Glossary
Adding reactions
A rule in Hess's Law stating that if two or more chemical reactions are added together to yield a net reaction, their individual enthalpy changes (ΔH) are also added to find the net enthalpy change.
Example:
Combining the ΔH values of several steps to find the overall ΔH for a complex synthesis reaction is an application of adding reactions.
Energy (E)
The capacity to do work or produce heat; it is a state function in thermodynamics.
Example:
The total energy stored in a battery before and after discharge is a state function, regardless of how it was discharged.
Enthalpy (H)
A thermodynamic property representing the total heat content of a system at constant pressure; it is a state function.
Example:
The enthalpy change for burning methane is the same whether it burns in one step or multiple steps.
Enthalpy change (ΔH)
The heat absorbed or released by a system at constant pressure during a chemical reaction.
Example:
A negative enthalpy change indicates an exothermic reaction, releasing heat to the surroundings.
Hess's Law
States that the total enthalpy change for a chemical reaction is the same, regardless of the path taken, as long as the initial and final conditions are the same.
Example:
Using a series of known reactions to calculate the Hess's Law enthalpy change for a target reaction that is difficult to measure directly.
Multiplying a reaction
A rule in Hess's Law stating that if a chemical reaction is multiplied by a coefficient, its enthalpy change (ΔH) must also be multiplied by the same coefficient.
Example:
If A → B has ΔH = +50 kJ, then 2A → 2B will have ΔH = +100 kJ, illustrating multiplying a reaction.
Pathway Dependent
A process or function whose outcome or value is influenced by the specific sequence of steps or route taken.
Example:
The amount of work done to move an object can be pathway dependent, as friction might vary with the path taken.
Pathway Independent
A process or function whose outcome or value is the same regardless of the specific route or sequence of steps taken.
Example:
The total displacement of a car from point A to point B is pathway independent, only depending on the start and end points.
Pressure (P)
The force exerted per unit area; it is a state function.
Example:
The pressure inside a sealed container of gas is a state function, regardless of how the gas was introduced.
Reversing a reaction
A rule in Hess's Law stating that if a chemical reaction is reversed, the sign of its enthalpy change (ΔH) must also be reversed.
Example:
If A → B has ΔH = +50 kJ, then B → A will have ΔH = -50 kJ, demonstrating reversing a reaction.
Spectator compounds/species
Chemical species that appear on both the reactant and product sides of intermediate steps in a reaction mechanism and cancel out when the steps are summed.
Example:
In a multi-step reaction, an intermediate like H₂O that is produced in one step and consumed in a subsequent step is a spectator compound.
State Functions
Properties of a system that depend only on its current state, not on the path taken to reach that state.
Example:
The change in elevation when climbing a mountain is a state function; it doesn't matter if you take a steep or winding path, the final height difference is the same.
Temperature (T)
A measure of the average kinetic energy of the particles in a substance; it is a state function.
Example:
The temperature of a cup of coffee is a state function, regardless of whether it was heated slowly or quickly.
Thermodynamics
The branch of science that deals with the relationships between heat and other forms of energy and work.
Example:
Studying how a chemical reaction releases heat into its surroundings falls under the realm of thermodynamics.
Volume (V)
The amount of space occupied by a substance; it is a state function.
Example:
The volume of a balloon at a given temperature and pressure is a state function, irrespective of how it was inflated.