Chemistry Lab/Thermodynamics

Thermodynamics is a topic for Chemistry Lab in the 2017 season. It was previously a topic in 2006.

Thermodynamics is a very broad topic, so a variety of problems were used. Although basic enthalpy problems were sometimes found, entropy and Gibbs free energy were significantly more common, as they were more advanced topics.

For supplemental information about thermodynamics, please see the Thermodynamics event page, which is an event dealing solely with thermodynamics. However, it is slightly more oriented to the physical aspect of thermo, rather than the chemical aspect.

First Law of Thermodynamics
Energy is conserved and can neither be created nor destroyed.

This law is sometimes represented as [math]\Delta E(universe) = 0[/math].

In terms of chemistry, this means that energy is transferred by means of heat or work.

As such, the first law is traditionally represented as [math]\Delta E = q + W[/math].

Internal Energy
Thermodynamics often divides the universe into two regions - a system and its surroundings.

Internal energy, or E, is a property of any system. E is the sum of all potential and kinetic energies in the system.

Unlike in engineering, chemistry views energy change from the perspective of a system. That is, increased internal energy represents a positive change in energy.

Work is equal to the product of pressure and change in volume. However, since expansion requires the system to expend energy, a negative term is required to reflect the internal energy of the system.

[math]W = -P\Delta V[/math], or [math]\Delta E = q - P\Delta V[/math].

Exothermic reactions require a decrease in internal energy, or a negative q, while endothermic reactions mean an increase in internal energy and a positive q.

Calorimetry
Heat flow can be measured using a device known as a calorimeter. Calorimeters are designed to hold heat while a chemical reaction is taking place. The reaction occurs in a solution, typically water, such that the change in temperature of the water can be used to calculate the heat flow between the chemical system and the water surrounding it. The most easily attainable calorimeter consists of two nested coffee cups; this provides good heat insulation and a constant pressure.

This equation is used to calculate the heat flow in a reaction: [math]q = m\cdot \Delta T\cdot C_p[/math]. C(p) represents the specific heat of the solution, or the amount of energy required to raise the temperature of a gram of a substance by 1 degree Celsius.

The equation is slightly altered for the calorimeter itself: [math]q = C\cdot \Delta T[/math]. In this case, C represents the heat capacity of the entire calorimeter, or the heat lost due to inefficiency.

The entire heat flow of the reaction is calculated by adding these two sums: [math]q = m\cdot \Delta T\cdot C_p + C\cdot \Delta T[/math].

However, remember that the an endothermic reaction for the calorimeter represents an endothermic reaction for the chemicals themselves.

[math]q(rxn) = -q(cal)[/math].

Enthalpy
Enthalpy represents the heat content of a system, capturing both the internal energy and the energy due to pressure.

[math]H = E + PV[/math].

[math]\Delta H = \Delta E + \Delta PV[/math].

If pressure is kept constant such as in a coffee cup calorimeter, change in enthalpy equals q.

[math]\Delta H = \Delta E + P\Delta V = q - P\Delta V + P\Delta V = q[/math].

Hess' Law is used to calculate enthalpy change simply from the enthalpies of formation or bond enthalpies.

[math]\Delta H = \Sigma \Delta H_f(products) - \Sigma \Delta H_f(reactants)[/math].

[math]\Delta H = \Sigma \Delta H_b(reactants) - \Sigma \Delta H_b(products)[/math]. The bond energies of the reactants represent the influx of energy from breaking bonds, while the bond energies of the products represent the outflow of energy from forming them.