#### A simple rule of thumb is ‘chemistry is the study of matter, and physics is the study of energy.’ This is an over-simplification, though, because matter and energy are irrevocably inter-twined. The energy transfers that take place during chemical reactions are studied extensively in thermodynamics, but energy terms and calculations are sprinkled throughout chemistry. For those students who haven’t had physics (or don’t remember it), this page has a brief summary of the types of energy, units, and calculations.

## Thermal energy:

All chemical reactions and many physical processes are either exothermic or endothermic. Before starting thermodynamics, a chemistry student should 1) understand the difference between the terms heat and temperature; 2) be able to convert temperature units; 3) be able to convert heat units; and 4) be able to calculate heat from temperature change information.

### Concept check

For the gas laws, standard conditions, STP, is defined as a temperature of 0 °C and pressure of 1 atm. In thermodynamics, the standard conditions for formation reactions are 298 K and 1 atm. Are these two temperatures the same? What is the temperature in °F?

No they are not the same. 0 °C is equal to 273 K and 32 °F; 298 K is equal to 25 °C and 77 °F.

A chemical cold pack utilizes an endothermic physical change, the dissolving ammonium nitrate in water. If the temperature of a cold pack containing 100 ml of water drops from 75 °F to 35 °F, how much heat was absorbed from the water? The specific heat capacity of water is 4.184 J/g°C.

The temperature change is 24 - 1.7 °C = 22 °C. Calculate heat using the formula q=ms ?T using the mass of water, 100 g. The answer is 9200 cal.

## Electromagnetic energy:

All we know about atomic and molecular structure we learned by observing light. Before starting quantum mechanics, Chemistry students should be able to 1) convert wavelength to frequency, and vice versa; 2) calculate the energy of a photon given its wavelength or frequency; 3) understand the difference between the mechanisms that light interacts with matter: black body radiation, emission and absorption; and 4) understand the relationship between the appearance of an element’s emission spectra and electron transitions.

### Concept check

A red laser pointer has a wavelength of 632.8 nm. What is its frequency?

The frequency is 4.738 x 10^{14} Hz. Note that nanometers are 1 x 10^{-9 }m.

Calculate the energy of a photon from a laser pointer with a wavelength of 632.8 nm. Planck’s constant is 6.626 x 10^{-34} Js.

The energy is 3.140 x 10^{-19} J for one photon. Note that the wavelength converted to frequency is 4.738 x 10^{14} Hz.