu2+ ions gradually disappears zinc metal loses electrons to copper ions copper ions gain electrons from the zinc

Oxidation-Reduction Summary:

Oxidation-Reduction Although some definitions of oxidation (loss of electrons) and reduction (gain of electrons) are easy to apply to many redox reactions, they are not easy to apply to others for example, the combustion of methane

Heat of Reaction In almost all chemical reactions, heat is either given off or absorbed Example: the combustion (oxidation) of carbon liberates 94.0 kcal per mole of carbon oxidized Exothermic reaction: Heat given off Endothermic reaction: Heat absorbed