Purpose The purpose of this activity is to determine the amount of energy produced or consumed during several chemical reactions.

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1 Chemistry Name: Date: KEY a Energy: Enthalpy of Thermochemical Reactions 1 Purpose The purpose of this activity is to determine the amount of energy produced or consumed during several chemical reactions. Introduction Chemical reactions that produce or consume heat are very important in daily life. Athletic injuries are often treated by cold packs (ammonium chloride + water) and hot packs (calcium chloride + water). Meals Ready to Eat (MREs) are used by soldiers and in emergency conditions (e.g., to feed people during power outages). In such reactions, it is very important to control the amount of heat transfer. The SI unit for energy is joule (J). On food packaging, energy is given in calories (C) with an upper case C, or 1 kilocalorie. One calorie is the amount of heat needed to raise the temperature of one gram of water by one degree Celsius. In chemistry, 1 C = 1,000 calories (c) and 1 calorie = J. An example of a thermochemical reaction is the melting of ice: H 2 O(s) H 2 O(l) H = 6.01 kj/mol This means that every mole of ice that is melted requires 6.01 kj. Because energy is conserved, when liquid water freezes, it releases the 6.01 kj/mol. Figure 1. Melting of one mole of ice requires 60.1 kj. Freezing of water releases the same amount of energy ( H = kj/mol). Preventing Frost Damage 2 When 1 kg of water freezes it releases 3.3 x 10 5 J of heat energy. Farmers use this principle to prevent frost damage to their orchards and other crops. When a frost is predicted, farmers will turn on the water sprinklers. As the water falls on the plants and starts to freeze, heat is released to the surroundings and plants. The heat helps the plants stay warm enough to prevent damage. This only works when the temperature does not drop much below freezing. Heat and enthalpy are two important terms that are commonly confused F:\330\330_sections\330_06_Energy\330_06_04_Enthalpy_Chemical_Reactions\330_06_04_1a_Enthalpy_Thermochemical_Reactions_HW_KEY.docx ( )

2 Chemistry a: Energy Enthalpy of Thermochemical Reactions p. 2 Heat (q) is one of two ways that energy can be transferred during thermodynamic processes. The other is work. ( E = q + w). Enthalpy ( H) is essentially a measure of the potential energy of the system. H = E + (PV), where E = change in internal energy. Most thermochemistry reactions done in the lab have a constant pressure (1 atmosphere). For constant pressure, H = qp. (The subscript p represents constant pressure.) For a thermochemical reaction: H = H(products) H(reactants) N.B. The change in enthalpy is for one mole of reaction. One interesting exothermic reaction is that produced by the bombardier beetle 3 : 1C 6 H 4 (OH) 2 (aq) + 1H 2 O 2 (aq) 1C 6 H 4 O 2 (aq) + 2H2O(l) H = 203 kj/mol Example 1. Thermochemical Equations 4 Problem. Calculate the heat evolved when 93.4 g of SO 2 is converted to SO 3. 2SO 2 (g) + 1O 2 (g) 2SO 3 (g) H = kj/mol Strategy. The equation shows that for every 2 moles of SO2 reacted, kj of heat are given off. (Note the negative sign.) Therefore, the conversion factor is: How many moles of SO 2 are in 93.4 g of SO 2? What is the conversion factor between grams and moles? Solution. (1) Calculate the number of moles of SO 2 in 93.4 g. (2) Calculate the number of kj produced by this exothermic reaction: H = = 144 kj Problems 1. Calculate the heat evolved when 266 g of white phosphorus (P 4 ) burns in air. 1P 4 (s) + 5O 2 (g) 1P 4 O 10 (s) H = kj/mol H = = 6.47 x 10 3 kj Chang & Overby, 6 th edition. Example 6.3.

3 Chemistry a: Energy Enthalpy of Thermochemical Reactions p The first step in the industrial recovery of copper from copper(ii) sulfide ore is by heating (roasting) is: 2CuS(s) + 3O 2 (g) 2CuO(s) + 2SO 2 (g) Calculate the heat evolved ( H) in kj per gram of CuS roasted. H = kj/mol The equation as written shows that kj of heat is released when two moles of CuS react. We want to calculate the amount of heat released when 1 g of CuS reacts. The heat evolved per gram of CuS roasted is: = kj/g CuS note: The sign for the heat evolved is positive because of the way the question was phrased. If it had asked for the amount of heat evolved. If it had asked for H, then the sign would be negative and the answer would be kj/g CuS. 3. The enthalpy associated with melting a solid to a liquid is called the heat of fusion ( H fus ). The enthalpy associated with the boiling of a liquid to a gas is called the heat of vaporization ( H vap ). a. How are the heats associated with melting and freezing related? The absolute values of H are the same. However, melting is endothermic, so the sign is positive. Freezing is exothermic, so the sign is negative. b. How are the heats associated with boiling and condensation related? The absolute values of H are the same. However, boiling is endothermic, so the sign is positive. Condensation is exothermic, so the sign is negative.

4 Chemistry a: Energy Enthalpy of Thermochemical Reactions p Solid sulfur is melted by the following equation: 1S(s) 1S (l) H fus = 17.7 kj/mol How many grams of sulfur can be melted by 29.0 kj of energy? = 52.5 g S 5. Determine the amount of heat (kj) given off when 1.26 x 10 4 g of NO 2 are produced according to the equation: 2NO(s) + 1O 2 (g) 2NO 2 (g) H = kj/mol Strategy: The thermochemical equation shows that for every 2 moles of NO 2 produced, kj of heat are given off (note the negative sign). We can write a conversion factor from this information kj 2 mol NO How many moles of NO 2 are in g of NO 2? What conversion factor is needed to convert between grams and moles? 2 Solution: We need to first calculate the number of moles of NO 2 in g of the compound. Then, we can convert to the number of kilojoules produced from the exothermic reaction. The sequence of conversions is: grams of NO 2 moles of NO 2 kilojoules of heat generated Therefore, the heat change is: 1 mol NO kj ( g NO ) kj g NO2 2 mol NO2 This is an exothermic reaction. The amount of heat given off is kj. 6. What is the change in enthalpy for the production of 1.00 mol SnO? 1Sn + 1SnO kj 2SnO (note: The reaction is endothermic so H = +92 kj) = 46.0 kj 7. What is the change in enthalpy for the reaction of 2.00 mol of iron(ii) oxide? 6FeO(s) + 1O 2 (g) 2Fe 3 O 4 (s) kj H = 635 kj

5 Chemistry a: Energy Enthalpy of Thermochemical Reactions p. 5 Extension 8. The Bunsen burners used in labs are fueled by natural gas (mostly methane, CH 4 ). The thermochemical equation for the reaction is: 1CH 4 (g) + 2O 2 (g) 1CO 2 (g) + 2H 2 O(l) Calculate H when: (a) 5.00 g of CH 4 react with an excess of oxygen. H = kj = 278 kj (b) 2.00 L of O 2 at 49.0 o C and 782 mmhg react with an excess of methane. (i) First, determine the moles of oxygen in the above equation. For this, we employ the ideal gas law. PV = nrt n = ( ) = mol O 2 (ii) Then, calculate the change in enthalpy associated with the amount of O 2 : = 34.7 kj (c) 2.00 L of CH 4 react with 5.00 L of O 2 in a reaction vessel kept at 25 o C and 1.00 atm. (i) First, determine limiting reactant. Volume and moles are directly related (Avogadro s law), so convert L of reactant to L of product (CO 2 or H 2 O). CH 4 : = 2.00 L CO 2 O 2 : = 2.50 L CO 2 CH 4 is the limiting reactant. (ii) Convert 2.00 L of CH 4 (or O 2 or CO 2 or H 2 O) to moles. PV = nrt n = = mol CH 4 (iii) Convert mole of CH 4 (or O 2 or CO 2 or H 2 O) to kj. = 72.8 kj