Quantitative Relationships in Chemical Reactions Chapter 7

Transcription

1 Quantitative Relationships in Chemical Reactions Chapter 7 The burning of charcoal releases heat (thermal energy) that grills our food. But the combustion of charcoal and fossil fuels also releases CO 2 (g), which affects our environment. The amount of CO 2 (g) and the thermal energy released are quantities discussed in this chapter.

2 Review: A chemical reaction is a rearrangement of atoms. The total number of atoms and the total charge must be conserved in any chemical reaction. For example: Ammonia is burned in air. 4 NH 3 (g) + 5 O 2 (g) 4 NO(g) + 6 H 2 O(g)

3 How to Read Chemical Equations

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5 Question: Does a balanced chemical equation give us the amounts of reactants and products that are present at the time? For example: Ammonia is burned in air. 4 NH 3 (g) + 5 O 2 (g) 4 NO(g) + 6 H 2 O(g)

6 Question: Does a balanced chemical equation give us the amounts of reactants and products that are present at the time? For example: Ammonia is burned in air. 4 NH 3 (g) + 5 O 2 (g) 4 NO(g) + 6 H 2 O(g)

7 Information Given by the Chemical Equation Coefficients show the atom/molecule/ formula unit ratio and the mole ratio of the reactants and products. Coefficients are sometimes called molar coefficients.

8 Consider the production of ammonia gas from its constituent elements. Write the balanced chemical equation. N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

9 Example 7-1, page 215 Calculate the number of moles of ammonia that can be produced from 5.00 mol of H 2 (g). N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

10 Example 7-2, page 216 Calculate the number of moles of ammonia that can be produced from 33.6 g of N 2 (g). N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

11 Example 7-3, page 216 Calculate the mass, in grams, of H 2 (g) that is needed to produce 119 g of NH 3 (g). N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

12 Example 7-4, page 217 How many molecules of N 2 are needed to react with 17.0 g of H 2? N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

13 FeS2, also known as fool s gold

14 Example 7-5, page 218 Some sulfur is present in coal in the form of pyrite (FeS 2 ; also known as fool s gold ). When it burns, it pollutes the air with the combustion product SO 2, as shown by the following balanced equation. 4 FeS 2 (s) + 11 O 2 (g) 2 Fe 2 O 3 (s) + 8 SO 2 (g) Calculate the mass, in grams, of SO 2 (g) produced by the combustion of 38.8 g of FeS 2.

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16 If a carpenter had 2 tabletops and 7 table legs, he could only build 1 four-legged table. The number of table legs is the limiting factor in the construction of four-legged tables. Similarly, in chemistry, the amount of product made in a chemical reaction may be limited by the amount of one or more of the reactants.

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19 Suppose you have a part-time job in a sandwich shop. One very popular sandwich is always made as follows: 2 slices bread + 3 slices meat + 1 slice cheese sandwich Assume that you come to work one day and find the following quantities of ingredients: 8 slices bread 9 slices meat 5 slices cheese How many sandwiches can you make? What will be left over?

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21 Limiting Reactant (Reagent) When reactants are mixed in exactly the mass ratio determined from the balanced equation, the mixture is said to be stoichiometric. If specific amounts of each reactant are mixed, the reactant that produces the least amount of product is called the limiting reactant. (or limiting reagent)

22 Consider the following container of N 2 (g) and H 2 (g):

23 In this case, the mixture of N 2 and H 2 contained just the number of molecules needed to form NH 3 with nothing left over. This is a stoichiometric mixture.

24 Now consider another container of N 2 (g) and H 2 (g):

25 In this case, H 2 is the limiting reactant. That is, the H 2 molecules are used up before all the N 2 molecules are consumed.

26 Limiting Reactant Example 1 (not in e-book) Nitrogen gas can be prepared by passing gaseous ammonia over solid copper(ii) oxide at high temperatures. The other products of the reaction are solid copper and water vapor. A sample containing 18.1 g of NH 3 (g) is reacted with 90.4 g of CuO(s). (a) Write the balanced chemical equation for the reaction. (b) Identify the limiting reactant. Justify your answer with a calculation. (c) Calculate the mass, in grams, of N 2 (g) that is formed.

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29 Limiting Reactant Example 2 (not in e-book) Methane is the main component of marsh gas. Heating methane in the presence of sulfur produces liquid carbon disulfide and gaseous hydrogen sulfide. In a laboratory, 120. g of methane is reacted with an equal mass of sulfur. (a) Write the balanced chemical equation for the reaction. (b) Identify the limiting reactant. Justify your answer with a calculation. (c) Calculate the mass, in grams, of carbon disulfide that is formed.

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32 THE CONCEPT OF PERCENT YIELD Actual Yield: The measured amount of product obtained in any reaction. Theoretical Yield: The calculated amount of product that would be obtained if all the reactant were converted to a given product. Percent Yield: The ratio of the two (actual : theoretical) expressed as a percentage. % yield = actual yield theoretical yield 100%

33 Problem 7-52, page 237 Gaseous ammonia reacts with oxygen to produce gaseous nitrogen monoxide and liquid water. (a) Write the balanced chemical equation. (b) When an 80.0 g quantity of NH 3 (g) is mixed with 200. g of O 2 (g), a 40.0 g quantity of NO(g) is obtained. Calculate the percent yield based on the limiting reactant.

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35 Example 7-8, page 226 In a given experiment, a 4.70 g quantity of H 2 is allowed to react with excess N 2. A 12.5 g quantity of NH 3 is obtained. Calculate the percent yield. N 2 (g) + 3 H 2 (g) 2 NH 3 (g)

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37 Example 7-9, page 226 When 25.0 g of zinc is added to an aqueous silver nitrate solution, the percent yield of silver is 72.3%. Calculate the mass of silver that is formed.

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39 Heat Energy in Chemical Reactions Thermochemical Equation: A balanced equation that includes heat energy. Can be represented in either of two ways 1) Heat is shown separately from the balanced equation using the symbol ΔH, delta H, which means change in enthalpy. For example, ΔH < 0, reaction is exothermic ΔH > 0, reaction is endothermic 2 H 2 (g) + O 2 (g) 2 H 2 O(l) ΔH = 572 kj

40 Heat Energy in Chemical Reactions Thermochemical Equation: A balanced equation that includes heat energy. Can be represented in either of two ways 2) Heat energy is shown as if it were a reactant or product. For example, 2 H 2 (g) + O 2 (g) 2 H 2 O(l) kj (exothermic) N 2 (g) + O 2 (g) kj 2 NO(g) (endothermic)

41 Change in Enthalpy Example 1 (not in e-book) The overall reaction in a commercial heat pack can be represented as 4 Fe(s) + 3 O 2 (g) 2 Fe 2 O 3 (s) ΔH = 1652 kj (a) How much heat is released when 4.00 mol iron is reacted with excess O 2? (b) How much heat is released when 1.00 mol Fe 2 O 3 is produced? (c) Calculate the change in enthalpy that accompanies the reaction of 1.00 g iron with excess O 2. (d) How much heat is released when 10.0 g Fe and 2.00 g O 2 are reacted?

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44 Additional problem from the e-book. Page 236 # 45 The thermite reaction was discussed in Problem For the balanced equation, ΔH = 850 kj. What mass of aluminum is needed to produce 35.8 kj of heat energy? 2 Al(s) + Fe 2 O 3 (s) Al 2 O 3 (s) + 2 Fe(l)

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