How does the Third Law of Thermodynamics affect chemical reactions at low temperatures?

In one line

The rule

The Third Law of Thermodynamics states: "The entropy of a perfect crystal at absolute zero is exactly zero."

Formula: S = 0 at T = 0 K, where S is entropy (a measure of disorder), and T is temperature in kelvin.

Let's understand it simply

Imagine you are rolling marbles on a smooth floor. At normal temperatures, the marbles move around, bump into each other, and spread out. This is like molecules in a reaction—they have energy, move fast, and can react.

Now, imagine the floor is covered in thick honey and it is freezing cold. The marbles hardly move. At very, very low temperatures, almost nothing moves. This is what happens to atoms and molecules as temperature drops near absolute zero (−273.15°C). They become perfectly ordered and stop moving. The Third Law tells us that at this point, the disorder (entropy) is zero.

How it works

1. At high temperatures, molecules have lots of energy and move around, so they are disordered (high entropy).
2. As temperature drops, their movement slows, and they become more ordered (entropy decreases).
3. Near absolute zero, in a perfect crystal (where every atom is in the right place), there is only one way to arrange the atoms—perfect order (entropy = 0).
4. For chemical reactions, the change in entropy (ΔS) between reactants and products becomes very small at low temperatures.
5. This affects the driving force for reactions, because reactions often depend on changes in entropy to happen.

The story behind it

The Third Law was first clearly stated by Walther Nernst, a German chemist, in the early 1900s. He was trying to understand how chemical reactions behave at very low temperatures, especially how much energy is needed to make them happen. Nernst's work helped chemists calculate how much energy is stored in chemicals, which is important for making new materials and medicines. He even won the Nobel Prize in Chemistry in 1920 for his work.

Where you see it in real life

• Making liquid oxygen or nitrogen: Factories cool air to very low temperatures to separate gases. The Third Law tells us how much energy is needed and how pure the gases can get.
• Super-cold chemistry labs: Scientists study reactions at temperatures close to absolute zero to make new materials or study quantum effects. The Third Law explains why some reactions almost stop.
• Cryopreservation: Freezing cells or tissues for later use. The Third Law helps us understand how molecules behave when almost all motion stops.
• Refrigerators and air conditioners: They can't reach absolute zero. The Third Law explains why it's impossible to cool something to exactly 0 K.
• Making superconductors: Materials that conduct electricity with zero resistance at very low temperatures. The Third Law helps predict their properties.
• Space science: Space is very cold. The Third Law helps scientists understand how molecules and reactions behave in space.

Why it matters

• Predicting reaction limits: It tells us which reactions can or cannot happen at very low temperatures.
• Calculating absolute entropy: Chemists use it to find the exact entropy of substances, which is needed for accurate calculations in thermodynamics.
• Designing industrial processes: Helps engineers know how much energy is needed to cool things or separate gases.
• Understanding why absolute zero is unreachable: No matter how hard we try, we can't remove all the energy from a system.

Worked example

Let’s see how the Third Law affects a reaction: Suppose you want to know if a reaction will happen at a very low temperature.

Given: ΔH = −50 kJ/mol (reaction gives off heat)
       ΔS = −150 J/mol·K (entropy decreases)
       T = 10 K (very low temperature)
Step 1: Write the formula for Gibbs free energy
        ΔG = ΔH − TΔS
Step 2: Convert ΔS to kJ (−150 J = −0.150 kJ)
Step 3: Plug in the numbers
        ΔG = (−50) − (10 × −0.150)
Step 4: Calculate TΔS
        10 × (−0.150) = −1.5
Step 5: ΔG = (−50) − (−1.5) = (−50) + 1.5 = −48.5 kJ/mol
Step 6: At low T, TΔS is small, so ΔG is mostly determined by ΔH

So, at low temperatures, the entropy term (TΔS) is very small. The reaction depends more on enthalpy (ΔH).

Common mistakes

• Thinking entropy is always zero at low T: Only a perfect crystal at absolute zero has zero entropy. Real substances have some disorder.
• Believing reactions always stop at low T: Some reactions can still happen if they give off enough energy (ΔH is very negative).
• Forgetting to convert units: Always match units (J and kJ) when using formulas.
• Assuming absolute zero can be reached: It is impossible to reach exactly 0 K in practice.

Remember it

At absolute zero, a perfect crystal has zero disorder. At very low temperatures, reactions slow down because there is almost no energy left to drive them.