1.Molecular orbital of o2
2. The first Law of thermodynamics. Enthalpy. Show the calculation of enthalpies of reaction based on Enthalpies of Formation.
The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can neither be created nor destroyed; energy can only be transferred or changed from one form to another. For example, turning on a light would seem to produce energy; however, it is electrical energy that is converted.
A way of expressing the first law of thermodynamics is that any change in the internal energy (∆E) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by the surroundings:
ΔE=q+w
The internal energy of a system of particles, U, is the sum of the kinetic energy in the reference frame in which the center of mass is at rest and the potential energy arising from the forces of the particles on each other.
This law says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system. Since both heat and work can be measured and quantified, this is the same as saying that any change in the energy of a system must result in a corresponding change in the energy of the surroundings outside the system. In other words, energy cannot be created or destroyed. If heat flows into a system or the surroundings do work on it, the internal energy increases and the sign of q and w are positive. Conversely, heat flow out of the system or work done by the system (on the surroundings) will be at the expense of the internal energy, and q and w will therefore be negative.
Endothermic - Reaction in which a system ABSORBS heat from its surroundings.
Standard-State Enthalpy of Reaction (H)
Three factors can affect the enthalpy of reaction:
The concentrations of the reactants and the products
The temperature of the system
The partial pressures of the gases involved (if any)
The isobaric process is shown in Figure (a), where the pressure of the system remains constant. Both the volume and temperature change. The isothermal process is shown in Figure (b), where the temperature of the system remains constant; therefore, by the ideal gas laws, the product of the volume and the pressure remains constant. An adiabatic process is shown in Figure (c), where there is no heat exchange with the outside world. An isochoric process is shown in Figure (d), where the volume of the system remains constant as the pressure and temperature change.
Enthalpy of formation (Hf)
The enthalpy associated with the reaction that forms a compound from its elements in their most thermodynamically stable states. These are measured on a relative scale where zero is the enthalpy of formation of the elements in their most thermodynamically stable states.
The standard-state enthalpy of reaction is equal to the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants:
Sample enthalpy of formation calculation
Compound Hf (kJ/mol-K)
B5H9(g) 73.2
B2O3(g) -1272.77
O2(g) 0
H2O(g) -241.82
The heat of formation of O2 is zero because this is the form of the oxygen in its most thermodynamically stable state.
In the reaction above 2 moles of B5H9 react with 12 moles of O2 to yield five moles of B2O3 and 9 moles of H2O. We find theHf by subtracting the sum of the enthalpies of the reactant from the sum of the enthalpies of the products:
