Chapter 6
24. Is the change in enthalpy for a reaction an extensive property? Explain the relationship between delta H for a reaction and the amounts of reactants and products that undergo reaction.
-delta H is an extensive property, meaning it depends on the quantity of reactants undergoing reaction. -delta H is usually reported for a reaction involving stoichiometric amounts of reactants and is dependent on the specific chemical reaction. -Ex: A + 2B --> C, the delta H of the rxn is usually reported as the amount of heat emitted or absorbed when 1 mole of A reacts with 2 moles of B to form 1 mole of C.
20. What is the change in enthalpy (delta H) for a chemical reaction? How is delta H different from delta E?
-delta H is the heat exchanged with the surroundings under conditions of constant pressure. It is equal to Qp, the heat at constant pressure. Conceptually, delta H and delta E are similar. They both represent changes in a state function for the system. Hoewver, delta E is a measure of all of the energy (heat and work) exchanged with the surroundings, and delta H = delta E + P(delta V)
6. State the first law of thermodynamics. What are its implications?
-first law = law of energy conservation. The total energy of the universe is constant. Because energy is neither created nor destroyed, and the universe does not exchange energy with anything else, its energy content does not change. -Has many implications, the most important of which is that with energy, you do not get something for nothing. The best we can do is break even.
15. What is heat capacity? Explain the difference between heat capacity and specific heat capacity?
-heat capacity: the quantity of heat required to change its temperature by 1 degree C. It's a measure of the system's ability to hold thermal energy without undergoing a large change in temperature. -the difference between heat capacity and specific heat capacity is that the specific heat capacity is the amount of heat required to raise the temperature of 1 gram of the substance by 1 degree C.
23. From a molecular viewpoint, where does the energy absorbed in an endothermic reaction go? Why does this reaction mixture undergo a decrease in temperature even though energy is absorbed?
-internal energy of a system is the sum of its KE and PE. It is this PE that absorbs energy in an endothermic reaction. -When some bonds break and new ones form, the electrons go from an arrangement of lower PE to one of higher PE, absorbing thermal energy in the process. This absorption of thermal energy reduces the KE of the system. This is detected by a drop in temperature.
22. From a molecular point of view, where does the energy emitted in an exothermic reaction come from? Why does the reaction mixture undergo an increase in temperature even though energy is emitted?
-internal energy of a system is the sum of its KE and PE. It is this PE that is the energy source in an exothermic reaction. Chemical potential energy arises primarily from the electrostatic forces between the protons and electrons that compose the atoms and molecules within a system. - In an exothermic reaction, some bonds break and new ones form, and the protons go from an arrangement of higher PE to one of lower PE. As they rearrange, their PE is converted into KE, the heat emitted in the reaction. This increase in KE is detected as increase in temperature
18. What is pressure-volume work? How is it calculated?
-the work caused by the expansion of volume -calculated the negative of the pressure that the volume expands against multiplied by the change in volume that occurs during the expansion (w = -P times delta V)
16. Explain how the high specific heat capacity of water can affect weather in coastal regions.
-Because water has such a high heat capacity, it can moderate temperature changes. This keeps coastal temperatures more constant. Changing the temperature of water absorbs or releases large quantities of energy for a relatively small change in temperature. -This serves to keep the air temperature of coastal areas more constant that the air temperature of inland areas.
2. What is energy? What is work? List some examples of each.
-Energy: the capacity to do work (ex: Kinetic energy, electrical energy, chemical energy, light energy) Work: the result of a force acting through a distance (ex: expansion of a cylinder, running of a marathon, moving an object)
12. What is heat? Explain the difference between heat and temperature.
-Heat: the flow of thermal energy caused by a temperature change. Thermal energy is a type of KE because it arises from the motions of atoms or molecules within a substance. -the higher the temp, the greater the motion of atoms or particles. -Heat is measured in units of energy (Joules, calories), while temperature is measured in units of kelvins, degrees F, or degrees C
26. What is Hess's Law? Why is it useful?
-Hess's law states that if a chemical equation can be expressed as the sum of a series of steps, then delta H for the overall reaction = the sum of the heats of reactions for each step. This makes it possible to determine delta H for a reaction without directly measuring it in lab. If you can find related reactions with known delta H that sum to the reaction of interest, you can find the delta H for that reaction.
17. IF two objects, A and B, of 2 different temperatures, come into contact, what is the relationship between the heat lost by one object and the heat gained by the other. What is the relationship between the temperature changes of the two objects?
-When 2 objects of different temperatures come in direct contact, heat flows from the higher temperature object to the lower temperature object. -the amount of heat lost by the warmer object = the amount of heat gained by the colder object. -The warmer object's temp will drop and the cooler object's temp will rise until they reach the same temperature. The magnitude of these changes depends on the mass and heat capacities of the 2 objects
8. What is a state function? List some examples.
A state function: a function whose value depends only on the state of the system, not on how the system arrived at that state -ex: pressure, volume, and internal energy
7. A friend claims to have constructed a machine that creates electricity but requires no energy input. Explain why you should be suspicious.
According to the first law of thermodynamics, a device that would continually produce energy with no energy input, sometimes known as a perpetual motion machine, cannot exist because the best we can do is break even.
14. Explain how the sum of heat and work can be a state function, even though heat and work themselves are not state functions
According to the first law of thermodynamics, the change in internal energy of the system must be the sum of the heat transferred and the work done (Delta E = w + q). The total change in internal energy is the difference between its initial energy and its final energy -the amount of work done and the amount of heat transferred are dependent on details of the path. In one path, more energy may be transferred through conversion of heat energy. In another path, more energy may be transferred through work. Work and heat are not state functions, but their sum (delta E) is constant.
21. Explain the difference between an exothermic and an endothermic reaction. Give the sign of delta H for each type.
An endothermic reaction has a positive delta H and absorbs heat from the surroundings. It feels cold to the touch. -exothermic reaction has a negative delta H and gives off heat to the surroundings. It feels warm to the touch.
10. If energy flows out of a chemical system and into the surroundings, what is the sign of delta E of the system?
Energy flowing out of the system is like a withdrawal from a checking account. Therefore, it carries a negative sign.
11. If the internal energy of the products of a reaction is higher than the internal energy of the reactants, what is the sign of delta E for the reaction? In which direction does energy flow?
If the reactants have a lower internal energy than the products, delta E of the system is positive and energy flows into the system from the surroundings.
19. What is calorimetry? Explain the difference between a coffee-cup calorimeter and a bomb calorimeter. What is each designed to measure?
In calorimetry, the thermal energy exchanged between the reaction (defined as the system) and the surroundings is measured by observing the change in temperature of the surroundings. -A bomb calorimeter is used to measure the delta E for combustion reactions. It includes a tight-fitting, sealed container that forces the reaction to occur at constant volume. -a coffee cup calorimeter is used to measure delta H of many different aqueous reactions. It consists of 2 styrofoam cups, one inserted into the other, to provide insulation from the lab environment. Because the reaction happens under conditions of constant pressure (open to the atmosphere), q reaction = q pressure = delta H reaction
9. What is internal energy? Is internal energy a state function?
Internal energy (E) of a system is the sum of the kinetic and potential energies of all of the particles that compose the system. It is a state function.
3. What is kinetic energy? What is potential energy? List some examples of each.
Kinetic Energy: associated with the motion of an object (ex: a moving billard ball, movement of gas molecules, a raging river) Potential energy: associated with the position or composition of an object (a billard ball raised above the surface of a table, a compressed spring, and bonds in molecules)
5. What is the SI unit of energy? List some other common units of energy.
The SI unit of measurement is defined as the Joule (J), with units Kg x (m^2/s^2). Named after the English scientist James Joule. -Other common units: Kilojoule(kJ), calorie (cal), the Calorie (Cal), and the kilowatt hour (kWh)
13. How is the change in internal energy of a system related to heat and work?
The internal energy (delta E) of a system is the sum of the kinetic and potential energies of all of the particles that compose the system. The change in the internal energy of the system must be the sum of the heat transferred (q) and the work done (w) ie: delta E = q + w
28. What is the standard enthalpy of formation for a compound? For a pure element in its standard state?
The standard enthalpy of a formation compound (∆H₋°) for a pure compound is the change in enthalpy when 1 mole of the compound forms from its constituent elements in their standard states.
27. What is a standard state? What is the standard enthalpy change for a reaction?
The standard state is defined as follows: for a gas, the pure gas at a pressure of 1atm. for a liquid or a solid, the pure substance in its most stable form at a pressure of 1atm and the temperature of interest (often taken to be 25°C); and for the substance in a solution, a concentration of exactly 1M. The standard enthalpy change (∆H°) is the change in enthalpy for a process when all reactants and products are in their standard states. The superscript degree sign indicates standard states.
29. How do you calculate ΔH°rxn from tabulated standard enthalpies of formation?
This equation essentially states that the standard enthalpy change of formation is equal to the sum of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants. and the standard enthalpy of formation values: ΔH fo[A] = 433 KJ/mol. ΔH fo[B] = -256 KJ/mol.
25. Explain how the value of delta H for a reaction changes upon each operation: a) multiplying the reaction by a factor b) reversing the reaction Why do these relationships hold?
a) delta H is multiplied by that same factor b) the sign of delta H is reversed -These relationships hold because delta H is a state function. Twice as much energy contained is twice the quantity of reactants or products. If the reaction is reversed, the final and initial states have been switched and the direction of heat flow is reversed.
30. What are the main sources of the energy consumed in the United States?
combustion of fossil fuels, which include petroleum, natural gas, and coal.
4. State the law of conservation of energy. How does it relate to to energy exchanges between a thermodynamic system and its surroundings?
law of conservation of mass: energy cannot be created or destroyed. It can only be transferred from one object to another, and it can assume different forms. -In an energy exchange, energy is transferred between the system and the surroundings. If the system loses energy, the surroundings gain energy, and vise versa
1. What is thermochemistry? Why is it important?
thermochemistry: the relationship between chemistry and energy. It's important because energy and its uses are important to society. It's important to understand how much energy is required or released in a process.