Chemistry 101 - Chapter 5 GRADE A+ GUARANTEED

Thermodynamics The study of energy and itrs transformations Thermochemistry The relationships between chemical reactions and energy changes Electrostatic Potential arises from the interactions between charged particles Eel = k(Q1xQ2)/d k = 8.99x10^9 J-m/C^2 the charges of Q1 and Q2 are typically on the order of magnitude of the charge of an electron (1.60x10^-19 C) When Q1 and Q2 are of the same charge, they will repel, making Eel a positive number When Q1 and Q2 are opposite charges, they will attract, and Eel will be negative A positively charged particle and a negatively charged particle are initially far apart. What happens to their electrostatic potential energy as they are brought closer together? As the distance decreases, the overall Eel will get more negative Fundamental principle of Thermochemistry Energy is released when chemical bonds are formed;energy is consumed when chemical bonds are broken. If the ions in the picture were allowed to move freely, they would ______________ and the potential energy of the system would _____________ (check image) a) attract b) decrease This is because, as long as the ions are not too far apart, they will fight to come together (highest point of potential energy); therefore as they do get closer, the potential energy decreases The First Law of Thermodynamics Energy can be converted, but never created nor destroyed Energy is conserved in any process; one way to express teh law is that the change in internal energy, DeltaE, of a system in any process is equal to the heat, q, added to the system, plus work, w, done on the system Delta E = q + w System the portion of the universe that we single out for study; A chemical reaction is an example of a system Surroundings everything that lies outside the system that we study; spectator ions or the container for the chemical reaction are examples of surroundings Anything that does not affect the chemical reaction Open System a system in which matter and energy can be exchanged with the surroundings ex: uncovered pot of boiling water Closed system systems that can exchange energy but not matter with their surroundings ex: mixture of H2 gas and O2 gas in a cylinder fitted with a piston Isolated System a system in which neither energy nor matter can be exchanged with the surroundings ex: insulated thermos containing hot coffee Is a human being an isolated, closed, or open system? Open because we exchange matter and energy with our surroundings Internal Energy E The sum of ALL kinetic and potential energies of the components of the system The change of internal energy (Delta E) is defined as the heat, added to the system, plus the work, done on the system by its surroundings ΔE = q + w ΔE = E final - E initial Change in energy and what does it mean? Thermodynamic quantities such as ΔE have three parts: 1) a number 2) a unit 3) a sign (1) and (2) together give the magnitude of the change, (3) gives the direction Apositive value of ΔE results when E final E initial, indicating that the system has gained energy from its surroundings. A negative value of ΔE results when E final E initial, indicating that the system has lost energy to its surroundings Note = there is an inverse relation to how much energy is in the surroundings compared to the system. If the internal energy increases, then the surroundings decrease. If the internal energy of the system decreases then the surroundings increase. What is the value of ΔE if Efinal equals Einitial? ΔE will be 0 Suppose a system receives a “deposit” of 50 J of work from the surroundings and loses a “withdrawal” of 85 J of heat to the surroundings. What is the magnitude and the sign of ΔE for this process? ΔE = q + w = 50 - 85 = -35 J Gases A(g) and B(g) are confined in a cylinder-and-piston arrangement like that in Figure 5.4 and react to form a solid product C(s): A (g) + B (g) --- C (s). As the reaction occurs, the system loses 1150 J of heat to the surroundings. The piston moves downward as the gases react to form a solid. As the volume of the gas decreases under the constant pressure of the atmosphere, the surroundings do 480 J of work on the system. What is the change in the internal energy of the system? ΔE = -1150 J + 480 J = -670 J A mixture of gases A2 and B2 are introduced to a slender metal cylinder that has one end closed and the other fitted with a piston that makes a gas-tight seal so that the gases are a closed system. The cylinder is submerged in a large beaker of water whose temperature is 25∘C, and a spark is used to trigger a reaction in the cylinder. At the completion of the reaction, the piston has moved downward, and the temperature of the water bath has increased to 28∘C. If we define the system as the gases inside the cylinder, which of the following best describes the signs of q, w, and ΔE for this reaction? ΔE = q + w q = 28C - 25C = 3 C a chemical process in which heat is released has a -q value work being done on the system is a +w The ΔE = q + w; because we know q is (-) and w is (+), we cannot predict the value of ΔE without more information Calculate the change in the internal energy for a process in which a system absorbs 140 J of heat from the surroundings and does 85 J of work on the surroundings. ΔE = q + w = 140 J - 85 J = +55 J Endothermic Process When the system absorbs heat from its surroundings Exothermic Process When the system loses heat to its surroundings When H2 (g) and O2 (g) react to form H2O (l), heat is released to the surroundings. Consider the reverse reaction, namely, the formation of H2 (g) and O2 (g) from H2O (l): 2H2O (l) -- 2H2 (g) + O2 (g) Is this reaction exothermic or endothermic? Endothermic State function A property of a system that is determined by its state or condition and not by how it got to that state Its value is fixed when temperature, pressure, composition and physical form are specified (P, V, T, E and H are state functions) If the battery is defined as the system, what is the sign on w in part (b)? Energy taken from the battery, w0 You lose 5 pounds over a 30-day period. Which of the following quantities act like a state function: the amount of calories you consume, your weight, or the number of calories burned through exercise? Your weight! Enthalpy A quantity defined by the relationship H = E + PV; The enthalpy change, ΔH, for a reaction that occurs at constant pressure is the heat evolved or absorbed in the reaction H= E + PV ΔH = qp If we define the system as the reactants and products, what is the sign of work in this process, positive or negative? Negative Pressure-Volume (PV) work Work performed by expansion of a gas against a resisting pressure w = -PΔV ΔV = Vfinal - Vinitial Pressure is usually given units of atm Volume is usually given units of L 1 L-atm = 101.3 J P = F/A F = force; A = area ΔV = A x Δh Magnitude of work = force x distance = F x Δh Magnitude of work = F x Δh = P x A x Δh CONTINUED.......