Energy and Thermochemical Equations

The first law of thermodynamics states that: energy cannot be created or destroyed. Energy can only change form. Chemically, that usually means energy is converted to work, energy in the form of heat moves from one place to another, or energy is stored up in the constituent chemicals. Heat is defined as that energy that is transferred as a result of a temperature difference between a system and its surroundings. Mathematically, we can look at the change in energy of a system as a function of both heat and work:

where DE is the change in internal energy of a system, q is the heat flowing into the system, and w is the work being done on the system. If q is positive, we say that the reaction is endothermic, that is, heat flows into the reaction from the outside surroundings. If q is negative, then the reaction is exothermic, that is, heat is given off to the external surroundings.

The total energy of the system is defined as the sum of kinetic and potential energies. Kinetic energy is the energy of motion -- the amount of energy in an object that is moving. Potential energy is stationary, stored energy. If you think of a ball sitting on the edge of a table, it has potential energy in the energy possible if it falls off the table.   Potential energy can be transformed into kinetic energy if and when the ball actually rolls off the table and is in motion. In a molecule potential energy is stored in it bonds.  When bonds are broken, that energy is released.

In descriptions of the energy of a system, you will also see the phrase "state properties" or "state functions". A state property is a property that does not depend on how the change takes place. Your bank balance is a state function. Your balance of $100  is completely unaffected by whether you received the money as a gift in one lump some or earned it for working 10 1-hour shifts. The two paths to the balance are different but the balance remains the same. The internal energy of a system, E, is also a state property.

Thermochemical equations bear a strong resemblance to regular chemical equations, but in addition to showing the ratio of reactant and product amounts they also show the amount of energy transferred and the direction of the transfer.  For example:

                                            2 H₂O(l)  +  2H₂(g)   2H₂(g)   +  O₂(g) DH_rxn = 572 kJ

shows that the decomposition is endothermic.  If the equation were written for 1 mole of water all of the other coefficients would need to change and the DH would be halved, or,

                                              H₂O(l)  +  H₂(g)   H₂(g)   +  1/2 O₂(g) DH_rxn = 286 kJ

The magnitude of the DH depends on the amounts of substances specified.

The magnitude of the DH is the same for both forward and reverse reactions but the sign is reversed, so that for the exothermic formation  of 2 moles of water from its elements DH is negative:

                                                2H₂(g)   +  O₂(g)   H₂O(l)  +  H₂(g) DH_rxn = - 572 kJ