Entropy

1. Entropy 301 Chemistry Page 48-49 of your workbook

2. Kaupapa / Learning Outcome • At the end of todays lesson I will be able to: • define entropy and describe entropy changes • distinguish between spontaneous and non-spontaneous reactions • describe enthalpy and entropy changes associated with spontaneity of reactions

4. What is entropy Entropy is defined as “The extent of randomness and disorder in a chemical system.” Symbol = S Unit = J. K-1.mol-1 (Joules per Kelvin per mole) Kelvin (K) is equal to ˚C + 273.15

5. What is entropy? It is a direct measure of the randomness or disorder of a system. Entropy describes the extent to which atoms, molecules or ions are distributed in a disorderly fashion in a given region in space. It is dependent on temperature and in nature an increase in entropy is favoured In other words all substances prefer to become disordered or more random

6. Magnitude of Entropy Highly ordered substances = Low entropy Highly disordered substances = High entropy What does ‘order’ mean? Order means the extent to which the particles in a substance are confined to a particular space

7. Example of order Water molecules in an ice cube As a solid the molecules are in a fixed, ordered structure = low entropy due to high order When the ice cube melts into a liquid The particles gain energy and are free to move throughout the liquid – increases disorder and increases entropy When the liquid water vaporises The particles gain a large amount of energy and freedom to move = large increase in entropy

8. Increasing Entropy Liquid Solid Liquid Gas Solution

10. Change in entropy The change in entropy is equal to the difference between the entropy if the products and reactants The change in entropy can be used along with the change in enthalpy to predict the spontaneity of a reaction

11. Entropy & Exothermic Reactions • When an exothermic reaction occurs in a system the heat transferred to the surroundings will be used to increase the movement of molecules. • Consequently there will be an increase in disorder in the surroundings and the entropy of the surroundings increases. Surroundings Heat Entropy System Exothermic System

12. Entropy & Endothermic Reactions • Endothermic reactions absorb heat from the surroundings.This causes a decrease in molecular motion resulting in a decrease in disorder in the surroundings. • Resulting in a decrease in entropy of the surroundings Surroundings Heat Entropy System Endothermic System

13. Question • Predict whether the entropy change (∆S) is greater than or less than zero for each of the following processes • Freezing liquid bromine • Evaporating a beaker of ethanol at room temperature • Dissolving sucrose in water • Cooling nitrogen gas from 80 oC to 20 oC System becomes more ordered so that ∆S < 0 System becomes more disordered so that ∆S > 0 System becomes more disordered so that ∆S > 0 System becomes more ordered so that ∆S < 0

14. Consider this Ice will spontaneously melt when the temperature goes above 0˚C at 1 atm of pressure. H2O(s) H2O(l) This process requires the input of energy to break the intermolecular bonds. Therefore it is an endothermic reaction yet its still spontaneous This is because the driving force is the increase in entropy to create an increase in disorder (∆S is positive)

15. Factors that effect entropy • Change in state Any change in state that increases the movement of particles will increase entropy 2. Amount of substance (mol) Whenever the amount of a substance increases during a reaction (more mol of product than reactant) the reaction becomes more random due to more particles moving around therefore entropy increases 3. Increase in temperature This provides the particles with more energy and therefore they move more which increases randomness therefore increases entropy

16. Predicting spontaneity of reactions The equation that relates enthalpy and entropy is known as Gibbs free energy

17. Gibbs free energy

18. At level 3 For Level 3 we need to be able to predict and explain whether a reaction will be spontaneous or not Summary:

19. Try this past examination question Hydrazine is often used as a rocket fuel. When liquid hydrazine undergoes combustion, it forms nitrogen and water: N2H4(ℓ) + O2(g) → N2(g) + 2H2O(g)ΔcH°( N2H4 (ℓ)) = –624 kJ mol–1 Explain why liquid hydrazine readily burns in oxygen. Your answer should consider both enthalpy and entropy changes.

20. Answer Enthalpy change: The combustion of liquid hydrazine is an exothermic process since ΔcH° is negative. Exothermic reactions form products that have lower energy than the reactants / energy is released and this favours the spontaneous / forward reaction. Entropy change: Exothermic reactions release heat to the surroundings, which makes the entropy change of the surroundings positive. As both the surroundings and the system gain entropy, this favours the spontaneous / forward reaction. OR The combustion reaction has more gas molecules in the products / goes from liquid to gas / increase in number of particles. Therefore the entropy of the system increases and this favours the spontaneous / forward reaction. As both enthalpy and entropy are favoured, then hydrazine readily burns / the reaction is spontaneous.

21. Achieved, Merit or Excellence • Achieved: • Recognises entropy increases which favour the reaction. OR • Recognises reaction is exothermic which favours the reaction. • Merit: • Partial explanation refers to both entropy and enthalpy changes. OR • Full explanation for enthalpy or entropy change. • Excellence • Full explanation