1. Enthalpy changes and entropy changes accompany chemical reactions. This topic demonstrates why some reactions and processes are spontaneous and others are not. ENTROPY

2. Spontaneous Reaction Example: a sparkler – Needs light from a flame for activation – Once lit, the available fuel combusts quickly and completely, releasing large amounts of energy as heat and light Spontaneous reaction is reaction that, given the necessary activation energy, proceeds without continuous outside assistance Spontaneous changes may not be instantaneous or happen rapidly; most are slow and need an input of energy to start

3. Spontaneous reaction Exothermic reactions tend to proceed spontaneously Endothermic reactions such as the electrolysis of water are nonspontaneous, occurring only when a continuous supply of energy is available. spontaneous nonspontaneous TED ED-What triggers a chemical reaction? http://ed.ted.com/lessons/what- triggers-a-chemical-reaction- kareem-jarrah

4. What is a spontaneous reaction?

5. Spontaneous Reaction However, some endothermic reactions like the dissolution of ammonium nitrate in water, are spontaneous even though the products are less energetically stable than the reactants. A physical property called entropy must also be taken into consideration. Why do reactions yielding less stable products occur spontaneously? Studies in thermodynamics have determined that enthalpy is not the only factor that determines whether a chemical or physical change occurs spontaneously.

6. Which room do you like to sleep in? The messy room on the right has more entropy than the highly ordered room on the left. The drive toward an increase in entropy is the natural direction for all processes.

7. Entropy Entropy is a measure of disorder of a system, and is given the symbol S. The units of S are: J K –1 mol –1. ordered disordered low entropy high entropy regular arrangement of particles random arrangement of particles

8. Entropy For example, consider a container with a partition, as shown on the left below. The system is very ordered, so it has a low entropy. When the partition is removed and the container is shaken, the molecules become mixed up and the system becomes more disordered (or the number of ways that energy can be distributed in the system has increased), so the entropy has increased. Entropy is the number of possible arrangements of the particles and their energy in a given system

9. Entropy In the liquid state, the particles are still quite close together but are arranged more randomly, in that they can move around each other The water molecules in ice are in fixed positions and can only vibrate about those positions The entropy increases when its solid form melts into a liquid The system with the higher entropy will energetically be the most stable (as the energy of the system is more spread out when it is in a disordered state

10. Entropy Entropy is the number of possible arrangements of the particles and their energy in a given system – A system consists of the molecules that are reacting in a chemical reaction. A system becomes more stable when its energy is spread out in a more disordered state Entropy changes that occur: – during a change in state, e.g. melting, boiling and dissolving (and their reverse) – during a temperature change – during a reaction in which there is a change in the number of gaseous molecules

11. Changing Entropy As a substance changes state from (s) → (l) → (g); its entropy increases as molecules become more disordered

12. Entropy change for reactions

13. Changing Entropy Increasing the temperature of a substance (without changing state) increases entropy e.g. when heating a gas, the number of different possibilities for arranging the energy over the molecules increases.

14. Changing Entropy When an ionic salt is dissolved in water (s) → (aq); the entropy increases as as the crystal breaks up and the ions find their way between the water molecules

15. Changing entropy If a reaction increases the number of gas molecules, the entropy increases as gases have higher entropies than solids or liquids Simpler substances with fewer atoms have lower entropy values than more complex substances When a process results in an increase in disorder, the entropy change for that process is positive Similarly, a decrease in disorder means that the entropy change is negative

16. Predicting entropy changes

17. Past Question

18. System and Surroundings can exchange energy and matter with surroundings universe = system + surroundings the system is the sample being observed In a chemical reaction, the system is usually made up of the reactants and products. the surroundings is everything else interactions between a system and its surroundings involve exchange of energy and matter can exchange energy, not matter, with surroundings cannot exchange matter or energy with surroundings Three types of systems based on this type of exchange are:

19. System and Surroundings You will know from experience that hot drinks spontaneously cool down to room temperature if you leave them long enough. Hot water molecules have more entropy than cold water molecules, so the entropy of the drink decreases over time as it cools. The coffee cup is not an isolated system, so the entropy of the surroundings must be increasing as the heat energy is released into the room.

20. Entropy changes in the surroundings

21. What triggers a chemical reaction?

22. Enthalpy, Entropy, and Spontaneous Change Changes in the enthalpy, ∆H, and entropy, ∆S, of a system help us to predict whether a change will occur spontaneously Exothermic reactions (-∆H) involving an increase in entropy (+∆S) occur spontaneously, because both changes are favoured Endothermic reactions (+∆H) involving a decrease in entropy (-∆S) are not spontaneous because neither change is favoured carbon dioxide and water will never recombine to form wood and oxygen after a fire has occurred.

23. Enthalpy, Entropy, and Spontaneous Change But what happens in cases where the energy change is exothermic (favoured) and the entropy decreases (not favoured)? Or when the energy change is endothermic (not favoured) but entropy increases (favoured)? In these situations, the temperature at which the change occurs becomes an important consideration as well as free energy When ΔH ∘ and ΔS ∘ have the same sign, the spontaneity of a reaction depends on the temperature.

24. Entropy and Spontaneous Change