Thermochemistry Chapter 6

The Nature of Energy Energy is the capacity to do work or produce heat. Energy is the capacity to do work or produce heat. Total energy of the universe is constant. Energy lost = Energy gained by something else. Potential Energy = energy due to position= mgh Kinetic Energy = energy of motion = ½ mv 2 Heat= transfer of energy due to temperature differences Work = force used to move an object a distance.

Heat is a transfer of energy System vs Surroundings System vs Surroundings State Function – a property that depends only on the present state of the system…not on the changes it has or will experience. State Function – a property that depends only on the present state of the system…not on the changes it has or will experience. Internal energy Internal energy Pressure Pressure Volume Volume Energy Energy System Surroundings Heat and work are NOT state functions!

Thermodynamic Quantities Consist of two parts 1) the number – indicates how much 2) the sign- indicates direction of flow HEAT q WORK w ENTHALPY H Internal Energy E Negative Values = flow out of system Positive Values = Flow into system

Heat Lost & Heat Gained Draw… …a graph of energy vs. reaction time for a reaction that gives energy to the surroundings AND a second one for a reaction that absorbs energy from the surroundings

Exothermic and Endothermic Exothermic Reaction Feels hot Feels hot Heat transferred to surroundings (lost by system) Heat transferred to surroundings (lost by system) Negative enthalpy and heat values Negative enthalpy and heat values Endothermic Reaction Feels cold Feels cold Heat transferred to system (gained by system) Heat transferred to system (gained by system) Positive enthalpy and heat values Positive enthalpy and heat values

PV Work Work (w) = Force*displacement W = F * d = F *  h W = P * A *  h P = Force/Area VOLUME! When pressure of system does not change  W = -P  V

Try Me Problem A balloon is inflated to its maximum capacity by heating. If the volume changes from 4.0 x 10 6 L to 4.5 x 10 6 L by addition of 1.3 x 10 8 J energy as heat. Assuming that the balloon expands against constant 1.0 atm pressure. Calculate Internal Energy. A balloon is inflated to its maximum capacity by heating. If the volume changes from 4.0 x 10 6 L to 4.5 x 10 6 L by addition of 1.3 x 10 8 J energy as heat. Assuming that the balloon expands against constant 1.0 atm pressure. Calculate Internal Energy. (1 L*atm = J) (1 L*atm = J)

How is enthalpy different? Enthalpy, H, is the amount of energy capable of doing work in a system. Enthalpy, H, is the amount of energy capable of doing work in a system. The amount of energy contained within the bonds of chemicals involved in the system. The amount of energy contained within the bonds of chemicals involved in the system. H = E + PV H = E + PV Answer Now Compare the equation for total internal energy with the equation for enthalpy listed above. How can you alternately define Enthalpy?

Enthalpies of Formation The enthalpy of formation (H f o ) for an element in its standard state is ZERO. Standard State is at 1 atm and 25 o C The more negative the value of H f o, the more stable the compound.  H rxn =  [n p (  H f o prod )]-  [n r (  H f o rct )]

Sample Problems Try Me 1 Try Me 1 Find the enthalpy for the reaction: 4NH 3(g) + HCl (l)  4NH 4 Cl (s) Try Me 2 Try Me 2 Find the enthalpy for the reaction: 2Al (s) + Fe 2 O 3(s)  Al 2 O 3(s) + 2Fe (s)

Experimental Determination of Heat & Enthalpy Specific/Molar Heat of Combustion Specific/Molar Heat of Combustion q = mc  Tq = nc  T -The heat needed to raise the temperature of 1 g (or 1 mol) of substance 1 degree K.

Another way to find  H rxn Experimentally! Experimentally! –  rxn =  H products –  H reactants How do you measure this stuff? How do you measure this stuff? CALORIMETRY! CALORIMETRY!

Hess’s Law Enthalpy is a state function Enthalpy is a state function The value will be the same regardless of how many steps are needed to complete the reaction. The value will be the same regardless of how many steps are needed to complete the reaction. Hess’s Law States: The enthalpies of individual steps in a reaction mechanism can be added together to calculate the enthalpy of the overall reaction.

Fundamentals for Applying Hess’s Law Reverse the reaction, reverse the sign on enthalpy. Reverse the reaction, reverse the sign on enthalpy. Multiply the reaction by a coefficient, multiply the enthalpy by the same coefficient. Multiply the reaction by a coefficient, multiply the enthalpy by the same coefficient. Add the reactions together, add the enthalpies together. Add the reactions together, add the enthalpies together.

Try Me! Overall: N 2 O 4(g) =>N 2(g) + 2O 2(g) Reaction Mechanism: NO 2(g)  ½ N 2(g) + O 2(g) H= kJ 2NO 2(g)  N 2 O 4(g) H= kJ This is fun! Let’s do some more!more