Enthalpy, ΔH, is an extensive property and state function that represents the total amount of heat within a chemical reaction or system.
Internal Energy, work and heat
The First Law of Thermodynamics states that the total energy in an isolated system is constant. The meaning of this definition is that energy can change from one form to another, but never be created nor destroyed. This relationship is illustrated by the equation:
U, sometimes shown as E, represents the internal energy of the system or chemical reaction. The variables of q and w represent heat and work respectively.
Heat and Enthalpy of Reaction
At constant temperature heat, q, is equal to the enthalpy of reaction, ΔHRxn.
Work is the amount of energy required to move an object against an external force. The most common form is pressure-volume work and is represented as:
The symbols of P and V represent pressure and volume respectively and by using the Ideal Gas Law we can incorporate moles, the gas constant R, and temperature.
Under constant volume, ΔV = 0. When placing it into the first equation for work we obtain w = 0.
Exothermic vs. Endothermic Processes
In an exothermic process a system or chemical reaction releases heat from the system to the surroundings. This causes the molecules to slow down as they lose thermal energy and begin to form bonds.
The releasing of thermal energy by the reactant molecules also produces an energy diagram where the products are lower in energy than the reactants.
In an endothermic process the system absorbs heat from the surroundings. By absorbing their thermal energy, the molecules speed up enough to break their bonds.
The absorbing of thermal energy by the reactant molecules also produces an energy diagram where the products are higher in energy than the reactants.
A thermochemical equation is a stoichiometric question that now incorporates the heat or enthalpy of a reaction, ΔHRxn. Whereas a normal stoichiometric reaction deals with a mole to mole ratio, a thermochemical equation deals with a ΔH to mole ratio.
Let’s take a look at a typical thermochemical equation practice question.
PRACTICE: How much heat (in kJ/mol) must be absorbed to produce 35.4 g NH3?
STEP 1: Calculate the molecular mass of NH3.
STEP 2: Convert the grams of NH3 into moles.
STEP 3: Perform a mole to ΔH comparison to find the amount of heat released or absorbed.
Using the balanced chemical equation allows you to determine the amount of heat, which in this case will be in units of kilojoules.
Enthalpy of Formation
In a formation equation 1 mole of a product is formed from the standard states of its elements. ΔHf is used to represent the enthalpy of formation for a compound or element.
For example, the formation of Ca(OH)2 comes from the standard states of carbon, oxygen and hydrogen combining together.
By using the enthalpies of formation for each compound we can calculate the enthalpy of reaction, ΔHRxn, by doing products minus reactants.
Enthalpy is also discussed in Chemical Thermodynamics, which looks at the spontaneity of chemical reactions and their connection to entropy and Gibbs Free Energy. In addition we deal with enthalpy when discussing Hess’s Law, calorimetry, specific heat capacity, and the standard states of elements.