Glossary
Change in Enthalpy (ΔH)
ΔH represents the heat absorbed or released by a system during a process at constant pressure, calculated as the difference between the final and initial enthalpy.
Example:
The ΔH for melting ice is positive, indicating that heat is absorbed from the surroundings to change it from solid to liquid.
Endothermic Reactions
Endothermic reactions are chemical processes that absorb heat from the surroundings, leading to a positive change in enthalpy (ΔH > 0).
Example:
The cold feeling when an instant ice pack is activated is due to an endothermic reaction absorbing heat from your skin.
Energy (E)
Energy is the fundamental capacity to do work or transfer heat, existing in various forms such as kinetic, potential, chemical, and thermal.
Example:
The energy stored in a battery can be converted into electrical energy to power a device.
Enthalpy (H)
Enthalpy is the total heat content of a system at constant pressure, encompassing its internal energy plus the product of its pressure and volume.
Example:
When considering the total energy stored in a block of ice, including the energy from its pressure and volume, you are thinking about its enthalpy.
Enthalpy of Reaction (ΔHrxn)
ΔHrxn is the specific amount of heat absorbed or released when a chemical reaction occurs under constant pressure, typically expressed per mole of reaction as written.
Example:
The ΔHrxn for the combustion of methane is a large negative value, showing that a significant amount of heat is released when methane burns.
Exothermic Reactions
Exothermic reactions are chemical processes that release heat into the surroundings, resulting in a negative change in enthalpy (ΔH < 0).
Example:
A hand warmer getting hot is an example of an exothermic reaction, as it releases heat to your hands.
Heat (q)
Heat is the transfer of thermal energy between objects or systems due to a temperature difference, always flowing from a hotter region to a colder one.
Example:
When you touch a hot stove, heat transfers from the stove to your hand, causing a burn.
Internal Energy (E or U)
Internal energy is the total kinetic and potential energy of all the particles within a system, representing the system's intrinsic energy content.
Example:
As water heats up, its molecules move faster and vibrate more, increasing its internal energy.
Negative ΔH
A negative ΔH signifies that a process is exothermic, meaning heat flows out of the system into the surroundings.
Example:
When natural gas burns, the negative ΔH indicates that the reaction releases heat, warming your home.
Positive ΔH
A positive ΔH indicates that a process is endothermic, meaning heat flows into the system from the surroundings.
Example:
The positive ΔH for photosynthesis means plants must absorb energy from sunlight to convert carbon dioxide and water into glucose.
Work (w)
Work is a form of energy transfer that occurs when a force acts over a distance, often seen in chemistry as expansion or compression of gases.
Example:
A piston moving in an engine as fuel combusts performs work by pushing against an external pressure.