CHAPTER 5 Chemical Reactions General, Organic, & Biological Chemistry Janice Gorzynski Smith

CHAPTER 5: Chemical Reactions Learning Objectives: Chemical vs Physical changes Chemical equations & balancing them Avagadro’s number, Moles, Molar Mass Stoichiometry Reaction percent yeild Limiting Reactant Percent concentration Redox Reactions: Identify oxidation, reduction, oxidizing agent, reducing agent Half reactions Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Chemical vs Physical Changes A physical change alters the physical state of a substance without changing its composition. A chemical change (a chemical reaction) converts one substance into another. Breaking bonds in the reactants (starting materials) Forming new bonds in the products Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

aA (physical state) + bB (state)  cC (state) + dD (state) Chemical Rxns Chemical Reactions aA (physical state) + bB (state)  cC (state) + dD (state) A, B = reactants C, D = reactants a, b, c, d = coefficients to indicate molar ratios of reactants and products Use modeling kit! CH4 and O2 CO2 and H2O Smith. General Organic & Biolocial Chemistry 2nd Ed.

Balancing Chemical Equations: Chemical Rxns Balancing Chemical Reactions Balancing Chemical Equations: Unbalanced equation: C4H10 + O2  CO2 + H2O Balanced equation: 2C4H10 + 13O2  8CO2 + 10H2O 8 molecules of CO2 2 molecules of C4H10 13 molecules of O2 10 molecules of C4H10 Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

Balancing Chemical Reactions Chemical Rxns Balancing Chemical Reactions HOW TO Balance a Chemical Equation Step [1] Write the equation with the correct formulas. The subscripts in a formula can never be changed to balance an equation, because changing a subscript changes the identity of a compound. Balance the equation with coefficients one element at a time. Step [2] Use modeling kit! Check to make sure that the smallest set of whole numbers is used. Step [3] Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Avagadro’s Number & The Mole A mole is a quantity that contains 6.02 x 1023 items. 1 mole of C atoms = 6.02 x 1023 C atoms 1 mole of CO2 molecules = 6.02 x 1023 CO2 molecules 1 mole of H2O molecules = 6.02 x 1023 H2O molecules Use modeling kit! The number 6.02 x 1023 is Avogadro’s number. 1 mol 6.02 x 1023 molecules 1 mol 6.02 x 1023 atoms Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Molar Mass The formula weight is the sum of the atomic weights of all the atoms in a compound, reported in atomic mass units (amu). The molar mass is the mass of one mole of any substance, reported in grams. Use modeling kit! The value of the molar mass of a compound in grams equals the value of its formula weight in amu. Smith. General Organic & Biolocial Chemistry 2nd Ed.

Mole Ratios Within Molecules: Mole Ratios Between Molecules: Chemical Rxns Stoichiometry: Mole Ratios Mole Ratios Within Molecules: AxBy Mole ratio of A:B = x:y Mole Ratios Between Molecules: aA + bB  cC + dD Mole ratio of A:B:C:D = a:b:c:d Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Stoichiometry aA + bB  cC + dD mass A mass B mass C mass D x ÷ MM x ÷ MM x ÷ MM x ÷ MM moles A moles B moles C moles D a:b b:c c:d a:c a:d Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Limiting Reactant aA + bB  cC + dD mass A mass B mass C mass D x ÷ MM x ÷ MM x ÷ MM x ÷ MM moles A moles B moles C moles D a:b Limiting Reactant: Compare moles A & moles B after applying mole ratio. The reactant with the least number of moles AFTER mole ratio considered is the limiting reactant. Use limiting reactant # moles to determine moles of products that form a:c a:d Use modeling kit! If A is the limiting reactant then use mole ratio between A:C and A:D to determine # moles of product Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Percent Yield Actual yield is determined experimentally, it is the mass of the product that is measured. Theoretical yield is the calculated mass of the products based on the initial mass or number of moles of the reactants. Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Concentration: % Composition by Mass Solution Stoichiometry: Often work with solutions when conducting reactions Determine the amounts needed to completely react just like any other stoichiometry problem. Use percent concentration to obtain mass of each substance. Use modeling kit! Smith. General Organic & Biolocial Chemistry 2nd Ed.

Oxidation is the loss of electrons from an atom. Chemical Rxns Oxidation & Reduction Oxidation is the loss of electrons from an atom. Reducing agents are oxidized Reduction is the gain of electrons by an atom. Oxidizing agents are reduced. Both processes occur together in a single reaction called an oxidation−reduction or redox reaction. Thus, a redox reaction always has two components, one that is oxidized and one that is reduced. Use modeling kit! A redox reaction involves the transfer of electrons from one element to another. Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Oxidation & Reduction Reactions Cu2+ gains 2 e− Zn + Cu2+ Zn2+ + Cu Zn loses 2 e– Zn loses 2 e− to form Zn2+, so Zn is oxidized. Use modeling kit! Cu2+ gains 2 e− to form Cu, so Cu2+ is reduced. Smith. General Organic & Biolocial Chemistry 2nd Ed.

Smith. General Organic & Biolocial Chemistry 2nd Ed. Chemical Rxns Oxidation & Reduction Reactions Cu2+ gains 2 e− Zn + Cu2+ Zn2+ + Cu Zn loses 2 e– Each of these processes can be written as an individual half reaction: Use modeling kit! Oxidation half reaction: Zn Zn2+ + 2 e− loss of e− Reduction half reaction: Cu2+ + 2e− Cu gain of e− 16 Smith. General Organic & Biolocial Chemistry 2nd Ed.