Chapter 4: Stoichiometry of Chemical Reactions. 4.1 Writing and Balancing Chemical Equations

Transcription

1 Chapter 4: Stoichiometry of Chemical Reactions 4.1 Writing and Balancing Chemical Equations A chemical equation represents or symbolizes a chemical reaction. o Substances are represents by their chemical formulas. o Relative amounts are indicated by numeric coefficients. The morphology or structure of a chemical equation: Note that an equation relates numbers of substances, not masses. Any number interpretation is valid: o Microscopic: equation gives the number of atoms and molecules. o Macroscopic: equation gives the number of moles of the substances. 1 Chem 180-Spring 2019

2 Stoichiometry requires a balanced chemical equation! o Dalton s Postulate: atoms are neither created nor destroyed in a chemical reaction (they are simply rearranged). o To balance a chemical equation, we need to achieve the same number and types of atoms on both sides of the equation, reactant & product. When given the reactants and products of a reaction, we may only change the coefficients to balance the equation! o Changing any of the chemical formulas changes the reaction. o The coefficients should be the smallest whole numbers that balance the equation. For now, the equations we balance are relatively simply and we can verify one is balanced by inspection: manually count the numbers of each element and see that the reactant numbers equal the product numbers. o Be sure to count atoms that appear in more than one formula. o A subscript only applies to the preceding element. o A subscript after a closing parenthesis applies to all the atoms inside the parentheses. o A coefficient applies to all the atoms following the coefficient. We can multiply coefficient by subscript to speed up our counting. o Note: If a formula is incorrect, you might not be able to balance! 2 Chem 180-Spring 2019

3 While balancing simple equations can involve some trial and error, systematic approaches can get the job done relatively quickly. o A balancing procedure might involve iteration (repetition) as balancing one element could unbalance another. o If you are rusty or unsure of yourself, the text s tabular approach helps organize the data. o I like to work from left to right balancing one element at a time. o Remember to keep polyatomic ions intact when balancing! count the N and O atoms on both sides change the coefficient of O on the left and recount change the coefficient of N on the left and recount Finished! (And always do one last recount to check!) 3 Chem 180-Spring 2019

4 N 2 + O 2 N 2 O 5 H 3 PO 4 + CaO Ca 3 (PO 4 ) 2 + H 2 O C 3 H 8 + O 2 CO 2 + H 2 O NO 2 + NH 3 N 2 + H 2 O Write and balance an equation for the combustion of thiosalicylic acid, C 7 H 6 O 2 S, used in the manufacture of indigo dyes. Start using state symbols in your equations: (s), (l), (g), (aq). You already know or can predict many of them. o All elemental metals except Hg are solid at room temperature (RT). o Elemental nonmetals are solids and gases, and one liquid, Br 2. o All ionic compounds are solid (b/c of strong ionic attractions). o Molecular compounds tend to be liquids, but light ones are gases. We may write reaction conditions above and below the arrow: 2 Ag 2 O(s) 4 Ag(s) + O 2 (g) 350 C CO(g) + 2 H 2 (g) CH 3 OH(g) 340 atm, ZnO, Cr 2 O 3 4 Chem 180-Spring 2019

5 When ionic reactants and products are involved, we can write three different equations to summarize the chemical changes taking place. o We see ionic equations a lot in lab! The three equations are: o Molecular (Formula): only uncharged compound formulas are shown. o Complete (Total) Ionic: (aq) strong electrolytes are dissociated. Strong electrolytes are soluble ionic compounds and strong acids. o Net Ionic: rewrite complete ionic and omit the spectator ions. Word eqn: When aqueous solutions of CaCl 2 and AgNO 3 are mixed, a reaction takes place producing aqueous Ca(NO 3 ) 2 and solid AgCl. Molecular Eqn: CaCl 2 (aq) + AgNO 3 (aq) Ca(NO 3 ) 2 (aq) + AgCl(s) CaCl 2 (aq) + 2 AgNO 3 (aq) Ca(NO 3 ) 2 (aq) + 2 AgCl(s) balanced! Complete Ionic Eqn: Ca 2+ (aq) + 2 Cl (aq) + 2 Ag + (aq) + 2 NO 3 (aq) Ca 2+ (aq) + 2 NO 3 (aq) + 2 AgCl(s) Net Ionic Eqn: 2 Cl (aq) + 2 Ag + (aq) 2 AgCl(s) Cl (aq) + Ag + (aq) AgCl(s) coefficients simplified! When carbon dioxide is dissolved in an aqueous solution of sodium hydroxide, the mixture reacts to yield aqueous sodium carbonate and liquid water. Write balanced molecular, complete ionic, and net ionic equations for this reaction. 5 Chem 180-Spring 2019

6 4.2 Classifying Chemical Reactions After studying the reactants and products of chemical reactions, chemists have found that many can be classified into one of several types: o Precipitation reaction: dissolved substances react to form solid product(s). o Acid-Base reaction: a hydrogen ion is transferred from one species to another. Also known as a Neutralization reaction. The hydrogen ion is transferred via a water molecule as the hydronium ion, H 3 O + (aq). o Oxidation-Reduction reaction: this will be examined later in the course. Several other classifications exist, but we will concentrate on these. Precipitation and acid-base reactions are two types of the doubledisplacement reaction in which two species swap their anions. o Generically, we can the double-displacement reaction as: AB + CD AD + BC o An example of the precipitation double-displacement reaction is: NaCl(aq) + AgNO 3 (aq) NaNO 3 (aq) + AgCl(s) o An example of the acid-base double-displacement reaction is: HCl(aq) + NaOH(aq) HOH(l) + NaCl(aq) In general, Acid + Base Water + Salt Given the reactants of a double displacement reaction; you should be able to: o Complete the Molecular Eqn (predict the products and their states), o Balance the Molecular Eqn, and o Write the balanced Complete Ionic and Net Ionic Eqns. Also identify the Spectator Ions. 6 Chem 180-Spring 2019

7 Precipitation Reactions Solubility rules are used to predict the states of the products, (aq) or (s). o You should use the following rules as they are provided with tests. o These rules are consistent with this in your text, although they are presented differently. Solubility guidelines for common ionic solids Follow these rules in the order listed. When you come to a rule that applies to your compound, STOP. Conflicts are thus avoided. 1. Salts of group 1 metal cations and the NH 4 + cation are soluble. 2. Nitrates, acetates, bicarbonates, chlorates, and perchlorates are soluble. 3. Chlorides, bromides, and iodides are soluble, except for those of silver, lead(ii), and mercury(i). 4. Sulfates are soluble, except for those of Ca 2+, Sr 2+, and Ba Carbonates, phosphates, sulfides, oxides, chromates, and hydroxides are insoluble (sulfides of group 2 cations and hydroxides of Ca 2+, Sr 2+, and Ba 2+ are slightly soluble). 6. If Rules 1 6 do not apply, the salt is probably insoluble. The steps to writing the Molecular, Complete Ionic, and Net Ionic Eqns. o To complete the Molecular equation for the reaction, apply double displacement to predict the products, and use the solubility rules to predict product states. Then balance the equation. There should be no ionic species in this equation that is, show no charges. o Write the balanced Complete Ionic equation by re-writing the Molecular equation, but substitute the strong electrolytes (soluble salts and strong acids) with their ions in solution. Use H 3 O + not H + for the strong acids! Be careful with subscripts and coefficients! o Identify the Spectator Ions in the Complete Ionic equation. Write the balanced Net Ionic equation by re-writing the Ionic equation, but omit the spectator ions. 7 Chem 180-Spring 2019

8 Complete and write the balanced molecular, complete ionic, and net ionic equations for: K 3 PO 4 (aq) + FeBr 2 (aq) If no ppt is formed, indicate this by writing no reaction or NR in the molecular eqn: <reactants> No Reaction <reactants> NR Acid-Base Reactions An acid is a molecular substance that dissolves in and reacts with water to produce H 3 O + (aq) ions. We call this process ionization. (Note the distinction from soluble salts whose ions dissociate.) o Strong acids are 100% ionized in aqueous solution: 100% HBr(aq) + H 2 O(l) H 3 O + (aq) + Br (aq) 8 Chem 180-Spring 2019

9 o Weak acids are not completely ionized; their reaction with water is reversible. HOOC-COOH(aq) + H 2 O(l) H 3 O + (aq) + HOOC-COO (aq) A base is an ionic (most commonly for us) or molecular substance that produces OH (aq) ions in water. o Strong bases are soluble metal hydroxides which generate hydroxide ions by dissociation: 100% KOH(s) K + (aq) + OH (aq) o Weak bases are molecular compounds that dissolve in and react with water to produce hydroxide ions by reversible ionization: NH 3 (aq) + H 2 O(l) NH 4 + (aq) + OH (aq) You should know these common strong acids and strong bases: 9 Chem 180-Spring 2019

10 To complete and write the three balanced equations for acid-base reactions, we follow the same steps we used for precipitation reactions. However, instead of forming a ppt, we form water! o When predicting the products, use H + from the acid. o When writing the ionic equations for strong acids, use H 3 O +. We implicitly add water on the reactant side to form H 3 O +, so do not forget to add H 2 O on the product side to keep the eqn balanced. o Weak acids are shown as molecules, not ionized, as reactants. Write the balanced Molecular, Complete Ionic, and Net ionic eqns for this strong acid strong base neutralization reaction: HNO 3 (aq) + Ba(OH) 2 (aq) Write the balanced Molecular, Complete Ionic, and Net ionic eqns for this weak acid strong base neutralization reaction: CH 3 COOH(aq) + NaOH(aq) 10 Chem 180-Spring 2019

11 4.3 Reaction Stoichiometry Now things get interesting! Solving stoichiometric problems can be challenging. Just think of it as being given an opportunity to excel! Stoichiometry refers to the quantitative calculations involving the relative amounts of reactants and products in a chemical reaction. Relative amounts are determined by the coefficients from the balanced chemical equation for the reaction. The fundamental unit for such calculations is the mole. Typical steps in stoichiometry are outlined below. The red box shows that all calculations include mole conversion from one species in the reaction to another. If we are not given moles of A, we must first find the moles of A If we are not asked for moles of B, we must then convert moles of B to the requested units. 1 mol A MM of A Step 2 MM of B 1 mol B y mol B x mol A = 1 mol A N A x and y are coefficients from the equation N A 1 mol B 11 Chem 180-Spring 2019

12 Let s dive right in! Potassium chlorate decomposes to solid potassium chloride and oxygen gas when heated. How many moles of O 2 are produced from the decomposition of 1.76 moles of potassium chlorate? How many grams of magnesium nitride are produced by the reaction of 3.82 g Mg with an excess of N 2? How many mg of H 2 are produced when one drop (0.05 ml) of hydrochloric acid solution (1.14 g/ml, 28.0% HCl by mass) reacts with an excess of aluminum to produce hydrogen gas and aqueous aluminum chloride. 12 Chem 180-Spring 2019

13 How many milliliters of M AgNO 3 (aq) are required to react completely with 175 ml of M K 2 CrO 4 (aq)? What mass of Ag 2 CrO 4 (s) is formed? 4.4 Reaction Yields So far, our stoichiometry has looked at reactions in which the stoichiometric amounts of reactants are present, that is, the relative amounts specified by the coefficients in the equation. (Or we were told one reactant was in excess.) But often reactants are not present in stoichiometric amounts, and one reactant runs out before the other. This limits the amount of product(s) formed. 13 Chem 180-Spring 2019

14 H 2 (g) + Cl 2 (g) 2 HCl(g) Identifying and solving limiting reactant problems: o In limiting reactant problems, you are given quantities of both reactants. o Pick a product, and convert each reactant to this product (moles or mass); that is, solve two stoichiometric equations. o The reactant that forms the smaller amount of product is the limiting reactant. o Suggestion: use the amount of the limiting reactant for other equation stoichiometry g H 2 and 154 g O 2 are allowed to react forming liquid water. Which gas is the limiting reactant, and what mass of the excess reactant remains? 14 Chem 180-Spring 2019

15 How many grams of barium sulfate will be formed from g of barium nitrate and g of sodium sulfate? What mass of the excess reactant is left over? Ba(NO 3 ) 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + 2 NaNO 3 (aq) Percent yield (% yield) = (actual yield / theoretical yield) * 100% What is the percent yield if the reaction of 25.0 g P 4 with 91.5 g Cl 2 produces 104 g PCl 3? 15 Chem 180-Spring 2019

16 How many grams of hydrogen chloride can be produced from g of hydrogen and 50.0 g of chlorine? H 2 (g) + Cl 2 (g) 2 HCl(g) If the theoretical yield for a reaction is 72.3 g of CCl 4, and 65.0 g of carbon tetrachloride was actually obtained, calculate the percent yield. (See example 9.13 on page 188 of your text for the complete calculation.) A sample of an alloy of iron and aluminum has a mass of 2.05 g. When the allow reacts with hydrochloric acid, g of hydrogen gas are collected. What is the percent by mass of iron in the alloy? 16 Chem 180-Spring 2019