Unit 14: Thermochemistry

1. Chemistry Unit 14: Thermochemistry

2. So how does this relate to a chemical equation? Heat of Reaction: is the quantity of heat released or absorbed during a chemical reaction • It is really just the different between the stored energy of the reactants and the products. • For example, if a mixture of hydrogen and oxygen is ignited, water will form and heat energy will release explosively. • This reaction does not tell you, however, if heat is absorbed or evolved – this can be done be writing a thermochemical equation, like this: 2H2 (g) + O2 (g)  2 H2O (g) + 483.6 kJ The heat absorbed or released during a chemical reaction at constant pressure is represented by ΔH

3. ΔH is ENTHALPY ΔH is the symbol for a quantity called enthalpy, which is the heat content of a system at constant pressure. It can be represented as –ΔH (for exothermic reactions) or +ΔH (for endothermic reactions). ENTHALPY CHANGES: ΔH = HPRODUCTS - HREACTANTS The sign of the enthalpy change reveals if heat as absorbed or released in the reaction:

4. Thermodynamics: So.. according to the 1st law of thermodynamics, energy is CONSERVED, but where does it go? • Well, that is answered in the 2nd law of thermodynamics, which states that energy tends to disperse. • Thermal energy cannot be transferred from a cold object to a hot object, because this would concentrate energy in one place rather than spreading it out. This concept helps explain why the energy from a fire disperses and spreads out from the source.

5. Disorder Any physical or chemical changes that, once begun, occurs with no outside intervention is called a spontaneous reaction. Examples Explosion, egg breaking, messy locker, spilled milk etcetc etc…. • In thermodynamics, the term for disorder is called entropy (S), which is defined as a measure of the number of possible ways that energy of a system can be distributed. • Relates to the freedom of the system’s particles to move and the number of ways they can be arranged. • Sometimes considered to be a measure of the disorder or randomness

6. Predicting Changes in Entropy Entropy changes associated with changes in state can be predicted. As solid water melts to become a liquid and evaporates to become a gas, the disorder or randomness of the particles increases. ENTROPY OF GAS > LIQUID > SOLID 2. The dissolving of a gas in a solvent always results in a decrease in entropy. The decrease in energy results from the gas being in a more ordered state. 3. Assuming no change in physical state occurs, the entropy of a system usually increases when the number of gaseous product particles is greater than the number of gaseous reactant particles. 2NO2 (g)  2 NO (g) + O2 (g) 2 moles of gas for the reactants is less than 3 moles of gas of the product so there is an increase in entropy. MORE MOLES THE BETTER !

7. Predicting Changes in Entropy With some exceptions, entropy increases when a solid or liquid dissolves in a solvent. When the solid dissolved into the liquid, the entropy increases because the organization of the solid is lost. 5. The random motion of the particles of a substance increases as its temperature increases.

8. Entropy (ΔS) In nature, ΔS universetends to be positive for reactions and processes under the following conditions: The reaction of process is exothermic (which means ∆Hsystem is negative) The entropy of the system increases, so ∆Ssystem is positive. So, if some reactions are spontaneuous, and some are non-spontaneous…how could you change the speed of those reactions? To do that, you would need to know about REACTION KINETICS!

9. Reaction Kinetics! Every chemical reaction proceeds at a definite rate, but can be sped up or slowed down by changing the conditions of the reaction. If you know the change in a product or reactant during a segment of time , you can calculate the average rate of the reaction. The reaction rate of a chemical reaction is generally stated as the change in concentration of reactants per unit of time as a reaction proceeds.

10. Reaction Kinetics! Reaction Rate - how fast a rxn happens Disappearance of reactant / time Appearance of product / time how fast? mol/sec or M/sec

11. Reaction Kinetics! Example: In a reaction between butyl chloride (C4H9Cl) and water, the concentration of C4H9Cl is 0.220 M at the beginning of the reaction. At 4.00 seconds, the concentration of C4H9Cl is .100 M. Calculate the average reaction rate over the given time period expressed as moles of C4H9Cl consumed per liter per second. Started with .220 M at 0 sec and ended with .100M after 4 seconds. This is a difference of .120M over 4 seconds. .120M = .03M per second 4.0 seconds

12. Collision theory In order for reactions to occur between substances, their particles must collide. This is known as collision theory. Collision Theory Summary The two molecules/particles must collide in order to react They must collide with the correct orientation. They must collide with enough energy to merge the valence electrons and disrupt the bonds of the molecules/particles.

13. First we know a collision must take place, but what else must occur? Not enough energy! Incorrect orientation! Correct orientation and energy! No rxn! No rxn! Rxn! The minimum amount of energy that reacting particles must have in order to form the activated complex and lead to a reaction is called the activation energy or Ea

14. with catalyst H H prod reac H = Energy Reaction Progress Catalysts lower A.E. Activated Complex or Transition State Energy of activated complex Breaking Bonds exothermic Ea Making Bonds Reactants Energy of reactants Reactants @ higher energy than products Products Energy of products What does the presence of a catalyst achieve? It lowers the Ea so it takes less energy to make the reaction occur.

16. Energy H Reaction Progress Activated Complex Products @ higher energy than reactants Products Ea endothermic Reactants How do the activation energies between endo and exo reactions compare? Endothermic reactions have much higher activation energies

17. Five Factors That Affect Reaction Rates Nature of Reactants Substances vary greatly in their tendencies to react. The rate of reaction depends on the particular reactants involved

18. Dust explosion! Five Factors That Affect Reaction Rates Surface Area …. Rate Because there are more frequent collisions 2. Surface Area:

19. Five Factors That Affect Reaction Rates 3. Temperature:speed (AKE) of particles Temp … Rate Increased K.E…More frequent & stronger collisions Exothermic rxns – still need to get them started

20. Concen. Five Factors That Affect Reaction Rates 4. Concentration:how much reactant in a given volume. Conc. … Rate . More frequent collisions

21. Five Factors That Affect Reaction Rates 5) Catalyst- lowers energy of activated complex -increases rate but unchanged after reaction -enzymes

22. I adrenaline beta-blocker Inhibitors No rxn! Block collisions