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Chemistry 101

Week 7: Thermochemistry

Introduction to Thermochemistry

Thermochemistry is the study of heat and energy associated with chemical reactions and physical transformations. It helps us understand how energy is transferred and converted in chemical systems. In this lesson, we will explore the following key concepts:

Energy, Heat, and Work

Energy is the capacity to do work or transfer heat. There are various forms of energy, such as kinetic, potential, thermal, electrical, and chemical energy. In thermochemistry, we focus primarily on thermal energy, which is the energy associated with the random motion of atoms and molecules.

Heat is the transfer of thermal energy between two objects due to a temperature difference. It always flows from a higher temperature object to a lower temperature object. The unit of heat is the joule (J).

Work is the energy transfer that occurs when an object moves against an opposing force. In chemistry, work often involves changes in pressure or volume. The most common type of work in thermochemistry is pressure-volume work, which occurs when a gas expands or contracts.

In the next section, we will explore how to measure heat using calorimetry.

Calorimetry

Calorimetry is the measurement of heat transfer in a chemical reaction or physical process. It involves the use of a calorimeter, which is an insulated device that minimizes heat exchange with the surroundings. The most common type of calorimeter is the coffee cup calorimeter, which consists of two nested styrofoam cups with a lid.

To measure the heat of a reaction using a coffee cup calorimeter:

  1. Measure a known volume of water and record its temperature.
  2. Add the reactants to the water and stir.
  3. Record the highest or lowest temperature reached.
  4. Calculate the heat absorbed or released by the water using: q = m × c × ΔT, where q is heat, m is the mass of water, c is the specific heat of water (4.18 J/g°C), and ΔT is the change in temperature.
  5. The heat absorbed or released by the reaction is equal in magnitude but opposite in sign to the heat absorbed or released by the water.

Enthalpy

Enthalpy is a thermodynamic quantity that measures the total heat content of a system at constant pressure. It is represented by the symbol H and has units of joules (J). The change in enthalpy (ΔH) during a chemical reaction is the heat absorbed or released by the reaction at constant pressure.

For a chemical reaction: reactants → products, the change in enthalpy is given by:

ΔH = H(products) - H(reactants)

If ΔH is positive, the reaction is endothermic and absorbs heat. If ΔH is negative, the reaction is exothermic and releases heat.

The standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 1 atm pressure and 25°C. By using standard enthalpies of formation, we can calculate the enthalpy change for any reaction using Hess's Law.

In the next lesson, we will apply these concepts to solve thermochemistry problems.