Thermodynamics Cont. Subtitle

Transcription

1 Thermodynamics Cont. Subtitle

2 System vs. Surroundings The system- the reactants and products of a reaction The surroundings- everything that surrounds a reaction Thermochemistry is concerned with the flow of heat from the system to its surroundings, and vice-versa ** Recall heat flows from hot bodies to cool bodies

3 Recap: Endothermic vs. Exothermic Heat flowing OUT of a system into its surroundings Exothermic System loses heat as the surroundings heat up Heat change of the system is negative Heat flowing INTO a system from its surroundings Endothermic System gains heat as the surroundings cool down Heat change of the system is positive

4 How to show the transfer of heat 3 ways Thermochemical equation Energy Diagrams H; Enthalpy

5 Exothermic Process Chemical: 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy Physical: H 2 O (g) H 2 O(l) + energy

6 Endothermic Process Chemical: Energy + 2HgO(s) 2Hg(l) + O 2 (g) Physical: Energy + H 2 O (s) H 2 O (l)

7 Exothermic & Endothermic Chemical Reactions Every chemical reaction has an energy change associated with it Exothermic reactions release energy Conversely, endothermic reactions absorb energy The Energy is stored in bonds between atoms

8 How to show the transfer of heat 3 ways Thermochemical equation Energy Diagrams H; Enthalpy

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11 How to show the transfer of heat 3 ways Thermochemical equation Energy Diagrams H; Enthalpy

12 Enthalpy The heat content a substance has at constant temperature and pressure Can t be measured directly because there is no set starting point The reactants start with a heat content The products end up with a heat content So we can measure how much enthalpy changes

13 Enthalpy Symbol is H Change in enthalpy is H If heat is released, the heat content of the products (the system) is lower H is negative (exothermic) If heat is absorbed, the heat content of the products (the system) is higher H is positive (endothermic H = H products -H reactants

14 Heat of Reaction The quantity of heat released or absorbed during a chemical reaction Difference between the stored energy of the reactants and the products Shown by a Thermochemical Equation- an equation that includes the quantity of heat released or absorbed during the reaction Ex: 2H 2 (g) + O 2 (g) 2H 2 O (g) kJ The quantity of heat for this (or any) reaction depends on the amounts of reactants and products. The quantity of heat released during the formation of water from H 2 and O 2 is proportional to the quantity of water formed. Producing twice as much H 2 O(g) would require twice as many moles of reactants and would release 2 x 483.6kJ

15 Thermochemical Equations The physical states of reactants and products must always be included in thermochemical equations because they influence the overall amount of heat exchanged 2H 2 O(g) kJ 2H 2 (g) + O 2 (g) The heat needed for the decomposition of H 2 O would be greater than 483.6kJ if we started with ice because extra heat would be needed to melt the ice and then change the liquid to a vapor

16 Stoichin Thermochemical Equations How much heat is evolved when 266 g of white phosphorus (P 4 ) burn in air? P 4 (s) + 5O 2 (g) P 4 O 10 (s) H= -3013kJ 266 g P 4 (s) 1 mol P 4 (s) 3013 kj 123.9g P 4 (s) 1 mol P 4 (s) Ans: 6470 kj

17 Heats of Fusion & Solidification Molar Heat of Fusion ( H fus ) the heat absorbed by one mole of a substance in melting from a solid to a liquid Endothermic, so H fus = + Molar Heat of Solidification ( H sol )- heat lost when one mole of liquid solidifies Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies H fus = - H sol

18 Heats of Vaporization and Condensation When liquids absorb heat at their boiling points, they become vapors Molar Heat of Vaporization ( H vap ) the amount of heat necessary to vaporize one mole of a given liquid ( H vap = +) Condensation is the opposite of vaporization Molar Heat of Condensation ( H cond )- amount of heat released when one mole of vapor condenses ( H cond = - ) H vap = H cond

19 Hess s Law If you add two or more thermochemical equations to give a final equation, then you can also add the heats of reaction to give the final heat of reaction Also called Hess s law of heat summation (yes, it s called Hess S law)

20 An Example of Hess s Law: Making Graphite from Diamonds We want C (s,diamond ) C (s,graphite ) We have: 1. C (s,diamond ) + O 2 (g) CO 2 (g) H = kj 2. C (s,graphite ) + O 2 (g) CO 2 (g) H = kj Switching equation 2 around and changing the sign of H, we get 1. C (s,diamond ) + O 2 (g) CO 2 (g) H = kj 2. CO 2 (g) C (s,graphite ) + O 2 (g) H = kj C (s,diamond ) C (s,graphite ) H = -1.9kJ

21 Hess s Law and Diamonds Also written as a thermochemical equation: C (s,diamond ) C (s,graphite ) + 1.9kJ Or a Potential Energy Diagram C (s,diamond) C (s, graphite)

22 Spontaneity, Entropy, & Gibbs Free Energy

23 Spontaneous Processes Spontaneous processes are those that can proceed without any outside intervention The gas in vessel B will spontaneously effuse in vessel A but once the gas is in both vessels, it will NOT spontaneously reverse

24 Spontaneous Processes Processes that are spontaneous in one direction are nonspontaneous in the reverse direction

25 Spontaneous Processes Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures Above 0 o C it is spontaneous for ice to melt Below 0 0 C the reverse process is spontaneous

26 Reversible Processes In a reversible process, the system changes in such a way that the system and surroundings can be put back in their original states by exactly reversing the process Changes are infinitesimally small in a reversible process

27 Irreversible Process Irreversible processes cannot be undone by exactly reversing the change to the system. All Spontaneous processes are irreversible

28 What Contributes to Spontaneity? Change in Enthalpy ( H): Heat/energy absorbed or given off Change in Entropy ( S): Entropy is a measure of randomness or disorder Temperature of the Reaction (T): What is the average kinetic energy of the particles in the reaction

29 Enthalpy s Contribution to Spontaneity Many spontaneous processes proceed with a DECREASE in energy, and are Exothermic (produces heat) at STP Endothermic (takes in heat) reactions that are nonspontaneous at room temp, often become spontaneous at higher temperatures Increase in energy often increases spontaneity

30 Enthalpy s Contribution to Spontaneity Endothermic (takes in heat) reactions that are nonspontaneous at room temp, often become spontaneous at higher temperatures

31 Entropy Entropy is a measure of randomness of a system First Law of Thermodynamics- Energy in the Universe is a constant. Energy cannot be created nor destroyed Second Law of Thermodynamics- Randomness or disorder in the universe is increasing What is Entropy (S)? It is a measure of molecular randomness or disorder. Change in entropy is denoted by the symbol S

32 Entropy on the Molecular Level Increasing Entropy Adding particles: adding more particles increases the collisions, and the randomness of motion Adding Energy/Increasing Temperature: velocity of particle motions is increased Increasing Volume: particles are allowed to roam in greater space, more random motion

33 Entropy and Physical States Entropy increases with the freedom of motion of molecules: S(g)> S(l)> S(s) Entropy in solutions: Dissolution of a solid: ions have more entropy, but SOME water molecuels have less entropy (they are grouped around ions) Usually, there is an overall increase in S Exception: very highly charged ions that make a lot of water molecules align around them)

34 Third Law of Thermodynamics At absolute zero, a pure substance exists as a perfect cyrstal, with no molecular movements (no kinetic energy) The entropy of a pure crystalline substance at absolute zero is 0

35 Gibbs Free Energy Gibbs Free Energy is defined as the energy in a system that is available to do useful work

36 Gibbs Free Energy & Spontaneity Can the sign of G tell us whether a reaction is spontaneous? If G is negative, the forward reaction is spontaneous If G is positive, the reverse reaction is spontaneous If G is 0, the system is at equilibrium A + B C

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