Quiz 7

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?

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?

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?

The standard enthalpy of formation (ΔH°f) 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, ΔH degree f = 0

27. What is the 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 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 be 25 degrees C); and for a substance in solution, a concentration of exactly 1 M. The standard enthalpy change (ΔH°) 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^(degree)rxn from tabulated standard enthalpies of formation?

To calculate ΔH°rxn 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: ΔH°rxn = Σ ΔH°f (products) - Σ nR ΔH°f (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)

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 the 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.

25. Explain how the value of ΔH for a reaction changes upon each operation. a. multiplying the reaction by a factor b. reversing the reaction Why do these relationships hold?

a. If a reaction is multiplied by a factor, the ΔH is multiplied by the same factor. b. If a reaction is reversed, the sign of ΔH is reversed. 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.

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

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

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.

ΔHrxn is an extensive property; therefore, it depends on the quantity of reactants undergoing the 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.

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

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.

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 relatively small change in temperature. This serves to keep air temperature of coastal areas more constant that the air temperature in inland areas.

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 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 the interest.

32. Explain global climate change. What causes global warming? What is the evidence that global warming is occurring?

One of the main products of fossil fuel combustion is carbon dioxide. CO2 is a greenhouse gas, meaning that it allows visible light from the sun to enter the Earth's atmosphere, but prevents heat (in the form of infrared light) from escaping. The result is that CO2 acts as a blanket, keeping Earth warm. However, because of fossil fuel combustion, CO2 levels in the atmosphere have been steadily increasing. This increase is expected to raise Earth's average temperature. Current observations suggest that Earth has already warmed by about 0.6°C in the last century, due to increase of about 25% in atmospheric CO2. Computer models suggest that the warming could worsen if CO2 emissions are not curbed. The possible effects of this warming include heightened storm severity, increasing numbers of floods and droughts, major shifts in agricultural zones, rising sea levels and coastal flooding, and profound changes in habitats that could result in the extinction of some plant and animal species.

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 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 from the laboratory environment. Because the reaction happens under conditions of constant pressure (open to the atmosphere), q(rxn) = q(p) = ΔH(rxn)

30. What are the main sources of energy consumed in the United States?

Most US energy comes from the combustion of fossil fuels, which include petroleum, natural gas, and coal.

31. What are the main environmental problems associated with fossil fuel use?

One of the main problems associated with the burning of fossil fuels is that even though they are abundant in the Earth's crust, they are a finite and nonrenewable energy source. Te other major problems associated with fossil fuel use are related to the products of combustion. Three major environmental problems associated with the emissions of fossil fuel combustion are air pollution, acid rain, and global warming. One of the main products of fossil fuel combustion is carbon dioxide, which is a greenhouse gas.

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

The heat capacity of a system is usually defined as the quantity of heat required to change its temperature by 1 degree Celsius. 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 1 degree celsius.

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

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