CHEM 1053: Energy

We're going to cover some terminology with regards to energy which we'll use throughout our videos. Energy is defined as the capacity to do work, and work is defined as a force acting over a distance. We can categorize energy as kinetic energy, which is the energy of motion, or potential energy, which is the energy of position or composition. If we brought a 10-kilogram mass to the top of a building, we would be giving it gravitational potential energy. Systems with high potential energy tend to change in ways to lower their potential energy. For the 10-kilogram mass, if it's pushed off the side of the building, the gravitational potential energy will be converted to kinetic energy as it falls. When it hits the ground, that kinetic energy will be converted to heat and sound and will be dissipated in the surroundings. According to the Law of Conservation of Energy, we cannot create or destroy energy, only change it from one form to another or transfer it from one object to another. So in an internal combustion engine, the combustion reaction between gasoline and oxygen molecules converts some of the chemical potential energy in the bonds of those molecules into thermal energy, pressure-volume work (which we'll discuss more later), and ultimately mechanical, electrical, and kinetic energies to move the car forward. The molecules in the exhaust--for example, carbon dioxide and water vapor--have lower chemical potential energy. The joule is the SI unit for energy. 1 joule is also 1 kilogram meter squared per second squared. But there are also other common units for energy. A calorie with a lowercase "c" is the amount of energy required to raise the temperature of 1 gram of water by 1°C. A capital "C" Calorie is equal to 1000 lowercase "c" calories and is the type of Calorie represented on a nutritional label in the United States. The nutritional labels of many other countries use kilojoules. We're going to discuss changes in energy for a process in terms of the system, which is something we'd like to study, and the surroundings, which is everything else in the universe. Since energy cannot be created or destroyed, if our system increases in energy, that energy had to come from the surroundings. So if a process causes energy to be transferred from the surroundings to the system, we'll consider it a positive change in energy. And if a process causes energy to be transferred from the system to the surroundings, we'll consider it a negative change in energy. We can also discuss transferring thermal energy, in particular, between the system and the surroundings. If a process transfers thermal energy from the system to the surroundings, we consider it an exothermic process. Changes in state are exothermic if they progress from a gas to a liquid to a solid. For example, if we wanted to freeze water to make ice, we would put the water in cold surroundings so that some of the thermal energy from the water would be transferred to the surroundings. Freezing is an exothermic process. So is condensation, which is changing from a gas to a liquid, and deposition, which is changing directly from a gas to a solid. Combustion reactions like burning propane in a stove are also exothermic. Dissolving an ionic solid into water can be exothermic or endothermic. If the solution gets warmer, then the process of dissolving released thermal energy, causing the water to increase in temperature. This is exothermic. If we dissolve an ionic solid in water and the solution gets colder, the dissolution process absorbed thermal energy from the water, so it is an endothermic process. In an endothermic process, thermal energy is transferred from the surroundings to the system. Changes of state from solid to liquid to gas are also endothermic. These include melting, which is changing from a solid to a liquid; vaporization, which is changing from a liquid to a gas; and sublimation, which is changing from a solid directly to a gas.