Study Guide

Kinetics Terms

Kinetics Terms

Reaction Kinetics

Measuring how fast a reaction occurs. Someone has to do it, right?

Chemical kinetics

1) The study of reaction rates, and 2) the cause of many sleepless nights for chemistry students worldwide.

Kinetics

Just like reaction kinetics, too. Favored by lazy people who don't want to write an extra work.

Equilibrium

If you ask us, equilibrium should've been spelled EQUALibrium. In equilibrium, every force that acts on an object is canceled out. An object in equilibrium will either stand still or move at constant speed (without acceleration) unless a force changes and puts it out of equilibrium, causing acceleration.

Collision theory

A theory (duh) that says reactants need to do two things for a reaction to occur: slam into one another with sufficient force, and do so at the right angle.

Activation energy

The amount of energy that is needed to kick off a chemical reaction and get the party started.

Transition State Theory

Reactions go through a path where they become less like reactants and more like products. Like adolescence for molecules.

Activated complex

An unstable, intermediate substance that forms as reactants transition into products in a chemical reaction. Sort of like the teenage years of a famous child actor.

Catalyst

A particularly handy molecule that speeds up the rate of a reaction by helping reactants make successful collisions. We call it the Cupid of the chemical reaction world. Ain't love grand?

Reaction rate

The rate at which reactants change into products. In other words, how fast a reaction happens.

Rate Law

Yep, a lot of laws in thermodynamics. This one is a mathematical representation of the rate of a specific reaction.

Rate constant

A value used in the calculation of a reaction rate. This value is always the same (surprise surprise) for a given reaction performed under the same conditions.

Reaction Order

Those crazy exponents above the concentrations in a rate law.

  • Zeroth (0th) Order–Exponent = 0
  • First (1st) Order – Exponent = 1
  • Second (2nd) Order – Exponent = 2

Overall Rate Law

Sum of reaction orders of all reactants.

Half-Life

The time required for a radioactive isotope to decay by half in number. If you have 100% carbon-14 in a substance, the half-life would be the time to 50% carbon-14.

Arrhenius Equation

Equation that relates the rate constant k to activation energy.

Frequency Factor (A)

Constant in Arrhenius equation, which describes how often molecules collide the "proper" way for a reaction to occur. Proper collisions sound very British, but frequency factors can be found anywhere.

Dynamic Equilibrium

Equilibrium where individuals may be changing, but the system as a whole is not changing.

Equilibrium constant

Represented by the symbol Keq, a numerical value that describes the ratio of products to reactants at equilibrium. Keq > 1 mean that the reaction is mostly products, so the forward reaction is favored. Keq < 1 mean that the reverse reaction is favored, because the reaction is mostly reactants.

LeChatelier's Principle

A disturbance to an equilibrium system is resisted by the system.

Thermodynamics

The study of the movement of heat.

First Law of Thermodynamics

This law states that energy is conserved in a closed system. The change in the internal energy, ΔU, of a system relates to energy, Q, in the form of heat, added to the system, and in the form of mechanical work, W, that leaves, or is done by the system. Want an equation? Here's an equation: ΔU = QW. Here are some important sign conventions to remember:

  • Heat, Q, added to the system is always positive (+)
  • Heat, Q, leaving the system is always negative (-)
  • Work, W, done by the system is always positive (+)
  • Work, W, done on the system is always negative (-)

Second Law of Thermodynamics

This law says that heat will always flow from warm to cold in a spontaneous process. Another way of putting it is that the entropy of a system can only increase or stay the same. Even though energy is conserved, we can never turn all of the available heat energy into mechanical work. Just like we can never turn back the clock. Well, as long as the clock is a metaphor for the passage of time, rather than an actual clock.

Third Law of Thermodynamics

Perfect crystal + absolute zero Kelvin = zero entropy

Internal energy

The internal energy of a system is the average energy of all of the molecules in that system. A warm object has high internal energy—its molecules zip about like there's no tomorrow. A cold object has slow-moving molecules, because of its low internal energy.

Heat

A means by which energy is transferred from a hot body to a colder body when the two are placed in thermal contact with one another.

Work

In physics, mechanical work is done any time a force moves an object—and lucky for us, the forces' union hasn't really negotiated a minimum wage yet, so we can get forces to do work all the time for pretty much free. Whenever a force F displaces an object a distance d, the work done is found by W = Fd. The unit of work is then a newton-meter, which works out to be exactly the same as a joule—the unit of energy.

Enthalpy

The change in the thermal energy of a system, also known as the heat of reaction. On the streets it goes by the symbol ΔH.

Entropy

A measurable quantity, often noted as ΔS for kicks, that represents the disorder in a system or surroundings. More technically, knowing the entropy of a system can help us determine the energy available for work in a process. That's right, Nature makes all of her randomness available for good use.

Free Energy

Thermodynamics version of a thumbs up or thumbs down for whether a reaction occurs naturally.

  • Reactions with a –ΔG value = Thumbs up.
  • Reactions with a +ΔG value = Thumbs down.

Spontaneity

Act of being fanciful, willing to make a sudden change

Endothermic

A process that absorbs heat.

Exothermic

Exothermic is the term used to describe a reaction that releases heat.

Joule

This is our standard unit for measuring energy, not to be confused with the rock we're rocking.