Glossary
Activation Energy
The minimum amount of energy required to initiate a chemical reaction. It represents an energy barrier that reactants must overcome to form products.
Example:
Even though combustion is spontaneous, you need to provide some activation energy, like a spark, to start a fire.
Cell potential
The potential difference between the two electrodes of an electrochemical cell, which measures the driving force for the redox reaction.
Example:
A positive cell potential for a galvanic cell indicates that the redox reaction is spontaneous and can produce electrical work.
Coupled Reactions
A strategy where a nonspontaneous reaction is made spontaneous by linking it to a highly spontaneous reaction, often by sharing an intermediate.
Example:
In biological systems, the nonspontaneous synthesis of ATP is often achieved through coupled reactions with spontaneous metabolic processes.
Electrochemistry
The study of the relationship between chemical reactions and electrical energy, involving redox reactions where electron transfer occurs.
Example:
Electrochemistry is fundamental to how batteries work, converting chemical energy into electrical energy.
Electrolytic Cells
Electrochemical cells that use electrical energy to drive nonspontaneous redox reactions. They require an external power source.
Example:
The process of electroplating, where a thin layer of metal is deposited onto an object, occurs in an electrolytic cell.
Energy
The capacity to do work or produce heat. In chemistry, it often refers to the internal energy of a system, which can be transferred or transformed.
Example:
When you burn a log, the chemical energy stored in the wood is converted into heat and light energy.
Enthalpy (ΔH)
A thermodynamic property representing the total heat content of a system. It measures the heat absorbed or released during a chemical reaction at constant pressure.
Example:
The combustion of propane is an exothermic reaction with a negative enthalpy (ΔH), meaning it releases heat to the surroundings.
Entropy (S°)
A thermodynamic property that measures the degree of disorder or randomness in a system. A higher entropy indicates greater disorder.
Example:
When ice melts into liquid water, its entropy (S°) increases because the molecules become more disordered and can move more freely.
Equilibrium
A state in which the forward and reverse reaction rates are equal, resulting in no net change in the concentrations of reactants and products.
Example:
When a saturated sugar solution has undissolved sugar at the bottom, it's at equilibrium because sugar is dissolving and crystallizing at the same rate.
Equilibrium Constant (K)
A value that expresses the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to the power of its stoichiometric coefficient.
Example:
A large equilibrium constant (K) for the Haber process indicates that at equilibrium, there will be a high concentration of ammonia product.
Galvanic Cells
Electrochemical cells that produce electrical energy from spontaneous redox reactions. Also known as voltaic cells.
Example:
A standard AA battery is a type of galvanic cell that generates electricity from a spontaneous chemical reaction.
Gibbs Free Energy (ΔG°)
A thermodynamic potential that measures the maximum reversible work that may be performed by a thermodynamic system at a constant temperature and pressure. It determines the spontaneity of a process.
Example:
A negative value for Gibbs Free Energy (ΔG°) indicates that a reaction, like the burning of glucose in your body, is spontaneous and can release energy.
High Entropy
A state characterized by a greater degree of disorder, randomness, or dispersal of energy within a system.
Example:
A gas expanding into a vacuum exhibits high entropy because its particles are spread out and highly disordered.
Kinetic Control
A situation where the rate of a reaction, rather than its thermodynamic favorability, determines the observed products or whether a spontaneous reaction actually proceeds at a noticeable speed.
Example:
Diamonds are thermodynamically unstable and should convert to graphite, but this conversion is under kinetic control and is extremely slow at room temperature.
Low Entropy
A state characterized by a greater degree of order, less randomness, or more concentrated energy within a system.
Example:
A perfectly ordered crystal lattice at absolute zero would represent a state of low entropy.
Nonspontaneous process
A process that will not occur naturally under a given set of conditions without continuous input of energy from the surroundings.
Example:
Water flowing uphill is a nonspontaneous process and requires a pump or external energy to occur.
Spontaneity
The tendency of a process to occur without continuous external intervention. A spontaneous process proceeds on its own under a given set of conditions.
Example:
Rusting of iron is a spontaneous process because it happens naturally over time when iron is exposed to oxygen and water.
Spontaneous process
A process that occurs naturally without continuous input of energy from the surroundings. It proceeds in a given direction under specified conditions.
Example:
A ball rolling downhill is a spontaneous process because it moves from a higher potential energy state to a lower one without external force.
Surroundings
Everything outside the system that can exchange energy or matter with the system.
Example:
If a hot coffee cup is your system, the air around it and the table it sits on are the surroundings.
System
The specific part of the universe being studied, where a reaction or process is taking place.
Example:
In a beaker containing a chemical reaction, the reactants and products within the beaker constitute the system.
Thermodynamics
The branch of chemistry and physics that deals with the relationships between heat and other forms of energy, and how energy changes affect the spontaneity of processes.
Example:
Studying thermodynamics helps predict if a chemical reaction, like the combustion of methane, will release or absorb energy and whether it will occur on its own.
Universe
Comprises both the system and its surroundings, representing the entirety of what is being considered in a thermodynamic analysis.
Example:
According to the first law of thermodynamics, the total energy of the universe remains constant.