Chapter 6 Review Questions – Flashcards

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.

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.

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=-P?V.

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 ?Erxn 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 ?Hrxn for many aqueous

a. ?H is the heat exchanged with the surroundings under conditions of constant pressure. ?H is equal to qp, the heat at constant pressure. Conceptually (and often numerically), ?H and ?E are similar: they both represent changes in a state function for the system.

However, ?E is a measure of all of the energy (heat and work) exchanged with the surroundings. ?H=?E+P?V

a. An endothermic reaction has a positive ?H and absorbs heat from the surroundings.

An endothermic reaction feels cold to the touch. An exothermic reaction has a negative ?H and gives off heat to the surroundings. An exothermic reaction feels warm to the touch.

a. The internal energy of a chemical system is the sum of its kinetic energy

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.

a. Multiplying the reaction by a factor: If a reaction is multipled by a factor, the ?H is multipled by the same factor. b. Reversing the reaction: If a reaction is reversed, the sign of ?H 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.

a. Hess's law states that if a chemical equation can be expressed as the sum of a series of steps, then ?Hrxn for the overall equation is the sum of the heats of reactions for each step.

This makes it possible to determine ?H for a reaction without directly

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 (?Ho) 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.

a. The standard enthalpy of formation (?Hof) 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, ?Hof =0.

a. To calculate ?Horxn subtract the heats of formations of the reactants multiplied by their