Thermochemistry

Kinetic Energy and Potential Energy Kinetic energy is the energy of motion: 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. The Nature of Energy

Units of Energy SI Unit for energy is the joule, J: sometimes the calorie is used instead of the joule: 1 cal = J (exactly) A nutritional Calorie: 1 Cal = 1000 cal = 1 kcal The Nature of Energy

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. Thermochemistry Terminology Work ( w ): product of force applied to an object over a distance. Heat ( q ): transfer of energy between two objects

Internal Energy Internal Energy: total energy of a system. Involves translational, rotational, vibrational motions. Cannot measure absolute internal energy. Change in internal energy, The First Law of Thermodynamics

Relating  E to Heat(q) and Work(w) Energy cannot be created or destroyed. Energy of (system + surroundings) is constant. Any energy transferred from a system must be transferred to the surroundings (and vice versa). From the first law of thermodynamics: The First Law of Thermodynamics

First Law of Thermodynamics Calculate the energy change for a system undergoing an exothermic process in which 15.4 kJ of heat flows and where 6.3 kJ of work is done on the system.  E = q + w

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. The First Law of Thermodynamics

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)

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

Chemical reactions can absorb or release heat. However, they also have the ability to do work. For example, when a gas is produced, then the gas produced can be used to push a piston, thus doing work. Zn(s) + 2H + (aq)  Zn 2+ (aq) + H 2 (g) The work performed by the above reaction is called pressure-volume work. When the pressure is constant, Enthalpy

Enthalpy

Enthalpy, H: Heat transferred between the system and surroundings carried out under constant pressure. Enthalpy is a state function. If the process occurs at constant pressure, Enthalpy

Since we know that We can write When  H, is positive, the system gains heat from the surroundings. When  H, is negative, the surroundings gain heat from the system. Enthalpy

Enthalpy => Heat of Reaction

For a reaction: 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 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 Enthalpies of Reaction

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 = heat capacity of 1 g of a substance. Calorimetry

Table 5.2: Specific Heats (S) of Some Substances at 298 K SubstanceS ( J g -1 K -1 ) N 2 (g)1.04 Al(s)0.902 Fe(s)0.45 Hg(l)0.14 H 2 O(l)4.184 H 2 O(s)2.06 CH 4 (g)2.20 CO 2 (g)0.84 Wood, Glass1.76, 0.84

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?

Constant Pressure Calorimetry Atmospheric pressure is constant! Calorimetry

Calorimetry Constant Pressure Calorimetry

Calorimetry Examples 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]

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 Hess’s Law

Another Example of Hess’s Law Given: C(s) + ½ O 2 (g)  CO(g)  H = kJ CO 2 (g)  CO(g) + ½ O 2 (g)  H = kJ Calculate  H for:C(s) + O 2 (g)  CO 2 (g)

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. 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. Enthalpies of Formation

Substance  o f (kJ/mol) C(s, graphite)0 O(g)247.5 O 2 (g)0 N 2 (g)0

Using Enthalpies of Formation to Calculate Enthalpies of Reaction For a reaction Note: n & m are stoichiometric coefficients. Calculate heat of reaction for the combustion of propane gas giving carbon dioxide and water. Enthalpies of Formation

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. Foods and Fuels

Fuels Fuel value = energy released when 1 g of substance is burned. Most from petroleum and natural gas. Remainder from coal, nuclear, and hydroelectric. Fossil fuels are not renewable. In 2000 the United States consumed 1.03  kJ of fuel. Hydrogen has great potential as a fuel with a fuel value of 142 kJ/g. Foods and Fuels

Thermochemistry