CHAPTER 10: CHEMICAL REACTIONS
I. Reactions & Equations A. Chemical reaction (rxn): process by which atoms of one or more substances are rearranged to form different substances B. Evidence of Reactions (video) 1. Energy is taken in or given off 2. Gases are given off (bubbles) 3. Permanent color changes 4. Odor 5. Precipitate (solid) is formed from solutions 6. Smoke and/or soot 7. Surface changes (ex: rust)
C. Representing Chemical Reactions 1. Chemical equation: condensed statement of facts about a chemical reaction; shows number & type of atoms present before & after reaction a. Obeys law of conservation of mass: same # of atoms before & after 2. General format: reactant 1 + reactant 2 product 1 + product 2 a. Reactants: substances present before the reaction b. Products: substances present after the reaction c. Symbols:
3. Writing an equation: a. Observation: hydrogen burns with oxygen to form water b. Word equation: hydrogen gas + oxygen gas water c. Skeleton equation: H 2 (g) + O 2 (g) H 2 O(l) d. Balanced equation: 2H 2 (g) + O 2 (g) 2H 2 O(l) i. Phase symbols are often dropped & substances are assumed to be in their normal state ii. 2H 2 + O 2 2H 2 O e. Example: i. Observation: carbon and sulfur react to form carbon disulfide ii. Word equation: solid carbon + solid sulfur liquid carbon disulfide iii. Skeleton equation: C(s) + S(s) CS 2 (l) iv. Balanced equation: C(s) + 2S(s) CS 2 (l)
D. Balancing Chemical Equations 1. Terminology: a. Coefficient: number placed to the left of the reactant or product; indicates the number of particles involved in the reaction i. Example: 2H 2 O, 3C 6 H 12 O 6, 5NaCl b. Subscript: number to the lower right of the element symbol; indicates the number of atoms of a particular element i. Example: CO 2, H 2 SO 4, Na 3 PO 4
2. Steps in balancing an equation: a. Write a skeleton equation using the correct formulas for all reactants & products **do NOT mess with subscripts after this point** b. Count the number of atoms of each element on each side of the equation c. Balance the equation using coefficients i. Atoms in = atoms out ii. iii. Start with the first element on the left & balance each element moving right -- save hydrogen & oxygen for last Coefficients must be in the lowest possible ratio (so if it s 2:4, reduce to 1:2) d. Check your work!
3. Examples: a. Write the balanced chemical equation for the reaction in which sodium hydroxide and calcium bromide react to produce solid calcium hydroxide. The reaction occurs in water. b. Propane (C 3 H 8 ) reacts with oxygen to form carbon dioxide and water. Write a balanced equation for this reaction.
Homework: p. 279: 1-3 p. 282: 4-6 (show all steps!) p. 283: 7-13 (13 show all steps!)
II. Classifying Chemical Reactions A. Five major types: 1. Synthesis: two or more substances combine to form one new substance a. Form: A + B AB b. Example: 2H 2 + O 2 2H 2 O c. May also involve two compounds combining to form one compound, or one compound and one element combining to form a compound
2. Combustion (oxidation reactions): oxygen combining with a substance to produce heat and usually light; typically burning a. Form: substance + oxygen product(s) + light + heat b. Example: CH 4 + 2O 2 CO 2 + 2H 2 O
3. Decomposition: one substance breaks down into simpler substances a. Form: AB A + B b. Example: 2HgO 2Hg + O 2 c. Usually require an energy source to occur
4. Single replacement: one element and one compound combine -- the single element replaces (switches places with) one of the elements in the compound. a. Form: A + BX AX + B b. Example: Zn + 2HCl ZnCl 2 + H 2
5. Double replacement: positive and negative ions of the two compounds are interchanged a. Form: AX + BY AY + BX b. Example: NaCl + AgNO 3 NaNO 3 + AgCl
B. Reactions may be more than one type 1. Example: C + O 2 CO 2 -- combustion & synthesis 2. Example: CH 4 + 2O 2 CO 2 + 2H 2 O -- combustion & single replacement
Homework Copy Table 10-3 p. 291: 24-30 Worksheet (both sides)
III. Reactions in Aqueous Solutions A. Aqueous solutions: solutions in which the solutes are dissolved in water 1. Three types: a. Molecular compounds that exist as molecules when dissolved in water (ex: table sugar in grain alcohol) b. Molecular compounds that form ions in water (ex: HCl dissociates into H + & Cl - ions) c. Ionic compounds that form ions in water (ex: NaOH dissociates into Na + & OH - ions; NaCl dissociates into Na + & Cl - ions)
B. 3 possible results when two aqueous solutions that produce ions are combined in double replacement reactions 1. May produce a precipitate: a solid produced in a chemical reaction a. Ex: 2NaOH(aq) + CuCl 2 (aq) 2NaCl(aq) + Cu(OH) 2 (s) 2. May produce water a. This will always contain an acid, which produces H + ions, and a base producing OH - ions b. No evidence of a chemical rxn is observable c. Ex: HBr(aq) + NaOH(aq) H 2 O(l) + NaBr(aq)
3. May produce a gas a. Evident due to bubbles of gas leaving the solution b. Gases commonly produced: O 2, CO 2, H 2 S, HCN c. Ex: 2HI(aq) + Li 2 S(aq) H 2 S(g) + 2LiI(aq)
Homework All of the following problems ask for the chemical, ionic, and net ionic equations we are ONLY doing chemical equations p. 294: #33-37 (#34 has no reaction, so for the products, put NR ) p. 296: #38-42 Worksheet
C. Simultaneous Chemical Reactions 1. Sometimes when 2 solutions are mixed, 2 or more reactions happen at the same time 2. Ex: sodium bicarbonate powder dissolves in hydrochloric acid to produce carbonic acid and aqueous sodium chloride; as the carbonic acid is formed, it immediately breaks down into water and carbon dioxide.
a. Equations: i. NaHCO 3 (aq) + HCl (aq) H 2 CO 3 (aq) + NaCl (aq) ii. H 2 CO 3 (aq) H 2 O (l) + CO 2 (g) b. These two reactions can be combined put all of the reactants on one side, and all of the products on the other i. NaHCO 3 (aq) + HCl (aq) + H 2 CO 3 (aq) H 2 CO 3 (aq) + NaCl (aq) + H 2 O (l) + CO 2 (g) c. Then cancel out anything that appears on both sides i. NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g)
Homework p. 299 #43-46 (chemical equations only) p. 299 #47, 49, 51
Review p. 303 Vocab (17 words) p. 304 Concept Map p. 304 Mastering Concepts #53-66, skip 65 p. 304-6 Mastering Problems #71-94 (do not need to write ionic or net ionic equations) p. 306 Mixed Review #95-100