loosely based on Chap 3 Sec 5 of Jespersen 7th edition Chemical Equations loosely based on Chap 3 Sec 5 of Jespersen 7th edition Dr. C. Yau Spring 2015 1 1

Writing Balanced Chemical Equations Read the question carefully and figure out which is/are the reactants and which is/are the products. Include physical states as much as possible. Balance the equation so that there are the same number of atoms of each element on the right side as on the left side by changing ONLY the coefficients of the equation. (Remember conservation of matter: No atoms are created nor destroyed in a chemical reaction.) 2

Do Practice Exercises 26 & 27 p.131 Example 3.13 p.130 Sodium hydroxide and phosphoric acid, H3PO4, react as aqueous solutions to give sodium phosphate and water. The sodium phosphate remains in solution. Write the balanced equation for this reaction. Do Practice Exercises 26 & 27 p.131 3

Write the balanced equation for the combustion of decene, C10H20 (l). Example Write the balanced equation for the combustion of decene, C10H20 (l). Note: Combustion (burning) always means a reaction of oxygen gas. Combustion of hydrocarbons (compound with only H and C) with excess oxygen gives CO2 and H2O. REMEMBER THIS! 4

Write the balanced equation for the combustion of decane, C10H22 (l). Example Write the balanced equation for the combustion of decane, C10H22 (l). Practice at home: (l) = liquid Write the balanced equations for the combustion of a) nonene, C9H18 (l) b) combustion of nonane, C9H20 (l) and c) nonanone C9H18O (l). 5

Working with Chemical Equations Be able to translate equations into words, both at the particulate level and at the macroscopic level. Ca (s) + 2H2O (l) Ca(OH)2 (aq) + H2 (g) particulate level: One Ca atom reacted with 2 molecules of H2O to form one f.u. of Ca(OH)2 and one molecule of H2. macroscopic level: One mol Ca atoms reacted with 2 mol of H2O molecules to form one mol Ca(OH)2 and one mol H2 gas.

NaOH(aq) + H3PO4 (aq) Na3PO4 (aq) + H2O (l) Example 3.14 p.132 How many moles of sodium phosphate can be made from 0.240 mol of sodium hydroxide by the following unbalanced reaction? NaOH(aq) + H3PO4 (aq) Na3PO4 (aq) + H2O (l) Useful tips: The coefficients of a balanced equation gives us conversion factors for moles to moles. Useful tips: When working with more than one substance in dim.analysis, you MUST GO THRU MOLES! Do Practice Exercises 28 & 29 p. 133 7

What does “go through moles” mean? It means somewhere in your dim. analysis you will be using a conversion of... Remember it is NOT...

Example 3.15 p. 134 Portland cement is a mixture of the oxides of calcium, aluminum and silicon. The raw material for its calcium oxide is calcium carbonate, which occurs as the chief component of a natural rock, limestone. when calcium carbonate is strongly heated it decomposes. One product, carbon dioxide, is driven off to leave the desired calcium oxide as the only other product. A chemistry student is to prepare 1.50 x 102 g of calcium oxide in order to test a particular "recipe" for Portland cement. How many grams of calcium carbonate should be used, assuming that ALL will be converted? 9

The heat from this reaction was used in welding train tracks. Example 3.16 p.135 The thermite reaction is one of the most spectacular reactions of aluminum with iron(III) oxide by which metallic iron and aluminum oxide are made. So much heat is generated that the iron forms in the liquid state. The heat from this reaction was used in welding train tracks. A certain welding operation requires at 86.0 g of iron each time a weld is made. What is the minimum mass, in grams, of iron(III) oxide that must be used for each weld? Also calculate how many grams of aluminum are needed. Do Practice Exercises 30 & 31 p. 136 10

The Thermite Reaction Fe2O3 (s) + 2Al (s)  2Fe (s) + Al2O3 (s) + HEAT The reaction requires ignition at a very high temperature. One method of ignition is to light a piece of Mg. Another method of ignition is to add conc sulfuric acid to a mixture of KClO3 and sugar. For those who took CHEM 108 on the Catonsville campus may remember the precautions when you worked with potassium chlorate in one of your experiments! http://www.youtube.com/watch?v=AlGFCWBeyDk&NR=1