Chapter 6 Chemical Reactions

Section 1: The Nature of Chemical Reactions Chemical reactions occur when substances undergo chemical changes to form NEW substances. Examples: Grow, ripen, decay , burn, rust, change color, produce a gas, digest Atoms are rearranged as bonds are broken and new ones are formed.

Reaction Format: H2O --- > H2 and O2 Reactants Products

Energy and Reactions Endothermic reactions: When more energy is required to break bonds in the reactants than is given off by forming bonds in the products Example: photosynthesis Products might feel colder

Exothermic reactions: More energy is released as the products form than is absorbed to break bonds in the reactants Examples: zinc reacting with HCl, reaction of isooctane and oxygen Temperature in the surroundings rises Quiz

Section 2: Reaction Types Synthesis reaction: substances join to form a more complex compound than the reactants A + B ---> AB 2Na + Cl2 ---> 2NaCl 2H2 + O2 ---> 2H2O

Decomposition Reaction More complex substances are broken apart to form simpler products AB ---> A + B H2O ---> H2 + O2

Combustion Oxygen is always a reactant, water is also a common product The oxidation reaction of an organic compound releasing heat CH4 + O2 ---> CO2 + H2O

Single-Replacement Reactions One element takes the place of another element in a compound AX + B ---> A + BX 3CuCl2 + 2Al ---> 2AlCl3 + 3Cu

Double-Replacement Reaction A compound forms from the apparent exchange of atoms between 2 compounds AX + BY  BX + AY Pb(NO3)2 + K2CrO4  PbCrO4 + KNO3

REDOX reaction Oxidation-reduction reaction: a chemical change in which a substance is oxidized (loses electrons) and another is reduced (gains electrons): Fe + O2  Fe2O3 Other examples: respiration and combustion Review Drawings

Diatomic Elements These elements, when alone, exist diatomically (2 atoms – share electrons): H2, N2, O2, Cl2, Br2, I2, F2

Section 3: Balancing Chemical Equations Law of Conservation of Mass: the number of atoms on the left side must equal the number of atoms on the right. Rules for balancing: ONLY coefficients (in front of the formula) can be added to either or both sides of the equation. NO chemical formulas can be changed (subscripts cannot be changed) Hints: Save solo elements for last. If there is a certain polyatomic ion on both sides, don’t count the atoms within the ion separately. Know diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2 .

Try some balancing: Quiz More . . .

Section 4: Rate of reactions Chemical reactions can be increased by: Increasing temperature Increasing the concentration of one substance in an equilibrium reaction favors the reaction that produces less of that substance: Le Chatelier’s principle Adding energy Increasing pressure Catalysts Increasing surface area

Equilibrium Chemical equilibrium is a state of balance in which the rate of reaction is equal in both directions and the concentration of products and reactants remains the same Different factors can affect equilibrium: Increasing temperature favors a reaction that absorbs energy Increasing pressure favors a reaction that reduces the production of gas Increasing concentration favors a reaction that produces less of that substance