THERMOCHEMICAL EQUATIONS
A Thermochemical Equation is a balanced stoichiometric chemical equation that includes the enthalpy
change, ΔH.
A thermochemical equation has two parts: a balanced chemical equation and the change in one or more
thermodynamic quantities (e.g., temperature, energy, or enthalpy) that occurs when that change
occurs.
The balanced equation can describe either a physical change a chemical change.
CHEMICAL THERMODYNAMICS- is the portion of thermodynamics that pertains to chemical reaction
-is expressed by a mathematical framework of thermodynamic equations which relate various
thermodynamic quantities and physical properties measured in a laboratory or production process.
A+B→C
ΔH = (±) #
Where {A, B, C} are the usual agents of a chemical equation with coefficients and “(±) #” is a
positive or negative numerical value, usually with units of kJ.
Endothermic: A + B + Heat → C, ΔH > 0
Exothermic: A + B → C + Heat, ΔH < 0
Energy changes which accompany chemical reactions are almost always expressed by thermochemical
equations.
Example:
CH4(g)+2O2(g)→CO2(g)+2H2O(l) (25°C, 1 atm pressure)ΔHm=–890kJ
Here the ΔHm (delta H subscript m) tells us whether heat energy is released or absorbed when the
reaction occurs as written, and also enables us to find the actual quantity of energy involved.
By convention, if ΔHm is positive, heat is absorbed by the reaction; i.e., it is endothermic. More
commonly, ΔHm is negative indicating that heat energy is released rather than absorbed by the reaction,
and that the reaction is exothermic.
EXAMPLE 2: HEAT ENERGY
How much heat energy is obtained when 1 kg of ethane gas, C2H6, is burned in oxygen according to the
equation:
2C2H6(g)+7O2(g)→4CO2(g)+6H2O(l)ΔHm=–
3120 kJ(3.7.3)(3.7.3)2C2H6(g)+7O2(g)→4CO2(g)+6H2O(l)ΔHm=–3120 kJ
Solution: The mass of C2H6 is easily converted to the amount of C2H6 from which the heat energy q is
easily calculated by means of Eq. (2). The value of ΔHm is –3120 kJ per 2 mol C2H6. The road map is
mC2H6 −→M nC2H6 −→−−ΔHm qmC2H6 →M nC2H6 →ΔHm q
so that
q=1×103 g C2H6×1 mol C2H630.07 g C2H6×−3120 kJ2 mol C2H6=−51 879 kJ=−51.88 MJ(3.7.4)(3.7.5)
The quantity ΔHm is referred to as an enthalpy change for the reaction. In this context the symbol Δ
(delta) signifies change in” while H is the symbol for the quantity being changed, namely the enthalpy.
Two more characteristics of thermochemical equations arise from the law of conservation of energy.
The first is that writing an equation in the reverse direction changes the sign of the enthalpy change. For
example,
H2O(l)→H2O(g)ΔHm=44 kJ
In the image above, the flames input energy into the water, giving it the energy necessary to transition
to the gas phase. Since flames provide the energy for the phase transition, this is an endothermic
reaction (energy is absorbed).
It tells us that when a mole of liquid water vaporizes, 44 kJ of heat is absorbed. This corresponds to the
fact that heat is absorbed from your skin when perspiration evaporates, and you cool off. Condensation
of 1 mol of water vapor, on the other hand, gives off exactly the same quantity of heat.
H2O(g)→H2O(l)ΔHm=–44kJ
It's counterintuitive, but the common summer occurrence seen above is actually exothermic. Since the
reaction isn't highly exothermic (like the combustion of CH4), we find it hard to associate with a release
of energy. Thermodynamics allows us to better understand on a micro level energy changes like this
one.
Rules for Using Thermochemical Equations
1. If a certain process has an enthalpy change DH, the reverse of that process has an
enthalpy change of -DH.
H2O (s)
H2O (l)
DH = 6.00 kJ
For example, melting one mole of ice to liquid water requires the input of 6.00 kJ of enthalpy.
Thus the liquid water has 6.00 kJ more enthalpy than the ice. If the reverse process, freezing the
water to ice, is to occur, the water has to lose that enthalpy. So freezing one mole of liquid water
to ice has an enthalpy change of -6.00 kJ.
Incidentally, as you can see, enthalpy and other thermodynamic changes depend a great deal on
the phases of the reactants and products in a thermochemical equation. Therefore, it is imperative
that the phases always be carefully specified when you are writing thermochemical equations.
Note: A process with a positive DH is called endothermic. A process with a negative DH is
called exothermic. Thus, the reverse of an endothermic process is exothermic, and the reverse of
an exothermic process is endothermic.
2. Multiplying a thermochemical equation by a constant also multiplies the thermodynamic
quantity by that constant.
If it takes 6.00 kJ of enthalpy to melt one mole of ice, then it will take 2*6.00 or 12.0 kJ to melt
two moles of ice, and 0.5*6.00 or 3.00 kJ to melt 0.5 moles of ice.
3. The thermodynamic quantity for the reaction applies as the equation is written. So, it can
be used as a stoichiometric ratio with any of the reactants or products in the reaction. For,
example, in the reaction:
H2 (g) + Cl2 (g)
2 HCl (g)
DHº = -183 kJ
You can use -183 kJ/mol H2, -183 kJ/mol Cl2 or -183 kJ/2 mol HCl as stoichiometric ratios in a
factor-label problem
Sources
Wikipedia (2018, February 23). Thermochemical equation. Retrieved from
https://en.wikipedia.org/wiki/Thermochemical_equation
ChemPages Netorials (n.d.). Thermodynamics: Thermochemical equations. Retrieved from
https://www.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/chemical/c
hemical3.htm
Chemistry LibreTexts (n.d.). Thermochemical equations. Retrieved from
https://chem.libretexts.org/Bookshelves/General_Chemistry/Book%3A_ChemPRIME_(Moore_e
t_al.)/03Using_Chemical_Equations_in_Calculations/3.07%3A_Thermochemical_Equations