Thermochemistry. Energy. 1st Law of Thermodynamics. Enthalpy / Calorimetry. Enthalpy of Formation

Transcription

1 Thermochemistry Energy 1st Law of Thermodynamics Enthalpy / Calorimetry Hess' Law Enthalpy of Formation

2 The Nature of Energy Kinetic Energy and Potential Energy Kinetic energy is the energy of motion: E k 1 2 m v 2 Potential energy is the energy an object possesses by virtue of its position. Potential energy can be converted into kinetic energy. Example: a bicyclist at the top of a hill.

3 The Nature of Energy SI Unit for energy is the joule, J: E k 1 m 2 1kg m 2 v 2 s 1J 2 kg sometimes the calorie is used instead of the joule: 1 cal = J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal Units of Energy 1m/s 2

4 Thermochemistry Terminology System: part of the universe we are interested in. Surrounding: the rest of the universe. Boundary: between system & surrounding. Exothermic: energy released by system to surrounding. Endothermic: energy absorbed by system from surr. Work ( w ): product of force applied to an object over a distance. Heat ( q ): transfer of energy between two objects

5 The First Law of Thermodynamics

6 The First Law of Thermodynamics Exothermic and Endothermic Processes Endothermic: absorbs heat from the surroundings. An endothermic reaction feels cold. Exothermic: transfers heat to the surroundings. An exothermic reaction feels hot.

7 Endothermic Reaction Ba(OH) 2 8H 2 O(s) + 2 NH 4 SCN(s) Ba(SCN) 2 (s) + 2 NH 3 (g) + 10 H 2 O(l)

8 The First Law of Thermodynamics State Functions State function: depends only on the initial and final states of system, not on how the internal energy is used.

9 Enthalpy => Heat of Reaction

10 Enthalpies of Reaction For a reaction: H H final initial Hproducts Hreactants Enthalpy is an extensive property (magnitude H is directly proportional to amount): CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) H = -802 kj 2CH 4 (g) + 4O 2 (g) 2CO 2 (g) + 4H 2 O(g) H = 1604 kj H

11 Enthalpies of Reaction When we reverse a reaction, we change the sign of H: CO 2 (g) + 2H 2 O(g) CH 4 (g) + 2O 2 (g) H = +802 kj Change in enthalpy depends on state: H 2 O(g) H 2 O(l) H = -44 kj

12 Calorimetry Heat Capacity and Specific Heat Calorimetry = measurement of heat flow. Calorimeter = apparatus that measures heat flow. Heat capacity = the amount of energy required to raise the temperature of an object (by one degree). Molar heat capacity = heat capacity of 1 mol of a substance. Specific heat = specific heat capacity (c) = heat capacity of 1 g of a substance. q specific heat grams of substance T

13 Table 5.2: Specific Heats (c) of Some Substances at 298 K Substance c ( J g -1 K -1 ) N 2 (g) 1.04 Al(s) Fe(s) 0.45 Hg(l) 0.14 H 2 O(l) H 2 O(s) 2.06 CH 4 (g) 2.20 CO 2 (g) 0.84 Wood, Glass 1.76, 0.84

14 If 24.2 kj is used to warm a piece of aluminum with a mass of 250. g, what is the final temperature of the aluminum if its initial temperature is 5.0 o C? q S m T

15 Calorimetry Atmospheric pressure is constant! Constant Pressure Calorimetry H q

16 Calorimetry Constant Pressure Calorimetry q rxn c (total mass of solution) T H rxn qrxn mol* *moles of speciesof interest

17 Calorimetry Examples CyberChem - Pizza 1. In an experiment similar to the procedure set out for Part (A) of the Calorimetry experiment, g of Mg(s) was combined with ml of 1.0 M HCl. The initial temperature was 25.0 o C and the final temperature was 72.3 o C. Calculate: (a) the heat involved in the reaction and (b) the enthalpy of reaction in terms of the number of moles of Mg(s) used. Ans: (a) 25.0 kj (b) 406 kj/mol ml of 1.0 M HCl at 25.0 o C were mixed with 50.0 ml of 1.0 M NaOH also at 25.0 o C in a styrofoam cup calorimeter. After the mixing process, the thermometer reading was at 31.9 o C. Calculate the energy involved in the reaction and the enthalpy per moles of hydrogen ions used. Ans: -2.9 kj, -58 kj/mol [heat of neutralization for strong acid/base reactions]

18 Hess s Law Hess s law: if a reaction is carried out in a number of steps, H for the overall reaction is the sum of H for each individual step. For example: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) H = -802 kj 2H 2 O(g) 2H 2 O(l) H= - 88 kj CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) H = -890 kj

19 Another Example of Hess s Law Given: C(s) + ½ O 2 (g) CO(g) CO 2 (g) CO(g) + ½ O 2 (g) H = kj H = kj Calculate H for: C(s) + O 2 (g) CO 2 (g)

20 Enthalpies of Formation If 1 mol of compound is formed from its constituent elements, then the enthalpy change for the reaction is called the enthalpy of formation, H o f. Standard conditions (standard state): Most stable form of the substance at 1 atm and 25 o C (298 K). Standard enthalpy, H o, is the enthalpy measured when everything is in its standard state. Standard enthalpy of formation: 1 mol of compound is formed from substances in their standard states.

21 Enthalpies of Formation If there is more than one state for a substance under standard conditions, the more stable one is used. Standard enthalpy of formation of the most stable form of an element is zero.

22 Enthalpies of Formation Substance o f (kj/mol) C(s, graphite) 0 O(g) O 2 (g) 0 N 2 (g) 0

23 Foods and Fuels Foods 1 nutritional Calorie, 1 Cal = 1000 cal = 1 kcal. Energy in our bodies comes from carbohydrates and fats (mostly). Intestines: carbohydrates converted into glucose: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O, H = kj Fats break down as follows: 2C 57 H 110 O O 2 114CO H 2 O, H = -75,520 kj Fats contain more energy; are not water soluble, so are good for energy storage.

24 Thermochemistry E k m v ( kg m s joule) Energy 1st Law of Thermodynamics q H rxn q P specific heat grams Enthalpy / Calorimetry T Hess' Law Enthalpy of Formation H rxn n H f products m H f reactants