Chemical Reactions and Equations

Chemical Reactions and Equations 5-1 5.1 What is a Chemical Reaction? A chemical reaction is a chemical change. A chemical reaction occurs when one or more substances is converted into one or more new substances. change : a substance that we start with that undergoes a : a new substance that forms during the reaction Products differ from reactants only in the arrangement of their component atoms. Physical evidence of a chemical reaction MAY include: - Change in color - Formation of a gas - Production of light - Formation of a solid - Change in heat 5-2 5.3 Writing Chemical Equations : a symbolic representation of a chemical reaction. A equation will show: the formulas for reactants and products the physical states of each substance - subscripts after formulas - (s), (l), (g) or (aq) relative numbers of reactants and products - expressed as coefficients in front of formulas special conditions required for the reaction - Written over arrow 5-3 1

5.3 Writing Chemical Equations When hydrogen gas is ignited in the presence of oxygen, an explosive reaction occurs producing gaseous water molecules. Is mass conserved? 5-4 5.3 Writing Chemical Equations : The number of atoms of each element is the same in the products as in the reactants. When a powdered mixture of aluminum metal and iron(iii) oxide is heated, it reacts to form liquid iron metal and aluminum oxide. Al (s) + Fe 2 O 3(s) Al 2 O 3(s) + Fe (l) The reaction is balanced with coefficients: 5-5 5.3 Writing Chemical Equations 1. Identify the reactants and products write their correct formulas Watch for diatomic molecules 2. Write a skeletal equation include physical states. 3. Change coefficients one at a time until the atoms of each element are balanced. start with the elements that occur least often save O & H for last if polyatomic ions are not broken up in a reaction, treat them as a unit 4. Double check balance are all the elements balanced? can coefficients be reduced? 5-6 2

5.3 Writing Chemical Equations When methane burns in the presence of oxygen, carbon dioxide and water are formed. Write and balance this reaction. 1-Identify the reactants & products and write their formulas: - Reactants: methane, CH 4 & oxygen, O 2 - Products: carbon dioxide, CO 2 & water, H 2 O 2-Write a skeletal equation including physical states: CH 4(g) + O 2(g) CO 2(g) + H 2 O (g) 3-Change coefficients one at a time until the atoms of each element are balanced. - First, we look at the carbon atoms. Already balanced. - Next, we look at the hydrogen atoms. CH 4(g) + O 2(g) CO 2(g) + 2 H 2 O (g) - Finally, we look at the oxygen atoms. CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (g) 4-Double check 5-7 : Balancing Equations Balance the following equations: A. Zinc reacts with aqueous hydrochloric acid to form aqueous zinc chloride and hydrogen B. Potassium chlorate decomposes into potassium chloride and oxygen C. Aluminum acetate solution reacts with potassium sulfate solution to form aqueous potassium acetate and solid aluminum sulfate D. Liquid hexane (C 6 H 14 ) reacts with oxygen to form carbon dioxide and water 5-8 Chemical reactions are rearrangements of atoms Some reactions fall within certain patterns, making it possible to anticipate products. 5-9 3

: Classifying Reactions Classify each of the following reactions as: decomposition, D combination, C single-displacement, SD double-displacement, DD 1.NH 3(g) + HCl (g) NH 4 Cl (s) 2.CuCl 2(aq) + Na 2 S (aq) CuS (s) +2 NaCl (aq) 3.NiSO 3(s) NiO (s) + SO 2(g) 4.Ca (s) + PbCl 2(aq) CaCl 2(aq) + Pb (s) 5-10 : Classifying Reactions 5-11 : Classifying Reactions 5-12 4

: Classifying Reactions 5-13 : Classifying Reactions 1. AlF 3(aq) + 3H 2 O (l) Al(OH) 3(s) + 3HF (aq) 2. BaCl 2(aq) + Na 2 SO 4(aq) BaSO 4(s) + 2NaCl (aq 3. Ca(OH) 2(s) CaO (s) + H 2 O (g) 4. Ca (s) + 2H 2 O (l) Ca(OH) 2(aq) + H 2(g) 5. CaO (s) + CO 2(g) CaCO 3(s) 6. Cl 2(aq) + 2NaI (aq) 2NaCl (aq) + I 2(aq) 7. Cu (s) + 2AgNO 3(aq) Cu(NO 3 ) 2(aq) + 2Ag (s) 8. Fe (s) + 2HCl (aq) FeCl 2(aq) + H 2(g) 9. H 2 SO 3(aq) H 2 O (l) + SO 2(g) 10. 2HgO (s) 2Hg (l) + O 2(g) 5-14 (CD C + D): a single compound breaks down into elements and/or simpler compounds. 2HgO (s) 2Hg (l) + O 2(g) Before After 5-15 5

5-16 : Predicting Products Predict the products and balance the equation for the following decomposition reaction. Platinum(IV) chloride decomposes to its elements 5-17 (A + B AB): two substances combine to form a single compound. 2 CO (g) + O 2(g) 2 CO 2(g) 5-18 6

Combination Reactions 5-19 : Predicting Products We can often predict the products of combination reactions involving metal and nonmetal element reactants because their products are ionic compounds. Predict the product and balance the reaction between aluminum and liquid bromine: Figure 5.16 5-20, (A + CD AD + C): a free element displace another element from a compound to produce a new compound and new free element. Cu (s) + 2AgNO 3(aq) Cu(NO 3 ) 2(aq) + 2Ag (s) 5-21 7

2Al (s) + Fe 2 O 3(s) Al 2 O 3(s) + 2Fe (s) Fe (s) + CuO (s) FeO (s) + Cu (s) What happens to the charge on the metal doing the displacing? 5-22 : Predicting Products Does copper displace silver or does silver displace copper? Which is more active? What does this mean? 5-23 5.4 Predicting Chemical Reactions Relative Activities: Cu > Ag Fe > Cu Al > Fe : a list of elements in order of decreasing reactivity used to predict if a single displacement reaction will occur. 5-24 8

: SD Reactions Will magnesium displace hydrogen? Mg (s) + 2HCl (aq) MgCl 2(aq) +H 2(g) Will aluminum displace hydrogen? 2Al (s) + 6HCl (aq) 2AlCl 3(aq) + 3H 2(g) Will iron displace zinc? 2Fe (s) + 3ZnCl 2(aq) 2FeCl 3(aq) +3Zn (s) Figure 5.21 5-25 : (CD + EF CF + ED) two compounds exchange ions to form two new compounds. There are three types: Precipitation Reactions Gas Formation Reactions Acid-Base Neutralization Reactions 5-26 : a double displacement reaction in which the formation of an insoluble solid called a precipitate is the driving force. BaCl 2(aq) + Na 2 SO 4(aq) BaSO 4(s) + 2NaCl (aq) What is the white solid in the test tube? 5-27 9

Which ions undergo a chemical change? How can we predict this? Na 2 SO 4 (aq) + BaCl 2 (aq) BaSO 4(s) + 2NaCl (aq) 5-28 Cmpds of alkalai metals, nitrates, acetates & ammonium ions Cmpds of chloride, bromide & iodide ions - except lead(ii), silver & mercury(i) ions Cmpds of sulfate ion - except strontium, barium, lead(ii) and mercury(i) ions Cmpds of hydroxide ion - except calcium, barium and strontium ions (only slightly) Cmpds of sulfide ion - except alkaline earth metals Cmpds of carbonate & phosphate ions 5-29 : Predicting Precipitation Reactions Predict the products of the reaction of lead(ii) nitrate solution and potassium iodide solutions and balance the equation: 5-30 10

: Predicting Precipitation Reactions Given that a precipitate forms when a barium chloride solution is mixed with a sodium chromate solution, predict the products and write a balance equation for this reaction. 5-31 : a double displacement reaction in which a gas is formed, which helps drive the reaction to completion. CaCO 3(s) + 2HCl (aq) CaCl 2(aq) + CaCO 3(s) + 2HCl (aq) CaCl 2(aq) + Metal carbonates react with acids to form, which decomposes to 5-32 A similar reaction occurs when a metal sulfite reacts with an acid MgSO 3(s) + 2HCl (aq) MgCl 2(aq) + MgSO 3(s) + 2HCl (aq) MgCl 2(aq) + Metal sulfites react with acids to form, which decomposes to 5-33 11

: A double displacement reaction in which the driving force is the formation of H 2 O. An acid reacts with a base to form an ionic compound (a salt) and water. HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O (l) Acid Base Salt Water 5-34 : Acid Base Neutralizations Calcium oxide is the white powder, lime. When added to water, it makes slaked lime, which is a solution of the base calcium hydroxide. If sulfuric acid is added to slaked lime, what products form? Write the balanced chemical equation for this reaction. 5-35 : Acid Base Neutralizations Complete and balance the reaction of hydrochloric acid with magnesium hydroxide: 5-36 12

: a 5 th type of reaction that does not always fall into one of the earlier categories. In a combustion reaction, a fuel reacts with oxygen in a reaction that burns to produce a flame. If the fuel contains carbon, most often carbon dioxide will be formed If the fuel contains hydrogen, most often water will be a product. CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (g) Write a balanced reaction for the combustion of each of the following hydrocarbons: 1. Propane, C 3 H 8 : Predicting Chemical Reactions 2. Octane, C 8 H 18 5-38 5.5 Representing Reactions in Aqueous Sol n : a form of chemical equation in which substances are represented as if they exist as molecules. ME are useful when using reaction patterns to predict products. When reactions occur in water, soluble ionic compounds exist as separate ions, not grouped together with each other as might seem so from the equation. AgNO 3(aq) + NaCl (aq) AgCl (s) + NaNO 3(aq) Sodium nitrate does not actually form - because it is soluble 5-39 13

5.5 Representing Reactions in Aqueous Sol n separate ions: AgNO 3(aq) + NaCl (aq) AgCl (s) + NaNO 3(aq) : shows soluble ionic compounds as Ag + (aq)+ NO 3 (aq) + Na + (aq)+ Cl (aq) AgCl (s) + Na + (aq)+ NO 3 (aq) Insoluble compounds (s), liquids (l) and gases (g) do not exist as ions and do not get separated in an ionic equation. : ions that occur on both sides of the equation. They remain unchanged through the reaction What are the spectator ions in the above ionic equation? 5-40 5.5 Representing Reactions in Aqueous Sol n Ag + (aq)+ NO 3 (aq) + Na + (aq)+ Cl (aq) AgCl (s) + Na + (aq)+ NO 3 (aq) Spectator ions: Na + & NO 3 - the spectator ions. : An ionic equation that leaves ou Ag + (aq) + Cl (aq) AgCl (s) 5-41 : ME, IE, & NIE Barium sulfate, used in the white pigment lithopone, can be prepared by mixing solutions of barium chloride and sodium sulfate. Write balanced molecular, ionic, and net ionic equations for this reaction. 5-42 14

: ME, IE, & NIE Write the net ionic equations from the following molecular equations: 1. Cu (s) + 2AgNO 3(aq) Cu(NO 3 ) 2(aq) + 2Ag (s) 2) 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2(g) 5-43 : ME, IE, & NIE What is the net ionic equation for the reaction of solutions of potassium chloride and sodium nitrate? 5-44 15