1.3.1 Entropy

Entrophy In addition to enthalpy (heat content), there is another important thermodynamic aspect of all chemical reactions – entropy (S).

E n t r o p y A measure of the amount of randomness or disorder in a system. The symbol for entropy: S The unit for entropy: J/ K·mole

Entrophy All substances, be they individual atoms of a single element or a molecule of a compound, possess some degree of disorder because particles are always in constant motion. Thus, S is always a positive number.

Entrophy Can you think of when S is theoretically equal to zero? S = 0 only for pure crystals at absolute zero ( 0 K or -273°C) This is known as the Third Law of Thermodynamics

Entrophy Can S ever be a negative number? Answer - no. A substance can not be less random than not random at all. We are typically concerned with how entropy changes during a chemical reaction, or with ΔS rather than S: ΔS = Sfinal – Sinitial Units become J/K, since now we are concerned with the entire system instead of just one mole

Entrophy What does the value of ΔS tell us about how entropy changes? Let's make up some numbers and see. Gas particles move much more than do particles in the liquid phase, making them more random or disordered. Let's give a gas particle an entropy value of 10 and a liquid particle an entropy value of 5 (because it is less random, it should have a smaller number).

Entrophy If a system changes from a gas state to a liquid state, it becomes less random, or more ordered, because liquid particles move about less randomly than do gas particles. Now calculate ΔS : ΔS = Sliquid – Sgas ΔS = 5 – 10 = –5

Entrophy A negative value of ΔS indicates a decrease in entropy— the system becomes less random A positive value of ΔS indicates an increase in entropy— the system becomes more random

The Law of Disorder a.k.a Second Law of Thermodynamics states that systems tend to become more random over time, not more ordered.

Predicting Entropy Changes You can predict entropy changes by looking at several factors in an equation. The following changes suggest an increase in entropy: Changes in state: solid → liquid liquid → gas solid → gas solid or liquid→ aqueous state (dissolving)

Entrophy An increase in the number of moles. If the product side has more moles than the reactant side, the system has become more random; Increasing the temperature. An increase in temperature increases the degree of randomness

Calculating Entropy Changes It is possible to calculate a value for ΔS. It is the same formula we used for Hess's Law, only now we are working with values for entropy instead of enthalpy: ΔS = ΣSproducts – ΣSreactants You'll find values for ΔS in the same Table of Thermochemical Data that you used for calculating ΔH.

Example: Calculate ΔS and state whether entropy increases (becomes more random) or decreases (becomes less random)? Do you predict a spontaneous reaction? 2 NO(g) + O2(g) →N2O4(g)

Practice Problems 1. Predict whether entropy is increasing (ΔS > 0) or decreasing (ΔS < 0)? Give a reason for your answer. steam condenses to water solid CO2 sublimes N2O4 (g) → 2 NO2 (g) water is heated from 25°C to 50°C C6H6 (l) + O2 (g) → 6 CO2 (g) + 3 H2O (l)

Practice problem #2 Calculate ΔS for the following reaction. Is entropy increasing or decreasing? Is the system becoming more random or less random? Based on entropy changes only, would you predict the reaction to be spontaneous or not? Na (s) + ½Cl2 (g) → NaCl (s)