Every reaction gives off or takes in energy. Explore the vocabulary of heat, work and enthalpy.
These terms were extracted from Chemistry by OpenStax, eCampusOntario (© 2016 Rice University; adapted by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License.
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unit of heat or other energy; the amount of energy required to raise 1 gram of water by 1 degree Celsius; 1 cal is defined as 4.184 J
Historically, energy was measured in units of calories (cal).
chemical reaction or physical change that absorbs heat
A reaction or change that absorbs heat is an endothermic process.
capacity to supply heat or do work
Chemical reactions, such as those that occur when you light a match, involve changes in energy as well as matter.
chemical reaction or physical change that releases heat
A change that releases heat is called an exothermic process.
transfer of thermal energy between two bodies
This chapter will introduce the basic ideas of an important area of science concerned with the amount of heat absorbed or released during chemical and physical changes—an area called thermochemistry.
extensive property of a body of matter that represents the quantity of heat required to increase its temperature by 1 degree Celsius (or 1 kelvin)
The heat capacity (C) of a body of matter is the quantity of heat (q) it absorbs or releases when it experiences a temperature change (ΔT) of 1 degree Celsius (or equivalently, 1 kelvin):
SI unit of energy; 1 joule is the kinetic energy of an object with a mass of 2 kilograms moving with a velocity of 1 meter per second, 1 J = 1 kg m²/s and 4.184 J = 1 cal
The SI unit of heat, work, and energy is the joule.
energy of a moving body, in joules, equal to (where m = mass and v = velocity)
Thermal energy is kinetic energy associated with the random motion of atoms and molecules.
energy of a particle or system of particles derived from relative position, composition, or condition
A battery has potential energy because the chemicals within it can produce electricity that can do work.
intensive property of a substance that represents the quantity of heat required to raise the temperature of 1 gram of the substance by 1 degree Celsius (or 1 kelvin)
Specific heat capacity depends only on the kind of substance absorbing or releasing heat.
intensive property of matter that is a quantitative measure of “hotness” and “coldness”
Temperature is a quantitative measure of “hot” or “cold.”
kinetic energy associated with the random motion of atoms and molecules
This chapter introduces many of the basic ideas necessary to explore the relationships between chemical changes and energy, with a focus on thermal energy.
study of measuring the amount of heat absorbed or released during a chemical reaction or a physical change
This chapter will introduce the basic ideas of an important area of science concerned with the amount of heat absorbed or released during chemical and physical changes—an area called thermochemistry.
energy transfer due to changes in external, macroscopic variables such as pressure and volume; or causing matter to move against an opposing force
Energy can be defined as the capacity to supply heat or do work.
device designed to measure the energy change for processes occurring under conditions of constant volume; commonly used for reactions involving solid and gaseous reactants or products
Bomb calorimeters require calibration to determine the heat capacity of the calorimeter and ensure accurate results.
device used to measure the amount of heat absorbed or released in a chemical or physical process
To do so, the heat is exchanged with a calibrated object (calorimeter).
process of measuring the amount of heat involved in a chemical or physical process
One technique we can use to measure the amount of heat involved in a chemical or physical process is known as calorimetry.
unit used for quantifying energy provided by digestion of foods, defined as 1000 cal or 1 kcal
Historically, energy was measured in units of calories (cal).
all matter other than the system being studied
These easy-to-use “coffee cup” calorimeters allow more heat exchange with their surroundings, and therefore produce less accurate energy values.
portion of matter undergoing a chemical or physical change being studied
Visit the Energy Systems tab to create combinations of energy sources, transformation methods, and outputs.
area of science that deals with the relationships between heat, work, and all forms of energy associated with chemical and physical processes
Thermochemistry is a branch of chemical thermodynamics, the science that deals with the relationships between heat, work, and other forms of energy in the context of chemical and physical processes.
sum of a system’s internal energy and the mathematical product of its pressure and volume
Chemists ordinarily use a property known as enthalpy (H) to describe the thermodynamics of chemical and physical processes.
heat released or absorbed by a system under constant pressure during a chemical or physical process
Enthalpy values for specific substances cannot be measured directly; only enthalpy changes for chemical or physical processes can be determined.
work done as a system expands or contracts against external pressure
A type of work called expansion work (or pressure-volume work) occurs when a system pushes back the surroundings against a restraining pressure, or when the surroundings compress the system.
internal energy of a system changes due to heat flow in or out of the system or work done on or by the system
(This is also one version of the first law of thermodynamics, as you will learn later.)
if a process can be represented as the sum of several steps, the enthalpy change of the process equals the sum of the enthalpy changes of the steps
If we have values for the appropriate standard enthalpies of formation, we can determine the enthalpy change for any reaction, which we will practice in the next section on Hess’s law.
compound composed only of hydrogen and carbon; the major component of fossil fuels
Many readily available substances with large enthalpies of combustion are used as fuels, including hydrogen, carbon (as coal or charcoal), and hydrocarbons (compounds containing only hydrogen and carbon), such as methane, propane, and the major components of gasoline.
total of all possible kinds of energy present in a substance or substances
The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E.
heat released when one mole of a compound undergoes complete combustion under standard conditions
Standard enthalpy of combustion () is the enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions.
enthalpy change of a chemical reaction in which 1 mole of a pure substance is formed from its elements in their most stable states under standard state conditions
The standard enthalpy of formation of CO₂(g) is −393.5 kJ/mol.
set of physical conditions as accepted as common reference conditions for reporting thermodynamic properties; 1 bar of pressure, and solutions at 1 molar concentrations, usually at a temperature of 298.15 K
A standard state is a commonly accepted set of conditions used as a reference point for the determination of properties under other different conditions.
property depending only on the state of a system, and not the path taken to reach that state
Internal energy is a type of quantity known as a state function (or state variable), whereas heat and work are not state functions.