Chapter 24 Chemical Reactions What is a chemical reaction? One or more substances are changed into new substances. Conservation of mass Mass of the products = mass of the reactants Reactants Produce Products Matter is not created or destroyed, just changes form. Have to have an equal number of atoms of each element on both sides. Chemical Formula vs Chemical Equation Chemical formula - tells what elements are in a compound and their ratios Chemical equations - used to describe a chemical reaction. Coefficients - Ex. 2NaCl or 3H 2 O (numbers tell # of molecules) Chemical Formulas Ex. NiCl 2 or NaOH (le@ers and subscripts) Chemical Equa3ons NiCl 2(aq) + 2NaOH (aq) à Ni(OH) 2(s) + 2NaCl (aq) Balancing Equations Ch 24.2 Balanced chemical equations: have the same number of atoms on both sides. 1
Putnam s 3 Steps: 1. Balance everything except H 2 O 2. Balance H 3. Balance O Atoms: Hg: O: HgO Hg + O 2 Atoms: Mg: O: Mg + O 2 MgO Atoms: Li: H: O: Li + H 2 O LiOH + H 2 Ch 24.3 Types of Reactions 1. Combustion 2. Synthesis 3. Decomposition 4. Single Displacement 5. Double Displacement 6. Acid/Base ** book doesn t talk about Quick background Common Polyatomic Ions NO 3- - nitrate OH - - hydroxide SO 4 2- - sulfate PO 4 3- - phosphate 2
1. Combustion Substance + Oxygen produce energy (as heat and light) A + O 2 AO + energy 2. Synthesis Reactions 2 substances combine to form a different substance. A + B AB 3. Decomposition Reactions A substance breaks down into 2 or more substances. AB A + B 2H 2 O à 2H 2 + O 2 4. Single Displacement Reaction One element replaces another in the equation. A + BC AC + B Ca + 2KCl à CaCl 2 + 2K Cl 2 + 2KI à I 2 + 2KCl Metal replaces metal Nonmetal replaces nonmetal 5. Double Displacement + ion of one compound replaces the + ion of another compound Sometimes get a precipitate a solid that separates from the liquid. AB + CD AD + CB 6. Acid/Base Reaction When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H+ cation of the acid combines with the OH- anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt. HCl + NaOH à NaCl + H 2 O 3
CFU: Name that reaction! 1. Cu(s) + 2AgNO 3 (aq) à Cu(NO 3 ) 2 (aq) + 2 Ag(s) 2. 2H 2 (g) + O 2 (g) à 2H 2 O(g) 3. 2C 2 H 6 + 7O 2 4CO 2 + 6H 2 O 4. Ba(NO 3 ) 2 (aq) + K 2 SO 4 (aq) à BaSO 4 (s) + 2KNO 3 (aq) 5. 2H 2 O(l) à 2H 2 (g) + O 2 (g) Card Sort Activity I will pair you with someone nearby. You will cut out one set of cards. Organize cards into 5 categories (the 5 reaction types discussed in class). Please note that columns may not be = Diatomic Molecules Predict products, Balance eq, and ID Reaction Type CH 4 + O 2 à Mg + O 2 à H 2 O 2 à Cu + AgNO 3 à Pb(NO 3 ) 2 + KI à Examples of Reactions 1. Combustion CH 4 + 2O 2 à CO 2 + 2H 2 O 2. Synthesis 2 Mg + O 2 à 2 MgO 3. Decomposition 2H 2 O 2 à 2H 2 O + O 2 (g) (MnO 2 as catalyst) 4. Single Displacement Cu + 2AgNO 3 à Cu(NO 3 ) 2 + 2 Ag 5. Double Displacement Pb(NO 3 ) 2(aq) + 2 KI (aq) à PbI 2(s) + 2 KNO 3(aq) Acid/Base vs. Red-ox Reactions Another way to classify reactions Not all reactions fit into these categories Goal: Identify whether a reaction is Acid/Base or Red-ox 4
Acid/Base Reactions Acid H + donor A chemical that adds H + to a solution EX: HX (HCl) Base H + acceptor A hydroxide (OH - ) EX: YOH (NaOH) HX + YOH à YX + H 2 O Acid + Base à Salt + Water Salt cation and anion getting together Millions of types of salts Acid/Base Reactions HCl (aq) + NaOH (aq) à NaCl (aq) + H 2 O (l) Acid + Base à Salt + Water 2 Types of reactions that yield H 2 O Combustion and Acid/Base! Red-ox Reactions Lose Electrons Oxidized goes Gain Electrons Reduced Reduced Red-ox Oxidized Red-ox Reactions Think back to synthesis 2Mg (s) + O 2(g) à 2MgO (One element gains e - and one loses e - ) Review oxidation states ion that will most likely form 2Mg (s) + O 2(g) à 2MgO (0) (0) (+2) (-2) ß oxidation states Mg lost e - so it is oxidized (LEO goes GER) O gained e - so it is reduced (LEO goes GER) Red-ox Reactions Cu (s) + 2AgCl (aq) à CuCl 2(aq) + 2Ag (s) What are the oxidation states? Which element lost a(n) electron(s)? Which element gained a(n) electron(s)? Ch 24.4 Chemical Reactions and Energy All chemical reactions release or absorb energy (heat, light, sound, electricity) Breaking bonds takes energy Bond formation releases energy EX: Dynamite combustion takes less energy to break the bonds than the energy released when new bonds are formed à Release of E and loud explosion! Exergonic Reaction releases E Endergonic Reaction absorbs E 5
Exergonic vs Endergonic Rxns Exergonic less E is required to break the original bonds than is released when new bonds formed (heat releasing) EX: glow sticks, heat packs, burning of wood, dynamite explosions, burning of fossil fuels Endergonic more E is required to break the original bonds than is released when new ones are formed (heat absorbing) EX: electroplating of metal, cold packs Ch 24.4 Chemical Reactions and Energy All chemical reactions release or absorb energy. Forms of E - heat, light, sound, electricity Breaking bonds takes energy Bond formation releases energy EX: Dynamite breaking bonds takes much less energy than the energy released when new bonds form. BOOM! Chemical Reactions and Energy Exothermic: Heat released due to exergonic reaction. Example: burning wood; iron rusting Energy comes from chemical bonds. Endothermic Heat is absorbed due to endergonic reaction. Example: electroplating, cold packs Reaction Rate Factors Catalysts and Inhibitors Shape and size of reactants Surface area Concentration of reactants Temperature Pressure IT COMES DOWN TO COLLISIONS. Apply to predict! 1. I have one beaker of hot water and one beaker of cold water. I add one drop of food coloring to the center of each container. What will happen? Describe how this parallels reaction rates. 2. I have a large pitcher of iced tea that I need to sweeten with several cubes of sugar. Describe two methods to speed up the dissolving time. Apply to predict, cont. 3. Adding a magnesium strip to vinegar will cause it to fizz. How could I test the affect temperature has on the reaction rate? What would you predict and why? 4. Adding an Alka Seltzer to water causes a chemical reaction. What are two ways to affect the reaction rate? 6
Discussion Questions 1. Pieces of zinc metal react with hydrochloric acid, producing bubbles of hygrogen gas and a solution of zinc chloride. Which of the following is NOT likely to increase the rate at which this reaction occurs? a. Grinding the zinc into a powder b. Diluting the hydrochloric acid c. Stirring the acid d. Heating the hydrochloric acid Discussion Questions 2. For the reaction of zinc metal with hydrochloric acid, explain why: a. Concentrated hydrochloric acid reacts at a faster rate than dilute hydrochloric acid. b. Zinc powder reacts faster than pieces of zinc. c. The reaction occurs faster at 60 degrees C than 20 degrees C. Catalyst vs. Inhibitors Catalyst: Speeds up a reaction w/o changing the reaction Lower the activation energy Often used in industry, such as polymerization to make plastics and fibers Inhibitor: Slows or prevents a reaction They combine with a reactant to prevent it from undergoing the original reaction Food preservatives prevent spoilage of certain foods (cereals and crackers) Inhibit catalysts? Pour lemon juice on cut fruit! 7