Enthalpy, entropy, and free energy

1. Enthalpy, entropy, and free energy 6.04

2. What does ‘Spontaneous’ Mean in Chemistry? • A spontaneous change is a change in a system that proceeds without a net input of energy from an outside source. • Needs some activation energy to get started • It may be a fast reaction, or slow reaction • Once it is started, it takes place without any continuous outside influence such as heating, cooling, stirring, etc.. • Spontaneous does not mean instantaneous or super fast!

3. What types of reactions can be ‘spontaneous’? • Exothermic reactions can be spontaneous. These reactions have a lower energy state than reactants • Negative enthalpy change • However, Endothermic reactions can also be spontaneous! These reactions will have a higher energy state than the reactants.  • Positive enthalpy change • Enthalpy is represented by ΔH

4. Entropy also affects the spontaneity of a reaction • Entropy is a measure of the disorder, or molecular randomness, of a system. • It is natural for things to become more disordered. If you throw a deck of cards on the floor, they are going to be randomly out of place. It will take effort to sort them and put them into increasing order of the deck. • Entropy is represented by ΔS • Entropy increases when ‘randomness’ increases. For example- solids are more ordered, and gases are more random. Therefore, gases have an increase in entropy compared to solids!

5. Predicting Entropy change of a system….

6. Lets Practice! • Freezing of water- Decrease in entropy • Dissolving salt in water- Increase in entropy because it increases the disorder! • Evaporating rubbing alcohol- Increase in entropy because it is turning into a gas and becoming more disordered! • CO2 + CaO  CaCO3 – Decrease in entropy. As a general rule, the side of the equation with more moles of gas has more entropy. Also, when the total number of moles decrease, entropy typically decreases as well! • 2KClO3  2KCl + 3O2- Increase in entropy. There are more moles in the product as well as a gas produced! Both signs of an increase in entropy!

7. Free Energy • Free Energy is represented by ΔG • A reaction, at a constant temperature and pressure, is spontaneous in the direction that gives a negative change in fee energy (-ΔG) • Temperature, enthalpy, and entropy all have an effect on the spontaneity of a chemical reaction. • Equation to calculate these values... • ΔG = ΔH – (T x ΔS) • The units for temperature used in the equation should ALWAYS be in Kelvin!

8. Great reference Table to determine if a reaction is sponaneous!

9. Lets Practice! • The melting of ice has an enthalpy value of ΔH = +333 kJ/kg. What is true about the spontaneity of this process? • A. Spontaneous at all temperatures • B. Spontaneous at low temperatures • C. Spontaneous at high temperatures • D. Not spontaneous at any temperature

10. Answer • C- Spontaneous at high temperatures • The melting of ice has a positive entropy change (melting from a solid to a liquid is always a positive entropy change) and a positive enthalpy change (ΔH = +333 kJ/kg). For this combination of entropy and enthalpy, the reaction will be spontaneous (-ΔG) at high temperatures.

11. Practice • The combustion of ethane gas has a negative enthalpy change and a positive entropy change. • A. Spontaneous at all temperatures • B. Spontaneous at low temperatures • C. Spontaneous at high temperatures • D. Not spontaneous at any temperature

12. Answer • A- Spontaneous at all temperatures • A negative enthalpy change and a positive entropy change are both “preferred” by nature, so this process is spontaneous at any temperature.

13. Practice • The process of freezing methane (CH4 (l) → CH4 (s)) has a negative enthalpy change (ΔH = -58.4 kJ/kg). • A. Spontaneous at all temperatures • B. Spontaneous at low temperatures • C. Spontaneous at high temperatures • D. Not spontaneous at any temperature

14. Answer • B- Spontaneous at low temperatures. • The freezing of methane has a negative entropy change (freezing from a liquid to a solid is always a negative entropy change) and a negative enthalpy change (ΔH = -58.4 kJ/kg). For this combination of entropy and enthalpy, the reaction will be spontaneous (-ΔG) at low temperatures.