Chem Quiz on Thermodynamics 11_07_24 PDF

## 4.1 Thermodynamics Terms and Characteristics **Open, Closed, and Isolated Systems** * **Open systems** - like an open water bottle - allow transfer of matter and energy. * **Closed systems** - like a closed water bottle - do not allow transfer of matter, but do allow transfer of energy. *...

## 4.1 Thermodynamics Terms and Characteristics **Open, Closed, and Isolated Systems** * **Open systems** - like an open water bottle - allow transfer of matter and energy. * **Closed systems** - like a closed water bottle - do not allow transfer of matter, but do allow transfer of energy. * **Isolated systems** do not allow transfer of matter nor energy. Bomb calorimeters are isolated systems due to being insulated. **State Functions vs Path Functions** **State Functions:** are variables that define how a system starts from how it ends. We calculate ```final - initial``` with state functions. * State functions ask only ```final - initial```, like ```ΔH```, ```ΔG```, ```ΔS```, ```ΔU``` **Example:** If you take a cup of water from your room at about 25°C, fly to the sun reaching about 5600°C, fly to the North Pole reaching -40°C, then return to your room, there was no net change in the temperature. **Path Functions:** follow the mechanism, or the path, for a given process. * Path functions are ```q``` and ```w``` (heat and work). **Example:** If you push a box around the room first to the door, then to the window, then back to the starting point, the amount of work (```w```) you put in depends on how far (and how hard) you pushed it each time. It depends on the path you took. We use ```PV``` work (```w = -PΔV```) rather than force-distance work (```w = Fd```), but the idea is the same. **Extensive vs Intensive Properties** **Extensive Properties:** an additive property of a system. * e.g., volume, mass, moles, energy **Example:** If you take two buckets with 1 L water each and combine them, now you have 2 L water. Very simple idea that combining two things adds them together. **Intensive Properties:** a property that defines a system. This is not additive. If you ratio two extensive properties, you get out an intensive property. * e.g., temperature, density, pressure **Example:** If you take two buckets of water, each at 25°C, and combine them, the temperature is still 25°C. It does not add to make it 50°C. Pressure and density are intensive because they are a ratio of two extensive properties (```P = force/area, p = mass/vol```). **Practice Exam Problem:** Which of the following statements is correct? a) Volume is an extensive property and temperature is an intensive property. **correct** b) Mass is an extensive property and moles is an intensive property. c) Both energy and temperature are extensive. d) Both pressure and density are extensive. ## 4.2 Thermodynamic Signs Signs are the most important fundamental concept in thermo!! Many questions are missed because of getting signs wrong. What you need to know: **BE THE SYSTEM**. **Generally:** * If something **LEAVES** the system, the sign is (-) * If something **ENTERS** the system, the sign is (+) **Specifically:** * ```q``` and ```w``` are negative when energy **leaves** the system * ```q``` and ```w``` are positive when energy **enters** the system * ```ΔG``` is negative = spontaneous; ```ΔG``` is positive = non-spontaneous * ```ΔS``` is positive = increasing disorder; ```ΔS``` is negative = increasing order * ```ΔH``` is negative = exothermic; ```ΔH``` is positive = endothermic **Pay attention to wording!! Examples:** * "Done on surroundings" is the same as "leaving the system" * "Done on the system" is the same as "entering the system" **Heat vs Work:** * Heat is chaotic energy, whereas work is a concerted flow of energy that can be easily controlled/guided * Both are **PATH** functions, meaning the methods you take to heat something, change its volume, etc affect the outcome **Practice Exam Problem:** Which one of the following statements is true? a) Work is the concerted flow of energy and is positive when it is done on the system. **correct** b) Heat is the concerted flow of energy and is positive when it is done on the system. c) Work is the concerted flow of energy and is positive when it is done on the surroundings. d) Heat is the concerted flow of energy and is positive when it is done on the surroundings. ## 4.3 and 4.7 First Law: Internal Energy and Enthalpy Theory These are theory questions with lots of words in T/F statements. * Know the first law, that ```ΔUuniv = 0``` means that energy is conserved * Know that ```ΔU = q + w``` * Know that ```U``` is all the sources of potential and kinetic energy in a system * Know that calorimetry allows ```∆Usys = -∆Usurrondings``` to measure unknown ```ΔU``` reactions * Know that ```∆H = AU + PAV``` * In a closed system, ```V``` is constant so ```ΔU = qv``` because no ```PΔV``` work * In an open system, ```P``` is constant so ```∆H = qp``` **Practice Exam Problem:** Which of the following statements is/are true concerning the first law of thermodynamics? 1) The internal energy (```U```) of the universe is conserved II) The internal energy of a system plus that of its surroundings is conserved III) The change in internal energy (```ΔU```) of a system and its surrounding can have the same sign a) I, III d) III only b) I only e) I, II **correct** c) I, II, III f) II only ## 4.4 Calculating Internal Energy (```ΔU = q + w```) This is a simple calculation: ```ΔU = q + w``` For which you are given ```q``` and ```w```. The only difficulty is **SIGNS**. | Entering | Leaving | | --------- | -------- | | w (+) | w (-) | | q (+) | q (-) | **Example:** You charge your phone battery to increase charge by 5 kJ, but the phone gives off 1 kl of heat in the process. What is the change in internal energy? **Solution:** Solve ```ΔU = q + w``` by first assigning signs to ```q``` and ```w```. * The **SYSTEM** is always the chemical reaction - here, the chemical reaction in the battery is the system. * You **INCREASE** charge of the battery, meaning ```w = + 5 kJ``` * "Gives off heat" means ```q = -1 kJ``` ```ΔU=q+w=5-1=4kJ``` **Practice Exam Problem:** A CD player and its battery together do 500 kJ of work, and the battery also releases 250 kJ of energy as heat and the CD player releases 50 kJ as heat due to friction from spinning. What is the change in internal energy of the system, with the system regarded as the battery and CD player together? a) - 750 kJ b) + 200 kJ c) -800 kJ **correct** d) - 200 kJ **Explanation:** * "do work" means ```w = -500 kJ``` * "releases energy" means ```q-250 kJ-50 kJ = - 300 kJ``` ```ΔU=q+w=-500-300-800 kJ``` ## 4.10 Energy Diagrams of Reactions This is the easiest question on the test. Be able to interpret the graph in order to determine if energy is entering (+) or leaving (-) the system and how much it changed by. **Energy enters so ΔU is positive.** **Energy leaves so ΔU is negative.** **Practice Exam Problem:** What is the change in enthalpy (```ΔH```) for the reaction from A → B given the reaction coordinate diagram below? **Explanation:** ```ΔH = Hproducts - Hreactants``` * Products (red) = 300 kJ * Reactants (blue) = 250 kJ ```ΔH = 300 – 250 = 50 kJ``` Increase in energy = endothermic a) -50 kJ; endothermic b) -150 kJ; exothermic c) 50 kJ; endothermic **correct** d) 300 kJ; exothermic