Notes: Chemical Reactions (text Ch. 8)

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1 U Name Per. Notes: Chemical Reactions (text Ch. 8) NOTE: This set of class notes is not complete. We will be filling in information in class. If you are absent, it is your responsibility to get missing information from a fellow classmate or the chemistry website: A chemical reaction is the rearrangement of atoms to form new substance(s). Reactant(s) appear on the left and product(s) appear on the right. Example: Bonds are broken in the diatomic molecules H2 and O2, and new bonds are formed between the hydrogen and oxygen atoms to form H2O. The SAME ATOMS are on both sides of the equation. Diagram: reactants product 2 H2 + O2 2 H2O 5 signs of a chemical reaction (this is a review from Matter and Change unit): 1. a gas is produced 2. light is produced 3. a solid (precipitate) is formed from 2 liquids 4. temperature change a. exothermic (hot- heat is released): Reactants Products + HEAT b. endothermic (cold- heat is absorbed: Reactants + HEAT Products 5. permanent color change Balancing Equations According to the Law of Conservation of Mass, matter is not created or destroyed in a chemical reaction. So the mass of all reactants must equal the mass of all products. (this is a review) Example 1: What is the total mass of reactants in the reaction below? 2 H2 + O2 2 H2O Answer: g? 72 g Example 2: What is the total mass of products in the reaction below? AgNO3 + NaCl AgCl + NaNO3 Answer: g 170 g 56 g ? Example 3: What is the mass of zinc produced in the reaction below? Ca + ZnCO3 CaCO3 + Zn Answer: g 40 g 125 g 100 g? 1

2 Because the Law of Conservation of Mass always holds in chemical reactions, we must always be sure we are working with a balanced equation. A balanced equation is one in which the number of atoms of each element is the same on both sides of the arrow. Example of an equation that IS NOT balanced: Atom inventory: Na + FeCl3 NaCl + Fe -Na- -Fe- -Cl- Example of an equation that IS balanced: Atom inventory: 3 Na + FeCl3 3 NaCl + Fe -Na- -Fe- -Cl- The big numbers that are used to balance the equation are called coefficients. 2 Fe(s) + 3 Cl2 (g) 2 FeCl3 (s) Coefficients Subscripts are the small numbers written in formulas; they stand for the number of atoms or polyatomic ions immediately to their left. Examples: FeCl3 3 is a subscript, meaning there are 3 Cl atoms Mg(NO3)2 3 is a subscript, meaning there are 3 O atoms; 2 is a subscript, meaning there are 2 NO3 (2 N atoms and 6 O atoms all together) ***When balancing equations, UONLY add coefficients U to balance. NEVER alter a chemical formula in the equation in order to balance (do not change SUBSCRIPTS!!).*** Example CaO + C CaC2 + CO 2

3 Writing Chemical Formula Equations from Word Equations: Symbols Used in Chemical Reactions (s) solid (l) liquid (g) gas (aq) aqueous solution (the substance is dissolved in water; example: NaCl(aq) is salt water) + separates 2 reactants or products, reads as reacts with on reactant side, reads as and on product side separates reactants from products; read as yields or produces Δ heat added to reactants (triangle appears above arrow) Catalysts are substances that increase the rate of the reaction but are not used up themselves. The formula for the catalyst is written above the arrow: (example MnO2) Catalysts are not used up in the reaction so they appear above the arrow MnO 2 2 KClO3 (s) 2KCl(s) + 3 O2(g) ***IMPORTANT to remember in writing chemical equations*** Diatomic elements: These 7 elements do not occur as single atoms. If they are not combined with another element(s), they must always be written as pairs (diatomic): I2, Br2, Cl2, F2, O2, N2, H2, NOT I, Br, Cl, F, O, N, H!!!!! To help you remember the 7 diatomic elements: I Bring Clay For Our New Hut, HOBrFINCl, or 7 (your teacher will show you the 7 on the periodic table). Example Solid sodium metal reacts with water to produce aqueous sodium hydroxide and hydrogen gas. 1. Turn it into a formula equation first (don t forget to add (s), (l), (g), or (aq)), review nomenclature notes if you need help with formula writing. 2. Balance! 3

4 Quick Guide to Naming/Formula Writing Type of Compound Molecular How to ID Naming Formula Writing N + N (or metalloid) prefix + name + no mono for first prefix prefix + name + ide prefixes become the subscripts *don t reduce *don t criss-cross Ionic M + N (or polyatomic ion) name + name + ide Only use No ide for (I, II, III, IV) polyatomic ions to indicate charge of elements with undefined charges Acid H + + anion ide: hydro ic acid Polyatomic ate: ic acid ions ite: ous acid criss-cross the ion s charges for new subscripts *reduce if necessary criss-cross the ion s charges for new subscripts *H + is always the cation Hydroxide Base cation + OH - cation name + hydroxide criss-cross the ion s charges for new subscripts *OH - is always the anion 4

5 U U I. 5 Types of Chemical Reactions We will learn 5 basic reaction types in this class. More types exist, but we will not go into them here. 1. Synthesis (also called Combination or Composition) Synthesis means 2 or more elements/simple compounds combine to form 1 compound. General form: A + B AB Identifying feature: only one Analogy: A boy and a girl come to the dance separately, but end up dancing together. ExampleU: 2 Mg + O2 2 MgO 2. Decomposition Decomposition means 1 compound is broken down into 2 or more simpler elements/compounds. General form: AB A + B Identifying feature: only one (opposite of Synthesis!) Analogy: A boy and a girl come to the dance together, but argue and stay apart the rest of the night. ExamplesU: 2 NaCl 2 Na + Cl2 2 KClO3 2 KCl + 3 O2 5

6 U 3. Single Replacement 1 element takes the place of another in a compound ( like replaces like ) General forms: A + BC B + AC (metal replacement) D + BC C + BD (halogen replacement) Identifying feature: 1 element + 1 compound on each side of the arrow Analogy: A boy and a girl are dancing, but then another boy cuts in and dances with the girl, leaving the first boy alone. Or a boy and a girl are dancing, but then another girl cuts in and dances with the boy, leaving the first girl alone. ExamplesU: Metal replacement: 2 Na + CuCl2 2 NaCl + Cu Halogen replacement: F2 + 2 KCl 2 KF + Cl2 BUT the boy/girl will not always be able to cut in. Sometimes the other boy/girl will not let them! We use an activity series to predict whether or not the replacement will occur. If an element is more reactive than (above) another element, it can replace that element. Metal Activity Series Halogen Activity Series (same order as on PT!) F (most reactive) Cl Br I (least reactive) Can Al replace Li? Can Cu replace Au? Can Br replace I? Can Cl replace F? ***Note: The activity series is only used for single replacement reactions.**** 6

7 U U 4. Double Replacement Ions from 2 ionic compounds switch places. General form: AB + CD CB + AD - A and C (the cations from each compound) switch places Identifying feature: 2 compounds on each side of the equation Analogy: 2 boy-girl pairs are dancing, and they switch partners. ExampleU: FeCl2 + Na2CO3 2 NaCl + FeCO3 Note: An acid-base (neutralization) reaction is a special type of double replacement reaction. It occurs when an acid and a base react to form an ionic compound (a salt) and water. Examples: HCl + NaOH + H2SO4 + KOH + 5. Combustion Also known as. Always follows the same form: Compound containing C and H (& sometimes O) + O2 CO2 + H2O Note: In a combustion reaction, the compound always burns in oxygen gas and always releases carbon dioxide and water. During incomplete combustion (a limited amt. of O2), carbon monoxide (CO) is also produced. ExamplesU: CH4 + 2 O2 CO2 + 2 H2O 2 C3H6 + 9 O2 6 CO2 + 6 H2O ***Trick for balancing tough combustion reactions: Balance the H first by placing a coefficient in front of the water. If that coefficient is ODD, double it and proceed with balancing C, then O. Always balance oxygen last!!! Example: C7H14 + O2 CO2 + H2O 7

8 II. Predicting Products For each of the following types of reactions: 1. predict the products 2. balance the equation using the lowest possible integers Your teacher will work these examples with you. 1. Synthesis A + B AB Example 1: Na + Br2 Example 2: Al + Cl2 2. Decomposition AB A + B Example 1: Example 2: Ag2O HgO 3. Single Replacement (check activity series!) A + BC B + AC D + BC C + BD (metal replacement) (halogen replacement) Example 1: Example 2: Cl2 + KF Ca + AlCl3 4. Double Replacement (includes acid-base reactions) AB + CD CB + AD Example 1: HCl + NaOH Example 2: AlBr3 + K2SO4 5. Combustion C and H (& sometimes O) compound + O2 CO2 + H2O Example 1: C3H8 + O2 Example 2: C11H24 + O2 8

9 III. Redox Reactions In redox reactions the oxidation number for an element changes in a chemical reaction. Oxidation Number: A number assigned to an element, based on the distribution of electrons. The same element can have very different properties in different oxidation states. Rules for Assigning Oxidation #s Examples Oxidation # 1 The oxidation number of any uncombined ELEMENT is 0 Na, O2 Na =,O2 = 2 The ox. # of an ION equals the charge of the ion Cl - Cl - = 3 The ox. # of elements in COMPOUNDS typically, but not always (unless noted otherwise), follow a trend on the periodic table: Group 1 = +1 ALWAYS LiF Li = Group 2 = +2 Group 13 = +3 CaCO3 Ca = Group 15 = -3 Group 16 = -2 (O usually -2) Group 17 = -1 (F ALWAYS -1) Transition metals AND Group 14 = multivalent 4 The ox. # of HYDROGEN in most compounds is +1 unless it is with a metal, in which case it is -1 5 The sum of ox.#s of all atoms in a NEUTRAL COMPOUND is 0 HF H2O H2O LiH LiF F = H = H= H= Li = + F = 0 CaCO3 Ca = C = O -2 (x3) = 0 The sum of ox. #s of all atoms in a POLYATOMIC ION equals the charge of the ion SO4 2- S = +O -2 (x4) = -2 Note: Additional rules/exceptions to these rules do exist, but are beyond the scope of this course. Oxidation is a reaction in which there is the loss of electrons Ex: Na Na + + e - Reduction is a reaction in which there is the gain of electrons. 0-1 Ex: Cl2 + 2e - 2Cl - LEO the lion says GER Loss of Electrons is Oxidation Gain of Electrons is Reduction OR OIL RIG Oxidation Is Loss (of electrons) Reduction Is Gain (of electrons) 9

10 Since oxidation is the loss of electrons and reduction is the gain of electrons, they must occur simultaneously. **Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction, or redox reaction for short.** Determine whether the following elements have been oxidized or reduced and label the reaction type. Example 1: 4Fe + 3O2 2Fe2O3 TYPE of Reaction: S/D/SR/DR Element Ox.# Reactants side Ox.# Products side Lose/Gain e - Oxidized/Reduced Example 2: Mg + 2HCl MgCl2 + H2 TYPE of Reaction: S/D/SR/DR Element Ox.# Reactants side Ox.# Products side Lose/Gain e - Oxidized/Reduced Example 3: HCl + NaOH NaCl + H2O TYPE of Reaction: S/D/SR/DR Element Ox.# Reactants side Ox.# Products side Lose/Gain e - Oxidized/Reduced ***Note: ALL of the reaction types we have learned are redox reactions EXCEPT for double replacement reactions.*** 10