Chapter 6 Review Questions

24. Is the change in enthalpy for a reaction an extensive property? Explain the relationship between ΔH for a reaction and the amounts of reactants and products that undergo reaction.

a. ΔHrxn is an extensive property; therefore, it depends on the quantity of reactants undergoing reaction. ΔHrxn is usually reported for a reaction involving stoichiometric amounts of reactants and is dependent on the specific chemical reaction. For example, for a reaction A+2B→C, ΔHrxn 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.

8. What is a state function? List some examples of state functions.

a. A state function is a function whose value depends only on the state of the system, not on how the system arrived at that state. Examples are pressure, volume, and internal energy.

14. Explain how the sum of heat and work can be a state function, even though heat and work are themselves not state functions.

a. According to the first law of thermodynamics, the change in the internal energy of the system (ΔE) must be the sum of the heat transferred (q) and the workd done (w): ΔE=q+w. The total change in internal energy (ΔE) is the difference between its initial energy and its final energy. The amount of the work done and the amount of heat transferred is dependent on the details of the path. In one path, more energy may be transferred through conversion to heat energy (if, for example, there is more friction). In another path, more energy may be transferred through work. Work and heat are not state functions, but their sum (ΔE) is constant.

10. If energy flows out of a chemical system and into the surroundings, what is the sign of ΔEsystem?

a. Energy flowing out of the system is like a withdrawal from a checking account; therefore, it carries a negative sign.

21. Explain the difference between an exothermic and an endothermic reaction. Give the sign of ΔH for each type of reaction.

a. An endothermic reaction has a positive ΔH and absorbs heat from the surroundings. An endothermic reaction feels cold to the touch. An exothermic reaction has a negative ΔH and gives off heat to the surroundings. An exothermic reaction feels warm to the touch.

16. Explain how the high specific heat capacity of water can affect the weather in coastal regions.

a. 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 than the air temperature in inland areas.

12. What is heat? Explain the difference between heat and temperature.

a. Heat is the flow of thermal energy caused by a temperature difference. Thermal energy is a type of kinetic energy because it arises from the motions of atoms or molecules within a substance. The higher the temperature, the greater the motion of atoms and molecules. Heat is measured in units of energy (e.g. joules, calories, and kilowatt-hours), while temperature is measured in units of Kelvins, degrees Celsius, and degrees Fahrenheit.

26. What is Hess's law? Why is it useful?

a. Hess's law states that if a chemical equation can be expressed as the sum of a series of steps, then ΔHrxn for the overall equation is the sum of the heats of reactions for each step. This makes it possible to determine ΔH for a reaction without directly measuring it in the laboratory. If you can find related reactions (with known ΔH) that sum to the reaction of interest, you can find ΔH for the reaction of interest.

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 ΔE for the reaction? In which direction does energy flow?

a. If the reactants have a lower internal energy than the products, ΔEsystem 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?

a. 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 ΔErxn for combustion reactions. The calorimeter includes a tight-fitting, sealed container that forces the reaction to occur at constant volume. A coffee-cup calorimeter is used to measure ΔHrxn for many aqueous reactions. The calorimeter consists of two Styrofoam coffee cups, one inserted into the other, to provide insulation form the laboratory environment. Because the reaction happens under conditions of constant pressure (open to the atmosphere). Qrxn=qp=ΔHrxn

25. Explain how the value of ΔH for a reaction changes upon each operation:

a. Multiplying the reaction by a factor: If a reaction is multipled by a factor, the ΔH is multipled by the same factor. b. Reversing the reaction: If a reaction is reversed, the sign of ΔH is reversed. c. Why do these relationships hold?: The relationships hold because H is a state function. Twice as much energy is contained in 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.

15. What is heat capacity? Explain the difference between heat capacity and specific heat capacity.

a. The heat capacity of a system is usually defined as the quantity of heat required to change its temperature by 1oC. Heat capacity (C) is a measure of the system's ability to hold thermal energy without undergoing a large change in temperature. The difference between heat capacity (C) and specific heat capacity (Cs) is that the specific heat capacity is the amount of heat required to raise the temperature of 1 gram of the substance by 1oC.

9. What is internal energy? Is internal energy a state function?

a. The internal energy (E) of a system is the sum of the kinetic and potential energies of all of the particles that compose the system. Internal energy is a state function.

13. How is the change in internal energy of a system related to heat and work?

a. The internal energy (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 (ΔE) must be the sum of the heat transferred (q) and the work done (w): ΔE=q+w.

23. From a molecular viewpoint, where does the energy absorbed in an endothermic chemical reaction go? Why does the reaction mixture undergo a decrease in temperature even though energy is absorbed?

a. The internal energy of a chemical system is the sum of its kinetic energy and its potential energy. It is this potential energy that absorbs the energy in an endothermic chemical reaction. In an endothermic reaction, as some bonds break and others form, the protons and electrons go from an arrangement of lower potential energy to one of higher potential energy, absorbing thermal energy in the process. This absorption of thermal energy reduces the kinetic energy of the system. This is detected as a drop in temperature.

22. From a molecular viewpoint, where does the energy emitted in an exothermic chemical reaction come from? Why does the reaction mixture undergo an increase in temperature even though energy is emitted?

a. The internal energy of a chemical system is the sum of its kinetic energy and its potential energy. It is this potential energy that is the energy source in an exothermic chemical reaction. Under normal circumstances, chemical potential energy (or simply chemical energy) arises primarily from the electrostatic forces between the protons and electrons that compose the atoms and molecules within the system. In an exothermic reaction, some bonds break and new ones form and the protons and electrons go from an arrangement of higher potential energy to one of lower potential energy. As they rearrange, their potential energy is converted into kinetic energy, the heat emitted in the reaction. This increase in kinetic energy is detected as an increase in temperature.

28. What is the standard enthalpy of formation for a compound? For a pure element in its standard state?

a. The standard enthalpy of formation (ΔHof) for a pure compound is the change in enthalpy when 1 mole of the compound forms from its constituent elements in their standard states. For a pure element in its standard state, ΔHof =0.

27. What is a standard state? What is the standard enthalpy change for a reaction?

a. The standard state is defined as follows: for a gas, the pure gas at a pressure of exactly 1 atm; for a liquid or solid, the pure substance in its most stable form at a pressure of 1 atm and the temperature of interest (often taken to the be 25oC; and for a substance in solution, a concentration of exactly 1 M. The standard enthalpy change (ΔHo) 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.

18. What is pressure-volume work? How is it calculated?

a. The work caused by an expansion of volume is simply the negative of the pressure that the volume expands against multiplied by the change in volume that occurs during the expansion: w=-PΔV.

29. How do you calculate ΔHorxn from tabulated standard enthalpies of formation?

a. To calculate ΔHorxn subtract the heats of formations of the reactants multiplied by their stoichiometric coefficients from the heats of formation of the products multiplied by their stoichiometric coefficients. In the form of an equation: i. ΔHorxn =Σnp ΔHof (products)-Σnp ΔHof(reactants)

17. If two objects, A and B, of different temperature come into direct 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? (Assume that the two objects do not lose any heat to anything else.)

a. When two 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 is equal to the amount of heat gained by the cooler object. The warmer object's temperature will drop and cooler object's temperature will rise until they reach the same temperature. The magnitude of these temperature changes depends on the mass and heat capacities of the two objects.

20. What is the change in enthalpy (ΔH) for a chemical reaction? How is ΔH different from ΔE?

a. ΔH is the heat exchanged with the surroundings under conditions of constant pressure. ΔH is equal to qp, the heat at constant pressure. Conceptually (and often numerically), ΔH and ΔE are similar: they both represent changes in a state function for the system. However, ΔE is a measure of all of the energy (heat and work) exchanged with the surroundings. ΔH=ΔE+PΔV