Free Energy

Whether a reaction happens at all, and how fast it happens once it starts, are two completely separate questions — and mixing them up is one of the most common MCAT traps in this part of general chemistry. Gibbs free energy is the tool that answers the first question. Chemical kinetics, the subject of this chapter, answers the second.

Key Takeaways

• Chemical kinetics studies reaction rates and the factors that influence them.

• Gibbs free energy (ΔG) combines enthalpy and entropy to predict spontaneity: ΔG = ΔH − TΔS.

• Negative ΔG means a reaction is spontaneous; positive ΔG means it's non-spontaneous.

• Spontaneous does not mean fast — many spontaneous reactions (including most biochemical ones) proceed so slowly they need catalysts like enzymes to occur on a meaningful timescale.

• Thermodynamics (will a reaction occur?) and kinetics (how fast will it occur?) are independent — thermodynamic favorability tells you nothing about reaction rate.

What Is Chemical Kinetics?

Chemical kinetics is the branch of chemistry concerned with understanding the rates of chemical reactions. It covers the mechanisms by which reactions occur, the factors that influence how fast those reactions proceed, the energy transferred during the process, and how different variables contribute to reaction rate. Before diving into rate mechanisms and rate laws later in this chapter, it's worth first drawing a clear line between kinetics and its close relative, thermodynamics — which is where Gibbs free energy comes in.

Gibbs Free Energy and the ΔG Formula

Gibbs free energy is a measure of the amount of energy available to do work in a chemical system. It's a thermodynamic quantity that combines enthalpy (ΔH) and entropy (ΔS) to predict whether a reaction will occur under constant temperature and pressure.

The formula is:

ΔG = ΔH − TΔS

This equation ties together the reaction's heat content (ΔH), the disorder of the system (ΔS), and the temperature (T) at which the reaction takes place. It gets a full, detailed treatment later, in the thermochemistry chapter — here, the goal is just to understand what ΔG tells you and how to use its sign.

Spontaneous vs. Non-Spontaneous Reactions

The change in Gibbs free energy (ΔG) determines whether a reaction will occur by itself, without outside assistance:

• If ΔG is negative, the reaction is spontaneous.

• If ΔG is positive, the reaction is non-spontaneous.

Why Spontaneous Doesn't Mean Fast

This is the single most important idea in this subtopic: whether a reaction is spontaneous has no bearing on how fast or slow it proceeds.

A spontaneous reaction (negative ΔG) is simply one that is energetically favorable — it doesn't need outside assistance to occur eventually. But "eventually" can mean anything from microseconds to millennia. In fact, nearly every biochemical reaction that enables life to exist is spontaneous, yet many proceed so slowly that, without the aid of enzymes and other catalysts, measurable reaction progress might not actually occur over the course of an average human lifetime. Spontaneity tells you a reaction is possible; it says nothing about when it will happen.

Thermodynamics vs. Kinetics

This distinction between "will it happen" and "how fast will it happen" is exactly the line between two major branches of chemistry:

• Thermodynamics is concerned with the energy changes that occur during a chemical reaction — whether or not a reaction will occur, and whether it's energetically favorable.

• Kinetics is concerned with the rate at which a reaction occurs — how quickly that reaction proceeds once it starts.

Thermodynamics can tell you a reaction is energetically favorable, but it cannot tell you anything about the rate at which that reaction will occur. The two are described by different tools: the change in Gibbs free energy tells you whether a reaction is spontaneous (thermodynamic information), while the rate of a reaction is described by a rate equation (kinetic information) — a tool this chapter builds out in detail in the sections ahead.

Common MCAT Mistakes

• Assuming a spontaneous reaction happens instantly. Spontaneity (negative ΔG) only means a reaction is energetically favorable and can proceed without outside energy input — it says nothing about the timescale, which can range from microseconds to millennia.

• Reading "negative ΔG" as "fast reaction." ΔG is a thermodynamic quantity; rate is a kinetic one. A large negative ΔG can pair with an extremely slow rate if the activation energy barrier is high.

• Treating thermodynamics and kinetics as the same branch of chemistry. Thermodynamics answers whether a reaction is energetically favorable; kinetics answers how fast it proceeds. Neither tool answers the other's question.

• Assuming spontaneous biochemical reactions don't need enzymes. Most reactions that sustain life are spontaneous but far too slow to matter on a biological timescale without a catalyst to speed them up.

MCAT-Style Concept Check

Question: A reaction has a large negative ΔG at body temperature but is observed to proceed extremely slowly without a catalyst present. Which of the following best explains this observation?

• A) The reaction must actually be non-spontaneous despite its negative ΔG value.

• B) Spontaneity and reaction rate are governed by independent thermodynamic and kinetic factors.

• C) A negative ΔG guarantees a fast reaction rate at any temperature.

• D) The reaction's rate-determining step must have a negative activation energy.

Answer: B

Explanation: ΔG (thermodynamics) determines only whether a reaction is energetically favorable, not how quickly it proceeds. A reaction can be strongly spontaneous and still be kinetically slow if it faces a high activation energy barrier — exactly why many spontaneous biochemical reactions require enzymes to proceed at a biologically useful rate.

FAQ

Does a negative ΔG mean a reaction happens fast?

No. A negative ΔG only means the reaction is spontaneous — energetically favorable and able to proceed without outside assistance. How fast it proceeds is a separate, kinetic question that ΔG doesn't address.

What is the difference between thermodynamics and kinetics?

Thermodynamics determines whether a reaction is energetically favorable (spontaneous), described by ΔG. Kinetics determines how fast a reaction proceeds once it starts, described by a rate equation. The two are independent of each other.

What does Gibbs free energy measure?

Gibbs free energy (ΔG) measures the energy available to do work in a chemical system, combining enthalpy (ΔH) and entropy (ΔS) at a given temperature via ΔG = ΔH − TΔS, to predict whether a reaction is spontaneous.

Why do many spontaneous biochemical reactions need enzymes?

Because spontaneity says nothing about speed. Many biochemical reactions have negative ΔG but proceed so slowly on their own that they wouldn't produce meaningful progress within a human lifetime — enzymes lower the activation energy so the reaction proceeds fast enough to sustain life.