Thermody-nizzle-amics A (Josh)^2 Production

Heating Shindig Amount of energy needed to change a given substance a given temperature depends on; Amount of energy needed to change a given substance a given temperature depends on; The mass you are heating (how much) The mass you are heating (how much) It’s specific heat (how well it can absorb energy) It’s specific heat (how well it can absorb energy) Some random stuff: Specific heat changes from phase to phase; Water has high specific heat (4.186 J/g*K); Metals have low specific heat Some random stuff: Specific heat changes from phase to phase; Water has high specific heat (4.186 J/g*K); Metals have low specific heat Q = MC  T Q = MC  T

Shifting Phaz0rz Phase change occurs at a constant temperature Phase change occurs at a constant temperature How much energy is required to change a substance from one phase or another depends on: How much energy is required to change a substance from one phase or another depends on: The mass of the substance present The mass of the substance present A constant called Heat of fusion (S L) or heat of vaporization (L G) A constant called Heat of fusion (S L) or heat of vaporization (L G) Q = KM Q = KM

Sample Heating Curve of Californium

Chemical Systems Chemical systems simply refer to the actual chemicals (the system) and it’s surroundings System Surroundings

Enthalpy is Hot (sometimes) Enthalpy refers to how much energy is stored within a chemical system Enthalpy refers to how much energy is stored within a chemical system When a reaction occurs, there is a change in enthalpy,  H When a reaction occurs, there is a change in enthalpy,  H A positive change in enthalpy,  H > 0, means that energy is entering the system. Such a process is known as endothermic. A positive change in enthalpy,  H > 0, means that energy is entering the system. Such a process is known as endothermic. A negative change in enthalpy,  H < 0, means that energy is leaving the system. This is an exothermic process. A negative change in enthalpy,  H < 0, means that energy is leaving the system. This is an exothermic process.

But wait… Q: If the energy in the system is going down during an exothermic process, why does it seem exothermic processes make things hot? A: Cuz I said so… and because you’re in the surroundings gaining the energy lost from the system.

How to Find Change in Enthalpy Bond Energies Bond Energies Heat of Formation Heat of Formation Hess’ Law Hess’ Law

Bond Energies Given a table of bond energies, find the total bond energy of the products and of the reactants. Remember coefficients Given a table of bond energies, find the total bond energy of the products and of the reactants. Remember coefficients  H = ∑Bond Energy Reactants  H = ∑Bond Energy Reactants - ∑Bond Energy Products - ∑Bond Energy Products Note: this is the only one where it’s reactants minus products Note: this is the only one where it’s reactants minus products

Heat of Formation This one’s even easier. First find the heat of formation of the products and reactants (It’ll be given, unless…) This one’s even easier. First find the heat of formation of the products and reactants (It’ll be given, unless…) All elemental substances (like O 2, Fe, etc.) have  H° F = 0 J All elemental substances (like O 2, Fe, etc.) have  H° F = 0 J To find enthalpy; To find enthalpy;  H ° rxn = ∑  H° F Products - ∑  H° F Reactants  H ° rxn = ∑  H° F Products - ∑  H° F Reactants

My Homedawg Hess Hess’ Law: If a reaction is the sum of two or more other reactions, the enthalpy change for the overall process is the sum of the enthalpy changes for the constituent reactions. Sweet.

Let’s make methane!?! Use these reactions to find  H° F of CH 4 Use these reactions to find  H° F of CH 4 C + O 2  CO 2  H = kJ C + O 2  CO 2  H = kJ H 2 + ½ O 2  H 2 O  H = kJ H 2 + ½ O 2  H 2 O  H = kJ CH 4 + 2O 2  CO 2 + 2H 2 O  H = kJ CH 4 + 2O 2  CO 2 + 2H 2 O  H = kJ

Stop defying Entropy; go combust right now! Entropy is a measure of disorder in a system. Entropy is a measure of disorder in a system. It is measured by how much energy a substance has compared to its absolute temperature. S = q/T (J/(mol * K)) It is measured by how much energy a substance has compared to its absolute temperature. S = q/T (J/(mol * K)) Chaotic things have greater entropy; gas has more entropy than liquid, and liquid more than solid. Stone’s hair has lots of entropy. Chaotic things have greater entropy; gas has more entropy than liquid, and liquid more than solid. Stone’s hair has lots of entropy. Potentially useful:  S rxn = ∑ S Products - ∑  S Reactants Potentially useful:  S rxn = ∑ S Products - ∑  S Reactants

Hippopotami can cause increases in entropy

How the Universe Works Everything moves towards maximum entropy and minimum enthalpy Everything moves towards maximum entropy and minimum enthalpy Basically, everything wants to spread its matter and energy across the universe Basically, everything wants to spread its matter and energy across the universe 2 nd Law of Thermodynamics: The Entropy of the Universe is always increasing. (don’t worry about the others) 2 nd Law of Thermodynamics: The Entropy of the Universe is always increasing. (don’t worry about the others)

The work of a True Hero, J. Willard Gibbs.  S Universe =  S System +  S Surroundings  S Universe =  S System -  H System /T  S Universe =  S System -  H System /T -T  S Universe =  H System - T  S System -T  S Universe =  H System - T  S System -T  S Universe is given as  G, change in Gibbs free energy. Since  S Universe is positive if  G is negative, if  G < 0, than a reaction is spontaneous (IE product favoring) Summary:  G =  H System - T  S System

Possible Reaction Types Exothermic and increasing entropy; always spontaneous Exothermic and increasing entropy; always spontaneous Endothermic and increasing entropy; spontaneous at higher temperatures Endothermic and increasing entropy; spontaneous at higher temperatures Exothermic and decreasing in entropy; spontaneous at lower temperatures Exothermic and decreasing in entropy; spontaneous at lower temperatures Endothermic and decreasing in entropy; never spontaneous Endothermic and decreasing in entropy; never spontaneous

Gibbs and Q and K Sorry, no proof this time  G =  G ° + RT lnQ R = J/ (Mol * K) If reaction is at equilibrium,  G = 0 so  G ° = -RT lnK

Applications

Throwing Hot Blocks of Metal into Water or Jello or maybe Ethanol Remember energy is conserved and the system will reach equilibrium at a common temperature Remember energy is conserved and the system will reach equilibrium at a common temperature Basic form Basic form M 1 C 1 (T f – T i1 ) = M 2 C 2 (T i2 – T f ) M 1 C 1 (T f – T i1 ) = M 2 C 2 (T i2 – T f )

Calorimeters Calorimeters measure reaction enthalpy by recording change in surroundings. Calorimeters measure reaction enthalpy by recording change in surroundings. Calorimeters will often have their own specific heat, but it factors in their mass so Q = C  T for calorimeters. It is assumed they don’t melt. Calorimeters will often have their own specific heat, but it factors in their mass so Q = C  T for calorimeters. It is assumed they don’t melt. Enthalpy is found by measuring the change in temperature of the calorimeter, calculating its increase in energy, and comparing that to how much stuff was reacted. Enthalpy is found by measuring the change in temperature of the calorimeter, calculating its increase in energy, and comparing that to how much stuff was reacted.

Chipotle Burritos Have Lots of Calories. Gana can eat three of them. Gana is a true hero just like J. Willard Gibbs.

Melting/Freezing and Evaporation/Condensation Points All phase changes are increase in entropy and endothermic or vice versa. The reaction has no favored side when  G is zero, therefore that temperature is the melting/freezing etc. point. From the earlier equation T phase change =  H phase change /  S phase change

FIN