Propane, C3H8, natural gas, CH4, and phosphorous, P4 react with oxygen O2, and these reactions release energy in the form of heat and light. No doubt, according to the principle of conservation of energy, energy is required to reverse the reactions. Thus, energy stored in chemicals (compounds) and energy released or absorbed in chemical reactions are called chemical energy, which also covers topics such as bond energy, ionization potential, electron affinity, electronegativity, lattice energy, etc.
For example, at standard conditions, the combustion of 1.0 mole hydrogen with oxygen releases 285.8 kJ of energy. We represent the reaction.
For the reverse reaction, 285.8 kJ/mol is required, and the sign for dH value changes.
|A Chemical Energy Level Diagram|
------------H2(g) + 1/2O2 | | | 286 kJ | | -286 kJ | | | ¯ ------------ H2O
Oil, gas, and food are often called energy by the news media, but more precisely they are sources of (chemical) energy -- energy stored in chemicals with a potential to be released in a chemical reaction. The released energy performs work or causes physical and chemical changes.
It is obvious that the amount of energy released in a chemical reaction is related to the amount of reactants. For example, when the amount is doubled, so is the amount of energy released.
The amount released is 0.15 mol * 285.8 kJ/mol = 42.9 kJ
The sudden release of energy causes an explosion.
A reaction that releases energy is called exothermic reaction. Energy is released in the form of heat, light and (pressure-volume) work. For example, when methane or propane is oxidized by O2, the heat released causes the gas to expand (explosion in some cases); releasing heat & light and doing work at the same time. In this case, the energy source came from chemical reactions instead of at the expense of internal energy described in the previous module.
Endothermic reactions absorb energy, and in all cases, the energy is supplied from another source, in the form of electrical energy, heat or light.
Many chemical reactions involve gases, and when a gas is formed, it displaces other gases by pushing them out against a pressure. Work, defined in Newtonian physics as a force times the distance along the force direction, is performed in such an action. The work is called the pressure-volume (P-V) work, which is a form of energy and it must be analyzed and its quantity included in chemical energy calculations.
The SI units for pressure are N m-2 and that of the volume is m3. Pressure times volume gives the unit of N m, which is the definition of joule,
In cases the pressure varies, an integral approach is required to evaluate the pressure volume work.
The molar mass of KClO3 is 123.5 g/mol and 8.2 g is 0.067 mol. Thus, the amount of oxygen produced is 0.10 (= 0.067*2/3) mol. Apply the ideal gas law to the pressure volume work (P V), w have
w = - D P V = - D n R T = - 0.10 mol*8.312 (J / (mol K))*298 K = - 248 J
The work done is due to the formation of gas O2 which expands against the atmosphere of 1.0 atm or 101.3 kPa. The volume changes of the solids are insignificant compared to that of the gas. In case both pressure and volume change, and the work is the difference of the pressure-volume product, DP V.
The enthalpy, usually represented by H is the energy released
in a chemical reaction under constant pressure, H = qP.
The enthalpy is a convenient property to evaluate for reactions taking
place at constant pressure. Enthalpy differ from internal energy, E,
Energy as the energy
input to a system at constant volume.
The energy released in a chemical reaction raises the internal energy,
E, and does work under constant pressure at the expense of
energy stored in compounds. Thus,
Like the internal energy, enthalpy is also a thermodynamic state function, depending only on the initial and final states of the system, but not on the rate of reaction.
In handbooks and textbooks, the standard enthalpy change is represented by Ho. For simplicity, we use dHo to represent the changes of standard enthalpy in our discussion to avoid (very) slow loading of the delta onto your computer.
When 1.0 mol or 22.4 L of CH4, at 273K and 1 atm, is oxidized completely, the standard enthalpy change is 890.4 kJ. One cubic meter is 1000 L (/22.4 = 44.6 mol). Thus, the standard enthalpy of change is,
Enthalpy is an important topic in thermodynamics. Various methods have been devised for the accurate measurement of heat of reaction under constant pressure or under constant volume. This link gives a more advanced treatment on enthalpy.
When the standard enthalpy is for a reaction that forms a compound from its basic elements also at the standard state, the standard enthalpy of reaction is called the standard enthalpy of formation, represented by dHof. Unless specified, the temperature is 298 K.
|Table of dHof|
|MgS|| -598 kJ/mol|
|NO2|| + 34|
In all the above equations of reaction, the right hand side has only one product and that its coefficient is 1. A general rule is to consider standard enthalpy of formation of all elements at the standard condition to be zero. Then, there is no need to write the complete equation for the tabulation of the standard enthalpy of formation. The above list can be simplified to give the table shown here.
A simple application of the standard enthalpy of formation is illustrated by Example 4.
Ten grams of nitrogen is less than 1 mol, and we carry out the calculation in the following manner:
1 mol N2 - 46.1 kJ 10 g N2 ---------- ---------- = - 32.9 kJ 28.1 g N2 0.5 mol N2Thus, 32.9 kJ is released when 10 g of N2 is consumed.
Standard enthalpies of formation and standard entropies are important thermodynamic data, and this link gives an extensive table of values for some key compounds.
The principle of conservation of energy leads to the formulation of the Hess law. It's application makes the enthalpy of reaction and standard enthalpy of formation very useful.