# Heat and Work

## Both heat and work refer to processes by which energy is transfered to or from a substance.

#### Key Points

• Heat is defined as the energy transferred from one system to another by thermal interaction. It is measured in joules.

• Work is the transfer of energy by any process other than heat.

• Heat and work are related. Work can be completely converted into heat, but heat can only be partially converted into work.

#### Terms

• Heat is defined as the energy transferred from one system to another by thermal interaction.

• the study of the energy and heat associated with chemical reactions and/or physical transformations

#### Figures

1. ##### An Illustration of Thermal Equilibrium

When the can of cola and the ice cube come into contact, heat is transferred from the cola can to the ice cube until both bodies reach thermal equilibrium.

2. ##### Heat is Transferred by Conduction or Radiation

Heat is transferred when an object with high thermal energy contacts an object with low thermal energy. Heat travels from the high energy body to the low energy body. Heat can also be transferred by radiation. A hot object can convey heat to any body in its surroundings via electromagnetic radiation in the infrared spectrum.

## Definition of Heat

Heat is defined as the energy transferred from one system to another by thermal interaction. It is measured in joules (J). Heat is transferred by conduction or radiation Figure 2. This means that when a high temperature body is brought into contact with a low temperature body, heat flows from the high temperature body to the low temperature body spontaneously until their temperatures are equivalent. The high temperature body loses thermal energy/triangle E and the low temperature body acquires the same amount of energy. The system is then said to be at thermal equilibrium Figure 1. Therefore, it can be said that heat is a process that involves the transference of thermal energy from a high temperature thermal body to a low temperature thermal body. The transfer of heat can be described by saying that triangle E joules of heat have passed from the high temperature body to the low temperature body.

## Definition of Work

Work is the transfer of energy by any process other than heat. Like heat, the unit measurement for work is joules. There are many forms of work among them including mechanical, electrical, and gravitational work. For our purposes, we are concerned with thermal work which is the work done in an enclosed chemical system. In this type of system, work is defined as the change in the volume (V) within the system (in liters) multiplied by a pressure (P) variable:

Heat and work are related. Work can be completely converted into heat, but heat can only be partially converted into work.

Scientists and engineers have been able to exploit the principles of thermochemistry to development technologies ranging from hot/cold packs to gasoline powered combustion engines.

Work can be defined in many terms, but in chemistry is most applicable when pressure and volume with gases. The product of volume (V) and pressure (P) is an energy. When volume changes (from a to b) at constant pressure, the energy change that results is a value of work:

$W=\int_a^b \! P \, \mathrm{d}V$

## Relationship Between Heat and Work

For a closed system, its change in internal energy (∆U) is related to heat (Q) and work (W) as:

$\Delta U=Q+W$

This means that the total energy within a system is affected by the sum of two possible transfers: heat and work.

#### Key Term Glossary

chemistry
The branch of natural science that deals with the composition and constitution of substances and the changes that they undergo as a consequence of alterations in the constitution of their molecules.
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closed system
A system that can exchange heat and work, but not matter, with its surroundings.
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combustion
a process wherein a fuel is combined with oxygen, usually at high temperature, releasing heat
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constant
Consistently recurring over time; persistent
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energy
a quantity that denotes the ability to do work and is measured in a unit dimensioned in mass × distance²/time² (ML²/T²) or the equivalent
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equilibrium
the state of a reaction in which the rates of the forward and reverse reactions are the same
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heat
Heat is defined as the energy transferred from one system to another by thermal interaction.
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internal energy
A property characteristic of the state of a thermodynamic system, the change in which is equal to the heat absorbed minus the work done by the system.
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joule
in the International System of Units, the derived unit of energy, work and heat; the work required to exert a force of one newton for a distance of one metre; also equal to the energy of one watt of power for a duration of one second; symbol: J
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liter
a non-SI metric system unit of volume equal to 1 cubic decimeter (dm^3), 1,000 cubic centimeters (cm^3) or 1/1,000 cubic meter (m^3)
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measurement
magnitude (or extent or amount) determined by measurement
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Pressure
the amount of force that is applied over a given area divided by the size of this area
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product
a chemical substance formed as a result of a chemical reaction
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substance
Physical matter; material.
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system
the part of the universe being studied, arbitrarily defined to any size desired
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temperature
A measure of cold or heat, often measurable with a thermometer.
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thermal
pertaining to heat or temperature.
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thermal energy
the part of the total potential energy and kinetic energy of an object or sample of matter that results in the system temperature
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thermochemistry
the study of the energy and heat associated with chemical reactions and/or physical transformations
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volume
A unit of three-dimensional measure of space that comprises a length, a width, and a height. It is measured in units of cubic centimeters in metric, or cubic inches or cubic feet in English measurement.
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work
In thermodynamics, work performed by a closed system is the energy transferred to another system that is measured by the external generalized mechanical constraints on the system.