General Chemistry/Thermodynamics/Introduction

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Thermodynamics is the study of the changes in energy that occur in reactions.

Systems[edit]

A system is the set of substances and energy that is being studied. If, for example, reactions are occurring in a jar, everything inside the jar is the system, and everything outside the jar is the surroundings. The surroundings are everything not in the system, which means the rest of the universe. The system and the surroundings together are called the "universe".

One important issue is what kind of interactions exist between the system and its surroundings. Some systems may exchange matter and heat with the surroundings (like a system of boiling liquid losing matter and heat with the vapor). This is called an open system. If there is just heat exchange occurring between the system and its surroundings it is called a closed system. No matter can enter or leave a closed system. And, finally, if there is no exchange of heat and no exchange of matter, the system is completely isolated.

Temperature vs. Heat[edit]

If you put a hot iron pot under cold water, it cools down very quickly. This is because iron has a low specific heat, which is the amount of energy required to increase the temperature of one gram of a substance by one degree Celsius (or Kelvin). Solid iron only requires 0.45 J/(g*K), while liquid water holds about nine times as much energy (heat) per degree (temperature). So when water receives energy from the pot, it only warms up by a few degrees, while the pot cools down substantially.

Thermochemical Equations[edit]

Equations don't just show chemicals; they also show how much energy is required or released. This energy is called enthalpy. There are two ways to write a thermochemical reaction, for example:

  • 2\hbox{H}_2 + \hbox{O}_2 \to 2\hbox{H}_2\hbox{O} + 571.6\ kJ
  • 2\hbox{H}_2 + \hbox{O}_2 \to 2\hbox{H}_2\hbox{O} \quad (\Delta H = -285.8~kJ)

Note that the number is negative in the second format. Also, notice how the first equation has double the energy because there are two moles of water. The second equation shows the amount of heat per mole, so the number of moles does not affect it.

From a thermochemical reaction, one can tell the molar heat of formation of a compound. Usually, this is measured at 25°C, and is written as ΔHf°. It is the amount of heat required to make one mole of the substance. The small ° symbol means "standard", as in Standard Temperature and Pressure.

Molar heat of combustion is the opposite of the molar heat of formation. It measures the amount of energy released when a mole of the compound is burned. It is written as ΔHc°.