Name Chemistry-PreAP Notes: Chemical Reactions Period Review: Some elements do not occur as single atoms when uncombined with other elements. They will bond with themselves, forming a molecule. In any chemical reaction, when you see these elements alone, they must be shown with a diatomic formula: Diatomic elements: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 I Bring Clay For Our New Hut OR HOBrFINCl And even though these are elements, the correct term for their particles is molecule, not atom. I. Describing Chemical Change A. Writing Equations Words can be used to describe, but that can become long and awkward, not to mention less useful. Chemists use to describe reactions. In chemical equations, and are used to represent the reaction. Symbols Used in Chemical Equations: Symbol Meaning combine with, reacts with yields, forms reversible reaction solid liquid gas dissolved in water, aqueous solution heat is supplied to the reaction precipitate is formed (2 possibilities) gas is released (2 possibilities) catalyst Ex 1: Solid iron reacts with oxygen gas to produce iron (III) oxide (rust). Ex 2: Solid sodium bicarbonate reacts with hydrochloric acid to produce aqueous sodium chloride, water, and carbon dioxide gas. 1
Practice 1. Write equations for each chemical reaction: a. Solid sulfur burns in oxygen gas to form sulfur dioxide gas. b. Heating potassium chlorate solid in the presence of the catalyst manganese(iv)oxide produces oxygen gas. Potassium chloride is left as a solid. 2. Write a sentence that describes each chemical reaction. a. KOH(aq) + H 2 SO 4 (aq) H 2 O(l) + K 2 SO 4 (aq) b. Na(s) + H 2 O(l) NaOH(aq) + H 2 (g) B. Balancing Chemical Equations In order to correctly chemical equations, always remember the law of conservation of. (In any chemical reaction, cannot be or.) An equation that gives the same of each type of on the left and right sides of the equation is called a equation. Numbers called are placed in front of the. When no coefficient is written, it is assumed to be. The tells you the number of,, or of that element/molecular compound/ionic compound used or produced in the reaction. The coefficient also tells you the number of *** *** of the element or compound used/produced in the reaction. Interpreting a balanced equation in two ways: Ex 1: 4 Fe (s) + 3 O 2(g) 2 Fe 2 O 3(s) Ex 2: C (s) + O 2 (g) CO 2(g) 2
Tips for Balancing Chemical Reactions Determine the correct formulas for all the reactants and products. Write formulas for reactants on left and formulas for products on right with in between. Separate multiple reactants and products with a + sign. Count the number of atoms of each element in the reactants and products. Balance the elements one at a time by using coefficients. - it s easier to begin with elements that appear only once. - never balance by changing subscripts within a formula. Once you determine correct formulas, they should never be changed. - many times you can treat polyatomics as one unit (if they appear on both sides of the equation) Check the numbers of each atom or polyatomic ion to be sure the equation is balanced Finally, make sure all the coefficients are in the lowest possible whole number ratio. Reduce if necessary. ALL coefficients must reduce if you do this! Ex 1: Write a balanced equation for the reaction of copper metal and an aqueous solution of silver nitrate to form aqueous copper (II) nitrate and silver. Ex 2: Balance the following reaction: Al (s) + O 2(g) Al 2 O 3(s) Practice: Write balanced chemical equations: 1. Potassium metal and water form aqueous potassium hydroxide and hydrogen gas. 2. Aqueous calcium hydroxide and sulfuric acid react to form aqueous calcium sulfate and water. 3. Balance the following equations. a. SO 2 + O 2 SO 3 b. Fe 2 O 3 + H 2 Fe + H 2 O c. P + O 2 P 4 O 10 d. Al + N 2 AlN e. C 2 H 6 + O 2 CO 2 + H 2 O f. C 3 H 8 + O 2 CO 2 + H 2 O 3
II. Types of Chemical Reactions A. Classifying Chemical Reactions There are 5 basic types of chemical reactions: 1. : two or more substances combine to create a more complex substance General Form: 2. : a complex substance is broken down into two or more simpler substances General Form: 3. : one element takes the place of another in a compound General Form: OR 4. : ions from two ionic compounds switch places General Form: 5. : a substance combines with oxygen and burns, releasing a large amount of energy in the form of light and/or heat General Form: (hydrocarbon combustion) Practice: Balance and identify type. a. Pb(NO 3 ) 2 + K 2 CrO 4 PbCrO 4 + KNO 3 b. C 3 H 6 + O 2 CO 2 + H 2 O c. Li + O 2 Li 2 O d. MgCO 3 MgO + CO 2 e. Cl 2 + KI KCl + I 2 END OF MATERIAL FOR 1 ST SEMESTER. SAVE THESE NOTES. YOU WILL BE TESTED ON THE INFORMATION IN THE ENTIRE PACKET IN JANUARY. 4
B. Predicting Products of Chemical Reactions 1. General Guidelines To predict the products of a chemical reaction, you must first recognize the of reaction: reactants probable type element + element 1 compound element + compound (aq) compound (aq) + compound (aq) hydrocarbon + oxygen You then need to be familiar with the mechanism of that particular reaction so you can predict the product(s). copper (I) oxide solid Type of reaction? Products? Write and balance. Practice Classify by reaction type and write a fully balanced equation by predicting products: 1) Sodium metal + chlorine gas 2) propane (C 3 H 8 ) gas + oxygen gas 2. Detailed Instructions: Single Replacement Reactions One element will replace another element in a compound IF it is more reactive. Otherwise, no reaction occurs. Activity Series of Metals --in order from most to least active in single replacement reactions --Hydrogen can also participate in single replacement reactions with metals Whether one will displace another from a compound can be determined by the relative of the two metals. The of metals lists metals in order of reactivity. (meaning metals at the bottom are less reactive than those at the top.) In a single-replacement reaction, a metal will any metal that is it in the activity series. 5
Ex 1: Ex 2: Ex 3: Mg (s) + Zn(NO 3 ) 2(aq) Mg (s) + AgNO 3(aq) Mg (s) + LiNO 3(aq) A can also replace another. This is usually limited to and they have their own activity series. The activity of halogens as you move up group 17 / 7A. (Fluorine is more active than.) Practice: Write a balanced equation for each SR reaction. a. Zn (s) + H 2 SO 4(aq) Halogen Activity Series b. Na (s) + H 2 O (l) c. I 2(s) + NaF (aq) d. Cl 2(g) + NaBr (aq) 3. Detailed Instructions: Double-Replacement Reactions All double-replacement reactions produce either a (abbreviated ppt, an insoluble solid made from 2 solutions), a gas, or water. To predict products, simply the cations (or anions) in each reactant. The ions switch partners. Ex 1: Pb(NO 3 ) 2(aq) + KI (aq) Ex 2: NaOH (aq) + HCl (aq) Ex Practice 3: HCl(aq) + NaOH(aq) 1. Write balanced equations for these DR reactions. a. NaOH (aq) + Fe(NO 3 ) 3(aq) More practice (iron (III) hydroxide is ppt) b. KOH (aq) + H 3 PO 4(aq) Write a balanced c. H equation 2 SO 4(aq) for + each Al(OH) of these: 3(aq) 6
Predicting products: mixed practice 1. potassium (s) + sulfur(s) 2. silver oxide (s) 3. calcium chloride(aq) + lead (II) nitrate(aq) 4. methane(g) + oxygen(g) (CH 4 ) 5. Chromium(s) + lead (II) nitrate(aq) Use Cr 2+ Beyond the Simple Examples: Now that you have the basics down, we will add some special cases of synthesis and decomposition reactions. Synthesis Reactions 1. metallic oxide + water metallic hydroxide (base) copper (II) oxide + water copper (II) hydroxide 2. nonmetallic oxide + water acid sulfur dioxide + water sulfurous acid 3. nitrogen gas + hydrogen gas ammonia gas (just memorize it) Decomposition Reactions (in the presence of heat) 1. metallic hydroxide (base) metallic oxide + water copper (II) hydroxide copper (II) oxide + water 2. acid water + nonmetallic oxide sulfuric acid water + sulfur trioxide Note: carbonic and sulfurous acids decompose as they form, without addition of heat: Carbonic acid water + carbon dioxide Sulfurous acid water + sulfur dioxide 3. metallic carbonate carbon dioxide + metallic oxide iron (II) carbonate carbon dioxide + iron (II) oxide 4. metallic chlorate metallic chloride + oxygen iron (II) chlorate iron (II) chloride + oxygen 7
REDOX REACTIONS 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. Rule Example 1 The oxidation number of any uncombined element is 0 Ox. # of Na (s) is 0 2 The ox. # of a monatomic ion equals the charge of the ion Ox. # of Cl - is 1 3 The more electronegative element in a binary molecular Ox. # of O in NO is 2 compound is assigned the # equal to the charge it would have if it were an ion 4 The ox. # of fluorine in a compound is always 1 Ox. # of F in LiF is 1 5 Oxygen has an ox. # of 2 unless it is combined with F (when it is +2), or it is in a peroxide like H 2 O 2 (when it is 1) 6 The ox. # of H in most compounds is +1 unless it is combined with a metal, in which case it is -1 7 In compounds, Group 1 & 2 elements and Al have ox. #s of +1,+2, +3, respectively Ox. # of O in NO 2 is 2 Ox. # of O in OF 2 is +2 Ox. # of O in Na 2 O 2 is 1 Ox. # of H in H 2 O is +1 Ox. # of H in LiH is 1 Ox. # of Ca in CaCO 3 is +2 8 The sum of ox. # of all atoms in a neutral compound is 0 Ox. # of C in CaCO 3 is +4 9 The sum of the ox. # of all atoms in a polyatomic ion equals the charge of the ion Ox. # of P in H 2 PO - 4 is +5 *See worksheets for more practice/examples of assigning oxidation numbers. Oxidation is a reaction in which the atoms or ions of an element experience an increase in oxidation state. An increase in oxidation state means that the oxidation number becomes more positive. Therefore, the species has lost electrons. 0 +1 Na Na + + e - Reduction is a reaction in which the oxidation state of an element decreases. A decrease in oxidation state means that the oxidation number becomes more negative. Therefore, the species has gained electrons. 0-1 Cl 2 + 2e - 2Cl - Since oxidation is the losing of electrons and reduction is the gaining of electrons, they have to occur simultaneously and the number of electrons produced in oxidation must equal the number of electrons acquired in reduction. **Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction, or redox reaction for short.** *See worksheets for practice in determining which types of reactions are redox and which are not. If 2 elements are experiencing a CHANGE in oxidation number from reactants to products, it s redox! Summarizing tip: All of the 5 basic reaction types we have learned are redox reactions except for one. What is it??? 8