Thermochemistry Equations and Formulas With Practice Problems

The video begins with an introduction to practice problems in thermal chemistry. It covers a scenario where a gas absorbs 2500 joules of heat while expanding from 3 liters to 7 liters at a constant pressure of 4 atm. The process is explained as endothermic, with the surroundings losing thermal energy. The concept of work done by the system during gas expansion is introduced, using the formula W = -PΔV, where W is work, P is pressure, and ΔV is the change in volume. The video emphasizes the sign convention for work done by or on the system and illustrates it with an example involving two cylinders.

This paragraph delves into the calculation of work done by a system during gas expansion, using the formula W = -PΔV, and explains the importance of the sign of ΔV in determining whether work is done by or on the system. It then transitions to calculating the change in internal energy (ΔE) of the system, using the relationship ΔE = Q + W, where Q is the heat absorbed and W is the work done. The difference between the conventions used in chemistry and physics for these calculations is highlighted, and an example calculation is provided to show how the internal energy of the system changes when it absorbs heat and does work.

The script moves on to calculate the heat energy absorbed by 70 grams of water as its temperature rises from 25°C to 60°C. The formula Q = mCΔT is introduced, where m is mass, C is the specific heat capacity of water, and ΔT is the change in temperature. The calculation results in 10250 joules, which is then converted to kilojoules, illustrating the high specific heat capacity of water and its ability to store thermal energy.

This section discusses the concept of specific heat capacity and its relation to a substance's ability to store thermal energy. It provides an example comparing two substances with different specific heat capacities and how they respond to the absorption of one joule of heat energy. The explanation highlights the difference between substances that are good at storing heat energy, like water, and those that are good at transferring heat energy, like metals.

The video script addresses the calculation of heat energy released when 72 grams of liquid water freezes into ice at 0°C. It recommends a conversion process using the molar mass of water and the heat of fusion, which is the energy released when one mole of water freezes. The calculation involves converting grams to moles and then using the heat of fusion to find the energy released, resulting in -24 kilojoules, indicating an exothermic process.

The script explains how to calculate the heat energy released during the complete combustion of 88 grams of propane with oxygen gas. It involves writing the balanced chemical equation for the combustion of propane and using the enthalpy of combustion, which is the amount of heat energy released when one mole of a substance combusts. The calculation converts the mass of propane to moles and then applies the enthalpy of combustion to find the total energy released, resulting in -4440 kilojoules.

The final part of the script focuses on calculating the standard enthalpy of combustion for methane using the standard heats of formation for methane, CO2, and H2O. It involves writing the balanced combustion reaction for methane and applying Hess's law to calculate the enthalpy change of the reaction. The calculation uses the given standard heats of formation and results in a negative value, indicating an exothermic reaction and the amount of heat energy released during the combustion of one mole of methane.