Energy. Energy is the capacity to do work or produce heat Law of Conservation of Energy (First Law of Thermodynamics) Units of energy

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1 Energy is the capacity to do work or produce heat Law of Conservation of Energy (First Law of Thermodynamics) Units of energy Calorie (based on water) Joule (more general 1 calorie = joules Energy

2 Energy Figure 6.3 Endothermic Process Figure 6.2 Exothermic Process

3 Potential energy Stored energy Energy due to position Kinetic Energy energy due to motion vibrational, translational, rotational KE = ½ mv 2 Potential and Kinetic Energy

4 Temperature Heat Work doesn t measure energy related to energy The measure of the average kinetic energy of the particles the transfer of energy between two objects due to a temperature Ways energy is transferred: heat (q), work (w) Energy changes on a system E = q + w force acting over a distance on the system work is positive by the system work is negative Temperature, Heat and Work. Place a rubber band loosely looped over the index fingers in contact with skin just above your upper lip. Now quickly stretch the rubber band. What do you experience? Now let the rubber band relax quickly. What do you feel now? When the rubber band is stretched quickly, work is done on it, causing its internal energy to rise. This rise reveals itself as a small increase in temperature. When the rubber band is allowed to quickly contract, it performs work and suffers a reduction in internal energy which produces a cooling sensation.

5 State Functions and Pathways Elevation state function Distance not a state function

6 Pathway Condition for the transfer of energy which is divided between work & heat The ratio of heat to work depends on the pathway The total energy change is independent of the pathway State Function (or property) A property of a system that depends on its present state not how it got that way A change in state function is independent of the pathway between two states Example: elevation versus distance Energy is a state function Work and heat are not state functions State Functions and Pathways. STATE FUNCTIONS Energy Enthalpy NOT STATE FUNCTIONS Work Heat

7 Internal Energy, E All the energy contained within a chemical system The sum of the kinetic and potential energies of all the particles in the system An increase in the internal energy of a system can take three forms An increase in temperature A phase change The initiation of a chemical reaction A decrease in the internal energy of a system will usually result in A decrease in temperature A phase change The total internal energy of a system cannot be determined Changes in internal energy can be determined Can be changed by a flow of work, heat or both E = q + w Internal Energy.

8 First Law of Thermodynamics the energy of the universe is constant energy can be converted from one form to another energy cannot be created or destroyed. In a chemical system the energy exchanged between system and surroundings can be accounted for by heat (q) and work (w) E = q + w Sign Conventions for Work and Heat Heat, Work and Energy. Process Sign Work done by the system on the surrounding - Work done on the system by the surrounding + Heat absorbed by the system form the surroundings (endo) + Heat absorbed by the surroundings from the system (exo) -

9 Heat, Work and Energy. Work refers to a force that moves an object over a distance Only pressure/volume work (the expansion/contraction of a gas) is significance in chemical systems. Only significant when there is an increase or decrease in the amount of gas present. Example 2NH 3 2N 2 +3H 2 There are more moles of gas in the product. The system can be thought of as doing work on the surroundings Pushing back the atmosphere. So work is negative.

10 Hot air Balloon Is w positive or negative? Work is done by the balloon Expanding work is negative Is q positive or negative? endothermic process Internal Energy E = q + (-w)

11 Heat, Work and Energy. For the expansion/contraction of a gas against constant external pressure w = - P V If there is no change in the total volume of gas before and after a reaction occurs, there is no significant work done by or on the system Then E = q because w=0

12 Enthalpy. H = E + P V = E (- P V ) consider w = - P V so. Enthalpy, H, can be considered to be energy with the work taken out. H = E (- P V ) substitute q + w or q + (- P V) for E H = q + (- P V) (- P V ) so H = q at constant external pressure For chemical reactions, H can be calculated theoretically and measured directly. Often is P V small even when a gas or volume change is involved so it is ignored and then H = E

13 Sample Questions from the AP Exam on H, E, q, w. A gas absorbs 28.5 J of heat then then performs 15.2 J of work. The change in internal energy of the gas is: a J b J c J d J e. None of these (b) E = q + w; 28.5 J 15.2 J = 13.3 J

14 Sample Questions from the AP Exam on H, E, q, w. Which of the following statements correctly describes the signs of q and w for the following exothermic process at 1 atmosphere pressure and 370 kelvin? H 2 O (g) H 2 O (l) a. q and w are both negative b. q is positive and w is negative c. q is negative and w is positive d. q and w are both positive e. q and w are both zero (c) An exothermic indications q is negative and the gas is condensing to a liquid so it is exerting less pressure on its surroundings indicating w is positive.

15 Sample Questions from the AP Exam on H, E, q, w. A system does 130. J of work on its surroundings while 70. J of heat are added to the system. What is the internal energy change for the system? E = q + w = -70. J J = -60. J

16 Sample Questions from the AP Exam on H, E, q, w. What will be the volume change if 125 J of work is done on a system containing an ideal gas? The surroundings exert a constant pressure of 5.2 atm. -w = -P V; 125J/-5.2atm J/L atm = V; the volume will decrease by 0.24 liters.

17 Sample Questions from the AP Exam on H, E, q, w. For the following reactions performed at constant external pressure, is work done on the system by surroundings, by the surroundings on the system, or is the amount of work negligible? a. Sn (s) + 2F 2 (g) SnF 4 (s) b. AgNO 3 (aq) + NaCl (aq) AgCl (s) + NaNO 3(aq) c. C (s) + O 2 (g) CO 2 (g) d. SiI 4(g) + heat Si (s) + 2I 2(g) a. w > 0; work is done on the system as 2 moles of gas reactant decrease to 0 moles of gas product b. w = negligible; no change in volume c. w = negligible; 1 mole of gas reactant to 1 mole of gas product indicates a volume change close to zero d. w < 0; 1 mole of gas reactant expands to moles of gas product therby doing work on the surroundings.

18 Sample Questions from the AP Exam on H, E, q, w. With the addition of heat and manganese dioxide as a catalyst, potassium chlorate at constant pressure will decompose according to the following equation 2KClO 3 (s) + heat 2KCl (s) + 3O 2 (g) Determine whether each of the values of H, q, w and E will be positive negative or unable to be determined. H is positive q is positive w is negative E cannot be determined from the information given. The sign of E depends upon the magnitudes of q and w.

19 Sample Exercise 6.1 Internal Energy p. 233 Calculate the E for a system undergoing an endothermic process in which 15.6 kj of heat flows and where 1.4 kj of work is done on the system work is done on the system Is w positive or negative? endothermic process Is q positive or negative? E = q + w

20 The Nature of Energy: Vocabulary Figure 6.4 The Piston, Moving a Distance Against a Pressure P, Does Work On the Surroundings P = F / A F = P A W = F D W = P A D W = P V or W = - P V Expanding work is done by the system work flows out of the system work is negative Contracting work is done to the system work is going into the system work is positive

21 Sample Exercise 6.2 Internal Energy p. 234 Calculate the work associated with the expansion of a gas from 46 L to 64 L at a constant external pressure of 15 atm. Look at the units easy math What does it mean? w = - P V For an ideal gas, work can occur only when its volume changes. So if a gas is heated at a constant volume, the pressure increases but no work occurs

22 Sample Exercise 6.3 Internal Energy, Heat & Work p. 234 A balloon is being inflated to its full extent by heating the air inside it. In the final stages of this process, the volume of the balloon changes from 4.00 x 10 6 L to 4.50 x 10 6 L by the addition of 1.3 x 10 8 J of energy as heat. Assuming that the balloon expands against a constant pressure of 1.0 atm, calculate E for the process. Look at the units easy math What does it mean? J = 1 L atm Notice: Which contributes more to the internal energy? Heat or work

23 Enthalpy, H. H = E + PV E is the internal energy, P is the pressure of the system, V is the volume of the system Enthalpy is a state function At A Constant Pressure E = q p + w E = q p - P V q p = E + P V H = E + PV H = E + P V where w = -P V rearrange PV = P V q p = H At constant pressure (where only PV work is allowed), the change in enthalpy, H of the system is equal to the energy flow as heat. For reactions at constant pressure H = H products - H reactants

24 Sample Exercise 6.4 Enthalpy p. 236 When 1 mole of methane (CH 4 ) is burned at a constant pressure, 890 kj of energy is released as heat. Calculate H for a process in which a 5.8 g sample of methane is burned at a constant pressure.

25 Calorimetry Coffee Cup Calorimeter Calorimeter a devise to experimentally determine the heat associated with a chemical reaction. Calorimetry the science of measuring heat based on observing the temperature change when a body absorbs or discharges energy as heat. Heat Capacity, C C = heat absorbed /increase in temperature Specific Heat Capacity J / C g or J/K g Molar Heat Capacity J / C mol or J/K mol

26 Specific Heat Capacities. p. 237

27 Constant-Pressure Calorimetry Atmospheric pressure remains constant during the process. For reactions in solution Assumptions Colorimeter does not absorb or leak any heat Solution treated like pure water with a density of 1.0 g/ml Energy released determined from temperature increase, mass of solution, specific heat capacity of solution. q = m C T

28 Figure 6.6 A Bomb Calorimeter The Nature of Energy: Vocabulary

29 Specific Heat

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