Chapter 5 Classification and Balancing of Chemical Reactions

Transcription

1 Chapter 5 Classification and Balancing of Chemical Reactions 5.1 Chemical Equations Chemical equations describe chemical reactions. - As words: hydrogen plus oxygen combine to form water - As a chemical equation: Reactants are written on the Products are written on the Necessary conditions are written above the arrow. Heat can be symbolized as. Law of Conservation of Mass: Bonds between atoms in reactants Break and rearrange. No atoms disappear or disappear For a balanced reaction: the number and kind of atoms on each side of the reaction must be the same. - The relationship between the quantities of chemical reactants and products is called - In a balanced chemical reaction, the numbers in front of the compounds are called Ch 5 Page 1

2 States of Matter The state of matter of each reactant and/or product is often included in parentheses after the formula of the substance. The state symbols we will use are: gas= solid= liquid= aqueous= 5.2 Balancing Chemical Equations Step 1: Write an unbalanced equation, using the correct formulas for all reactants and products. For example, hydrogen must be not H since it is diatomic. Step 2: Add appropriate stoichiometric to balance the number of atoms of each element. Subscripts in chemical formulas cannot be changed because doing so would change the of the substances in the reaction Begin with elements that appear in only compound or formula on each side of the equation. Leave until last. Treat polyatomic ions as a, not as individual atoms. For example, balance not sulfur atoms and oxygen atoms Unbalanced Reaction Balance for first. Ch 5 Page 2

3 Now check balance for. Finally check for Step 3: Make sure the number of atoms of each element must be the same on each side of the equation. H, S, O, Na on the left H, S, O, Na on the right Step 4: Make sure that coefficients are reduced to their If you came up with 2 H 2 SO NaOH 2 Na 2 SO H 2 O You would need to divide all coefficients by. Problem: Which of the equations below are balanced? 1) CH 3 CH 2 OH + 3 O 2 2 CO H 2 O 2) Ca(OH) 2 + Na 3 PO 4 CaPO NaOH a. Both equations are balanced. b. Neither equation is balanced. c. Only equation 1 is balanced. d. Only equation 2 is balanced. Ch 5 Page 3

4 Problem: Balance the following reactions. Remember to use state symbols. a) Aluminum metal in oxidized by oxygen to form aluminum oxide powder. b) Barium metal when dropped into water produces a solution of barium hydroxide and hydrogen gas. c) Ethane gas is burned in oxygen to form carbon dioxide and water vapor. d) Solutions of magnesium hydroxide and aluminum nitrate are mixed to form a solution of magnesium nitrate with an aluminum hydroxide precipitate. Problem: When the equation below is balanced, what should the coefficients be? C 5 H 12 + O 2 CO 2 + H 2 O Ch 5 Page 4

5 5.3 Precipitation Reactions and Solubility Guidelines Although it is possible to classify reactions into many detailed categories, we will categorize them into four major categories to help predict products. Precipitation Reactions A process where a (aka precipitate) is formed. Most are double displacement reactions. General Format is AB + CD AD + CB - Anions and cations change partners! - Remember to always write the first. - It can help to write the of the products before writing the Remember to the final equation! Problems: Predict the products and balance the final equations when the following aqueous solutions are mixed. Name all reactants and products. 1. Ba(NO 3 ) 2 (aq) + Na 2 SO 4 (aq) 2. KOH(aq) + Al(NO 3 ) 3 (aq) All aqueous solutions are given the state symbol (aq). We need more information before we can add state symbols to the products to see which is the precipitate (ppt.) Ch 5 Page 5

6 Predicting State Symbols Whether a precipitate will form, depends on the of the products. - Solubility is the amount of a compound that will dissolve in a given amount of solvent at a given. - If a substance has low solubility in water (akainsoluble), then it is likely to form a precipitate. Product gets an state symbol. - If a substance has high solubility in water, (aka soluble) then no precipitate will form. Product gets an state symbol. General Rules on Solubility 1. These are just guidelines, there are always exceptions The table in the textbook is very limited. A more complete set of solubility ruled is presented on the next page. Ch 5 Page 6

7 Solubility Rules for Some Ionic Compounds in Water Soluble Ionic Compounds use state symbol (aq) 1. All lithium (Li + ), sodium (Na + ), potassium (K + ), rubidium (Rb + ), cesium (Cs + ) and ammonium (NH 4 + ) salts are SOLUBLE. 2. All nitrate (NO 3 - ), acetate (C 2 H 3 O 2 - ), chlorate (ClO 3 - ), and perchlorate (ClO 4 - ) salts are SOLUBLE 3. All chloride (Cl - ), bromide (Br - ), and iodide (I - ) salts are SOLUBLE. Except Those Containing: use state symbol (s) No common ones No common ones Pb +2, Ag +, & Hg 2 +2 are NOT soluble. Mercury (II) iodide (HgI2) is also NOT soluble. 4. All fluoride (F - ) salts are SOLUBLE. Mg +2, Ca +2, Sr +2, Ba +2, & Pb +2 are NOT soluble. 5. All sulfate (SO -2 4 ) salts are SOLUBLE. Ca +2, Sr +2, Ba +2, Pb +2, Ag +, Hg 2 +2, are NOT soluble. Not Soluble Ionic Compounds Except Those Containing: use state symbol(s) use state symbol (aq) 6. Hydroxide (OH - ) and oxide (O -2 ) Li +, Na +, K +, Rb +, Cs +, NH + 4, & Ba +2, are soluble. compounds are NOT SOLUBLE (Ca 2+ and Sr 2+ are moderately soluble) 7. Sulfide (S -2 ) salts are NOT SOLUBLE Li +, Na +, K +, Rb +, Cs +, NH 4 +, & Ba +2 are soluble. 8. Carbonate (CO 3-2 ), phosphate (PO 4-3 ), Li +, Na +, K +, Rb +, Cs +, & NH 4 + are soluble. chromate (CrO -2 4 ), oxalate (C 2 O -2 4 ) & sulfite (SO3 2- ) salts are NOT SOLUBLE Soluble compounds are defined as those that dissolve to the extent of 1 g or more per 100 g water. NOT Soluble compounds are further classified as: - Slightly soluble, which dissolve to the extent of 0.01 g to 1 g per 100 g water. - Insoluble, for which less than 0.01 g per 100 g water will dissolve. SOLUTIONS MADE FROM THE ABOVE SPECIES, WHEN SOLUBLE, ARE FOUND TO EXIST AS CHARGED PARTICLES AND THUS CONDUCT ELECTRIC CURRENT. THEY ARE CONSIDERED ELECTROLYTES. WRITE THEM IN IONIZED FORM IN AQUEOUS SOLUTIONS. RULE SUMMARY OF STRONG AND WEAK ELECTROLYTES EXCEPTIONS 1. Most acids are weak electrolytes Common strong acids (strong electrolytes) are HCl, HBr, HI, HNO3, H2SO4, HClO3, and HClO4 2. Most bases are weak electrolytes Strong base hydroxides (strong electrolytes) are those of Li, Na, K, Rb, Ca, Sr, and Ba. 3. Most soluble salts are strong electrolytes. Important weakly ionized salts are HgCl2, Hg(CN)2, CdCl2, CdBr2, CdI2, and Pb(C2H3O2)2. Ch 5 Page 7

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9 Problem: Lets add state symbols to the previous two reactions. Ba(NO 3 ) 2 (aq) + Na 2 SO 4 (aq) 2 NaNO 3 + BaSO 4 3 KOH(aq) + Al(NO 3 ) 3 (aq) 3 KNO 3 + Al(OH) 3 Problem: What are the solubilities of Na 3 PO 4 and PbI 2 in water? a. soluble, soluble b. soluble, insoluble c. insoluble, insoluble d. insoluble, soluble 5.4 Acids, Bases, and Neutralization Reactions Acid base neutralization reactions are processes in which an (a compound with as the cation) reacts with a, (a compound with as the anion) to yield plus an ionic compound called a. A neutralization reaction is a special form of double displacement reaction. HA(aq) + MOH(aq) H 2 O(l) + MA(aq or s) Ch 5 Page 9

10 Bases other than hydroxide: - Aqueous ammonia - carbonate ion, - bicarbonate ion also react as bases. Problem: Write balanced neutralization equations for the following reactions. a) Solutions of lithium hydroxide and sulfuric acid were mixed. b) Solutions of phosphoric acid and calcium hydroxide neutralize each other. (note: This is both a neutralization & precipitation reaction.) c) A potassium carbonate solution reacts with hydrochloric acid to give water and a gas that would smother a flame and potassium chloride solution. d) Aqueous ammonia is mixed with nitric acid. Ch 5 Page 10

11 Problem: Determine if the following reactions are acid base (neutralization) reaction, precipitation reactions, or both. 1) AgNO 3 (aq) + NaBr(aq) AgBr(s) + NaNO 3 (aq) 2) H 2 SO 4 (aq) + Sr(OH) 2 (aq) SrSO 4 (s) + 2 H 2 O(l) 3) H 2 SO 4 (aq) + 2 KOH(aq) 2 H 2 O(l) + K 2 SO 4 (aq) 5.5 Redox Reactions Defining Oxidation and Reduction Oxidation reduction reactions or reactions, are processes in which one or more electrons are from one atom to another. Historically referred to the combination of an element with oxygen. referred to the removal of oxygen from an oxide to yield the element. Example: The formation of rust is a redox reaction. Today the words have taken on a much broader meaning. Oxidation is the of one or more electrons. Reduction is the of one or more electrons. Ch 5 Page 11

12 oss of lectrons is xidation ain of lectrons is eduction memory tool: ( the lions says ) Oxidation - the loss of electrons; increase in oxidation number. Reduction - the gain of electrons; decrease in oxidation number. Think paint ROLR The two processes occur simultaneously Redox Reactions: e - are from one substance to another. therefore: For the rusting of iron we said that Fe lost 3 e- but that O only gained 2 e -. What happened to the last e -? We must balance redox reactions for 4 Fe(s) + 3 O2(g) 2 Fe2O3(s) Fe (loose 3 e- each) = e - lost O (gain 2 e - each) = e - gained Oxidation and reduction always occur together. When one substance loses an electron (is oxidized), another substance must gain that electron (be reduced). Ch 5 Page 12

13 The substance that causes the reduction is a. The substance causes the oxidation is an. - The substance oxidized is the reducing agent. - The substance reduced is the oxidizing agent. Oxidizing Agent: causes oxidation undergoes reduction gains electrons becomes more negative (or less positive) Reducing Agent: causes reduction undergoes oxidation loses electrons becomes more positive (or less negative) Problem: Balance the following redox reactions. Identify what was oxidized, what was reduced, the oxidizing agent and the reducing agent. Cu(s) + Ag + (aq) Cu 2+ (aq) + Ag(s) is the reducing agent since it caused Ag + to be reduced to Ag. is the oxidizing agent because it caused Cu to be oxidized to Cu 2+. Ch 5 Page 13

14 Al (s) + Br 2 (l) AlBr 3 (s) Becomes Al (s) + Br 2 (l) AlBr 3 (s) Note: the substance oxidized the substance reduced the oxidizing agent the reducing agent are all on the side! Problem: Identify the substance oxidized, the substance reduced, the oxidizing agent and the reducing agent in each of the following: a) Cu 2+ + Fe (s) (aq) Cu (s) + Fe 2+ (aq) b) Mg (s) + Cl 2(g) 2 MgCl (s) c) 2 Al (s) + Cr 2 O 3(s) 2 Cr (s) + Al 2 O 3(s) Ch 5 Page 14