Quantities in Chemical Reactions

Transcription

1 Quantities in Chemical Reactions The Meaning of a Balanced Equation C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O (g) The balanced equation tells us: 1 molecule of propane reacts with 5 molecules of oxygen 3 molecules of carbon dioxide and 4 molecules of water are produced. The balanced equation ALSO tells us: 1 mole of propane reacts with 5 moles of oxygen 3 moles of carbon dioxide and 4 moles of water are produced

2 6.1 The Meaning of a Balanced Equation : The process of determining the amount of a reactant or product from another reactant or product based on their relationship in a balanced equation. Any problem that gives quantity of one chemical and asks for quantity of another. - have a - use from a balanced equation Mole-Mole Conversions C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O (g) A mole ratio is used to relate the number of moles of one reactant or product to another. Mole ratios are obtained from the coefficients in the balanced equation. For example, the mole ratio of O 2 to C 3 H 8 is 5:1 or:! #$% & ' ( #$% ) * +, The mole ratio of O 2 to C 3 H 8 allows us to calculate the amount of O 2 that will react with any amount of C 3 H 8 that reacts. The mole ratio is used as a conversion factor in a dimensional analysis equation. If 0.40 mol C 3 H 8 reacts: 6-4 2

3 : Mole-Mole Conversions Benzene (C 6 H 6 ) burns in air according to the following equation: 2 C 6 H 6(g) + 15 O 2(g) 12 CO 2(g) + 6 H 2 O (g) 1. What is the mole ratio of O 2 to C 6 H 6? 2. How many moles of O 2 are required to react with each mole C 6 H 6? 3. How many moles of O 2 are required to react with 0.38 mole of C 6 H 6? 6-5 : Mole-Mole Conversions Benzene (C 6 H 6 ) burns in air according to the following equation: 2 C 6 H 6(g) + 15 O 2(g) 12 CO 2(g) + 6 H 2 O (g) 1. What is the mole ratio used to determine the moles of CO 2 that will be produced when 2.3 mol O 2 reacts? 2. How many moles of CO 2 will be produced? 6-6 3

4 : Mole Ratios How many moles of Cl 2 are required to prepare 0.63 mol AlCl 3? 2 Al (s) + 3 Cl 2(g) 2 AlCl 3(g) Mass-Mass Conversions We don t measure reactants and products in moles, but we commonly measure their mass. The balanced equation gives us mole ratios, NOT mass relationship. How do we convert grams of reactant or product to moles? A + BX B + AX MM Mass A A Mole ratio MM Moles A Moles B B Mass B : What mass of chlorine gas is required to react with 9.20 grams of sodium? 2 Na (s) + Cl 2(g) 2 NaCl (s) 6-8 4

5 : Mass-Mass Conversions 2 H 2(g) + O 2(g) 2 H 2 O (g) If g of hydrogen was used in the liftoff during a shuttle launch, then: 1. What mass of oxygen gas was consumed? 2. What mass of water vapor was produced? 6-9 : Mass-Mass Conversions When aluminum metal is exposed to oxygen gas, a coating of aluminum oxide forms on the metal according to the reaction: 4 Al (s) + 3 O 2(g) 2 Al 2 O 3(s) If a sheet of pure aluminum gains g of mass when exposed to air: 1. What mass of O 2 reacted with the Al? 2. What mass of Al is reacted? 3. What mass of Al 2 O 3 is formed?

6 6.4 Limiting Reactants : The reactant which runs out before the other(s) and limits the amount of product that can be formed. Can you identify the excess reactant in each case? Limiting Reactants What is the limiting reactant? Cu (s) + 2 AgNO 3(aq) Ag (s) + Cu(NO 3 ) 2(aq)

7 6.4 Limiting Reactants In a chemical reaction, the balanced equation tells us the relative number of molecules (or moles) that combine in the reaction. If reactants are not present in this ratio, then there will be a limiting reactant and excess of the other reactant : Limiting Reactants Complete the after picture. 1. How many H 2 O molecules form? 2. What is the limiting reactant? 3. What is in excess? 2 H 2(g) + O 2(g) 2 H 2 O (g)

8 6.4 Limiting Reactants If a problem provided quantities of more than one reactant, then you must determine the limiting reactant before solving the problem. 1. Calculate the amount of product that can be produced, using the information provided for each of the reactants. 2. Compare the calculated amount of product produced, selecting the LEAST amount The reactant that produces the least amount of product is the limiting reactant The other reactant(s) are the excess reactant(s) : What is the maximum mass of AlCl 3, that can be produced if 0.40 g Al is mixed with 0.60 g Cl 2? 2 Al (s) + 3 Cl 2(g) 2 AlCl 3(s) Percent Yield : the predicted amount of product calculated using stoichiometry. : The amount of product actually obtained in the lab different than theoretical yield due to errors in procedure or limitations of reaction. usually less than the theoretical yield but can be more

9 6.5 Percent Yield : describes how much of a product is actually obtained relative to the amount that should form assuming complete reaction of the limiting reactant. -./ = :.;/ <=>>% : A student was synthesizing aspirin in the laboratory. Using the amount of limiting reactant, she calculated the amount of aspirin that should form as 8.95 g. When she weighed her aspirin product on the balance, its mass was 7.44 g Energy Changes When methane reacts with oxygen when you use a gas stove, it s obvious that an energy change is occurring. Heat is released to the surroundings and is used to heat water and cook food. All chemical and physical changes are accompanied by energy changes

10 6.6 Energy Changes : Energy is neither created nor destroyed but rather transferred or transformed. Energy released in a reaction, is transferred to surroundings. Amount of energy released is equal to amount absorbed by surroundings. Energy can also be converted from one type to another. 6.6 Energy Changes Efficiency is the amount of useful work that is achieved from an energy conversion. No transfer or transformation is 100% efficient

11 6.6 Energy Changes Chemical reactions involve energy changes Most chemical reactions are exothermic they release energy to the surroundings. 2 H 2(g) + O 2(g) 2 H 2 O (g) Some reactions are endothermic, absorbing energy from the surroundings and cooling them Energy Changes Is energy absorbed or released by the reaction in each case? What happens to the temperature of the surroundings?

12 6.6 Energy Changes Heat is energy that is transferred between two objects because of a difference in their temperatures. In chemistry, quantities of energy (and heat) are usually expressed with units of joules (J) or calories (cal). 1 cal = J Nutritionists use the Calorie (Cal) which is a kilocalorie (kcal), or 1000 cal. 1 Cal = 1000 cal = 1 kcal : One serving of this cereal has 190 Calories. What is the energy content in units of joules? Energy Changes When heat is added to a substance, the substance increases in temperature. How much the temperature increases is dependent upon the identity of that substance. : The amount of heat required to increase the temperature of 1 gram of a substance by 1 degree celsius. Specific heat units: J/(g ºC) or cal/(g ºC) Specific heat depends on the substance and the physical state of the sample. The specific heat of water (l) is J/(g ºC). Other values are given in Table

13 6-25 : Specific Heat 1. How much heat is required to increase the temperature of 1.00 g of liquid water by 10.0 ºC? 2. How much heat is required to increase the temperature of g of water by 1.00 ºC?

14 : Specific Heat heat = mass specific heat temperature change q = m C ΔT 1. How much heat must be added to 15.0 g of water to increase its temperature from 25.0 to 75.0 ºC? 2. What is the heat change when 15.0 grams of water cools from 75.0 to 25.0 ºC? Energy Changes q system + q surroundings = 0 : The specific heat of aluminum is J/(g ). If 156 g of aluminum at 75.0 is cooled to 25.5, 1. how much heat is transferred? 2. What is the sign of q, and what is its significance?

15 : Specific Heat If a 1 kj quantity of heat was added to each of the following 50.0-gram substances, which would increase in temperature by the greatest amount? Aluminum: J/(g ºC) Copper: J/(g ºC) Lead: J/(g ºC) Heat Changes in Chemical Reactions : devise used to determine the heat change of a system by measuring the heat change of its surroundings. q system + q surroundings = 0 In this calorimeter, an insulated cup is used so the surroundings is limited to what is inside the cup

16 : Calorimetry A 92.0-g piece of copper pipe is heated and then placed into this calorimeter with g of water at C. The final temperature of the mixture is C. What is the heat change (q) of the pipe? 6-31 : Calorimetry A sample of a metal alloy is heated and then placed in g of water held in a calorimeter at 22.5 C. The final temperature of the water is 29.0 C. Was the initial temperature of the alloy greater than or less than the initial temperature of the water? What is the heat change of the alloy?

17 6.7 Heat Changes in Chemical Reactions The heat change for a chemical reaction can be determined in the same way as the heat change for the piece of copper pipe, if the reaction takes place in solution. q reaction + q surroundings = 0 If the reaction does not take place in solution, such as a combustion reaction, then a bomb calorimeter is required. : When an exothermic reaction takes place, the water in the calorimeter absorbs the heat from the reaction and increases in temperature. q rxn + q cal = 0 q rxn = q cal 6-33 : Heat Changes in Chemical Reactions The balanced equation for the combination reaction of hydrogen gas and solid iodine is: H 2(g) + I 2(s) 2 HI (g) The heat change, q, for this reaction is kj per mole of iodine that reacts. Is this reaction endothermic or exothermic? What is the energy change when 2.50 mole of iodine reacts?

18 : Bomb Calorimeter When 5.00 grams of methanol is burned in a calorimeter, the temperature of the calorimeter increases from 20.0 to 35.0ºC. The heat capacity of the calorimeter is 7.70 kj/ ºC. What is the heat of combustion per gram of methanol? 6-35 Combustion of Fuels

19 : Combustion of H 2 The heat of combustion of H 2 is -286 kj/mol. How much heat is released when a balloon filled with g H 2 is burned?