17.1 Enthalpy. Energy at constant Pressure. Dr. Fred Omega Garces. Chemistry 201. Miramar College. 1 Enthalpy. January 13

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1 17.1 Enthalpy Energy at constant Pressure Dr. Fred Omega Garces Chemistry 201 Miramar College 1 Enthalpy

2 Thermodynamics: Importance A spontaneous process, whether a chemical change, a physical change, or just a change in location, is one that occurs by itself under specified conditions, without a continuous input of energy from outside the system. The freezing of water, for example, is spontaneous at 1 atm and -5 C. A spontaneous process such as burning or falling may need a little push to get started - a spark to ignite gasoline, a shove to knock a book off your desk - but once the process begins, it continues without external aid because the system releases enough energy to keep the process going. In contrast, for a nonspontaneous process to occur, the system must be supplied with a continuous input of energy. Under a given set of conditions, if a process is spontaneous in one direction, it is not spontaneous in the other. How can the direction of a spontaneous change be determine? How can the net direction of any reaction be predicted? Is the energy change of a process the key to predicting spontaneity? (Stilberberg) 2 Enthalpy

3 First Law of Thermodynamics In Thermochem, the First Law of Thermodynamics was introduced. * Energy is neither created nor destroyed, it is constant in the universe. The internal energy (E) of a system can be change by addition or removal of heat (q) or work (w) ΔE = q + w But according to the First Law of Thermo, Therefore, ΔE univ = ΔE sys + ΔE surr = 0 constant q sys = - q surr and w sys = - w surr * Any modern statement of conservation of energy must take into account mass-energy equivalence, therefore the first law is qualified to include the mass-energy of the universe is constant. 3 Enthalpy

4 Nomenclature Consider a chemical system of interest. Reactant g Product ΔH reaction i) CH 4 + O 2 g CO 2 + H 2 O + E ΔH combustion ii) C + 1/2 O 2 g CO ΔH formation * iii) H 2 O (s) g H 2 O (l) ΔH fusion iv) H 2 O (l) g H 2 O (g) ΔH vaporization v) CuSO 4 (s) + 5H 2 O g CuSO 4 5H 2 O (s) ΔH hydration * ΔH formation = Heat involve when a product is formed from its elements. i.e., C + O 2 g CO 2 4 Enthalpy

5 Scale and Reference When measurements are taken, there must be some sort of reference. Some measurements use scales that are absolute, i.e., Temperature your age. Others only need a point of reference, i.e., time & date, atmospheric pressure or elevation. Energy and enthalpy has been defined but not in term of absolute value. What convention should be used? Seems like this is easy to do. However 5 Enthalpy

6 What is a good Energy Reference Scale Consider: A park car. Is the energy of this car zero? What if it is parked on a slope and the brakes are released, where did the kinetic energy originate? Does a stationary object truly have zero energy? How about the atoms which make it up? Surely there are electrons that are moving, which means it has some energy. and when you consider that the Earth is rotating and the universe is moving how do we quantify energy? Potential energy Equilibrium position in Valley 6 Enthalpy

7 Standard State Because all matter will have some energy content Via - vibration or electron dynamics, assigning matter an absolute energy is elusive!!! Energy and enthalpy are state function. This means only changes in their properties are important. (Unlike temperature which has an absolute scale - Kelvin) Reference used: Standard State Standard state: 1 atm and 298 K (25 C) (In chem200, STP was at 0 C.) Furthermore, phase of substance is also important. standard state applies to: liquid or solid in pure state 1 atm. 1.0 M X - super zero indication that the variable is at standard state ΔE, ΔH, ΔG, S 7 Enthalpy

8 ΔH formation : Calculation Consider the following reaction: i) C (graphite) + O 2 (g) g CO 2 (g) ΔH f =? ii) N 2(g) + O 2 (g) g NO 2 (g) ΔH f =? Std heats of formation ΔH f: Heat absorbed when one mole of substance in its standard state is formed from the most stable form of the element in their standard state. Note: The standard heat of formation for elements in its most stable form (structure and phase) is defined as zero. ΔH f = 0: Br 2 (l), O 2 (g), C (graphite), Na (s) ΔH f 0 : H 2 O (l), O 2 (l), C (diamond), NaCl (s) 8 Enthalpy