Breakdown, Discussion & Help With Sample Problems: What You Should Know

6.1 Forms of Energy and Their Interconversion

A couple of sets of tricky definitions are found in Section 6.1: heat (q)--which is the energy tgransferred between a system and its surroundings as a result of differences in their temperatures only; and work (w)--which is the energy is transferred when an object is moved by a force.

Thus, the total change in a system's internal energy is delta E = q + w.

Also, the term system is the part of the universe whose change we are going to measure; everything else that is relevant to the change is defined as the surroundings.

Finally, one of the major canons of thermodynamics is the notion of "products minus reactants". In other words, if the reaction of A results in the formation of B...and B has more energy than A...in order to determine the energetic outcome of the reaction...since B's energy is greater than A's...the net energy change for the reaction has a positive sign.

On the other hand, if B has less energy than A...then the net energy change for the reaction has a negative sign.

Another important concept introduced in Section 6.1 is that of the joule, the metric system unit of energy.

One joule is equal to one kilogram meter squared per second squared...and a wise CHEM 200 student reads the top of p 225 (Silberberg) and understands why those units are what they are.

Another important concept discussed in 6.1 is that of the state function. A state function is a property of the system that can be determined completely by its current state, regardless of how it got to that state. In other words, state functions, and the quantities that determine them, are path independent.

Yet another important concept discussed in 6.1 is the first law of thermodynamics, which states that the total energy of the universe is constant. The implication of the first law is simple yet critical: as defined above, the energy of the system and the energy of the surroundings remains constant: energy is conserved.

6.2 Enthalpy: Heats of Reaction and Chemical Change

Using enthalpy changes in chemical reactions enables scientists to ignore what is referred to as PV work, because the change in enthalpy is the change in energy of a reaction plus the product of the constant pressure and the change in volume...which means that enthalpy (delta H) is a state function.

But CHEM 200 students can often "get by" with the knowledge that enthalpy changes in chemical reactions (delta H) are also referred to as heats of reaction.

In addition, reactions in which the enthalpy of the system is decreased (releasing heat to the surroundings) are referred to as exothermic reactions...and the sign of the enthalpy change is negative.

On the other hand, reactions in which the enthalpy of the system is increased (removing heat to the surroundings) are referred to as endothermic reactions...and the sign of the enthalpy change is positive.

click here for a few words about SAMPLE PROBLEM 6.2 (page 230)

Also contained in this section (commencing at the bottom of p. 230) is an interesting discussion that aims to answer the question "In an exothermic chemical reaction, where does the heat come from?"

In essence, the answer is that the total bond energy of the reactants is greater than the total bond energy of the products. Refer to pages 232-3 for additional explanation of this issue.

6.3 Calorimetry: Laboraatory Measurement of Heats of Reaction

Several important terms are introduced in Section 6.3, including specific heat capacity and molar heat capacity.

Also discussed in 6.3 is the practice of calorimetry. A calorimetry experiment utilizes a device called a calorimeter, which enables researchers to measure the heat released (or absorbed) by a physical or chemical process.

click here for a few words about SAMPLE PROBLEM 6.3 (page 234)

click here for a few words about SAMPLE PROBLEM 6.4 (page 235)

6.4 Stoichiometry of Thermochemical Equations

The essential point within Section 6.4 is that energy can be added to the long list of quantities that need to be balanced, when carrying out and/or evaluating a chemical reaction.

click here for a few words about SAMPLE PROBLEM 6.6 (page 238)

6.5 Hess's Law of Heat Summation

...simply stated: the enthalpy change of an overall process is the sum of the enthalpy changes of its individual steps. From Silberberg (p 239): "Hess's law provides a way of calculating the enthalpy changes of an enormous number of reactions because it allows us to imagine that a reaction occurs through steps for which we know the enthalpy changes."

click here for a few words about SAMPLE PROBLEM 6.7 (page 240)

6.6 Standard Heats of Reaction

In yet another complication, it turns out that enthalpy changes vary with reaction conditions. Therefore, a series of standardizes delta values have been developed...which are calculated when all of the reactants and products are in their standard state.

The definition of the standard state of a substance is found on page 241 of Silberberg.

click here for a few words about SAMPLE PROBLEM 6.9 (page 244)

exercises such as numbers 6.24, 6.37, 6.47, 6.55, 6.64, and 6.75 are all straightforward problems of the type that are often found on CHEM 200 exams