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A Thermochemical Equation is a balanced stoichiometric chemical equation that includes the enthalpy change, ΔH. In variable form, a thermochemical equation would look like this: A chemical equation is a symbolic representation of a chemical reaction. ...
Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ...
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.
Understanding Aspects of Thermochemical Equations
Enthalpy (H) is the transfer of energy in a reaction (for chemical reactions it is in the form of heat) and ΔH is the change in enthalpy. ΔH is a state function. Being a state function means that ΔH is independent of the processes between initial and final states. In other words, it does not matter what steps we take to get from initial reactants to final products—the ΔH will always be the same. ΔHrxn, or the change in enthalpy of a reaction, has the same value of ΔH as in a thermochemical equation, but is in units of kJ/mol being that it is the enthalpy change per moles of any particular substance in the equation. Values of ΔH are determined experimentally under standard conditions of 1atm and 25°C (298K). Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ...
The word mole when used alone, has several possible meanings in the English language: Mole (animal): a small burrowing mammal. ...
As discussed earlier, ΔH can have a positive or negative sign. A positive sign means that the system uses heat and is endothermic. The negative value means that heat is produced and the system is exothermic. Endothermic can mean: In biology, an endotherm is a type of animal that can control its body temperature. ...
Exothermic has several meanings, including: In biology, an exothermic or poikilothermic animal is one that requires external sources of heat (usually sunlight) to maintain its internal temperature: for example, reptiles. ...
 Image File history File linksMetadata Endothermic. ...
Since enthalpy is a state function, the ΔH given for a particular reaction is only true for that exact reaction. Physical states (of reactants or products) matter, as do molar concentrations.
This matter of ΔH being dependent on physical state and molar concentration means that thermochemical equations must be stoichiometrically correct. If one agent of the equation is changed through multiplication, then all agents must be proportionally changed, including ΔH. (See Manipulating Thermochemical Equations, below.)
Thermochemical equation’s multiplicative property is largely due to the First Law of Thermodynamics, which says that energy can be neither created nor destroyed, a concept commonly known as the conservation of energy. It holds true on a physical or molecular scale. The first law of thermodynamics, a generalized expression of the law of the conservation of energy, states: // Description The essence of the First Law of Thermodynamics declares: energy cannot be destroyed. ...
Manipulating Thermochemical Equations
Coefficient Multiplication Thermochemical equations can be changed, as mentioned above, by multiplying by any numerical coefficient. All agents must be multiplied, including ΔH. Using our thermochemical equation of variables as above, we get the following example.
A + B → C
ΔH= (±) #
Lets say that we need to multiply A by two in order to use the thermochemical equation (as in addition, below). We must then multiply all the agents in the reaction by the same coefficient, like so:
2A + 2B → 2C
2ΔH= 2[(±) #]
This is again logical when the First Law of Thermodynamics is considered. Twice as much product is produced, so twice as much heat is removed or given off.
It should also be noted that division of coefficients works in the same way.
Hess’s Law: Addition of Thermochemical Equations Hess’s Law states that the sum of the energy changes of all the thermochemical equations included in an overall reaction is equal to the overall energy change. Since ΔH is a state function and so is not dependent on how the reactants become the products, we can use several steps (in the form of several thermochemical equations) to find the ΔH of the overall reaction.
Example:
Reaction (1) C(graphite, s) + O2(g) → CO2(g)
This reaction comes about via two steps (a reaction sequence):
C(graphite, s) + ½O2(g) → CO(g)
ΔH= -110.5kJ
CO(g) + ½O2(g) → CO2(g)
ΔH= -283.0kJ
We want to add these two reactions together to get Reaction (1) so that we can find ΔH, so we check to make sure that agents in the reaction sequence not present in (1) cancel each other. In this example, CO(g) is not in (1) and cancels. We add the reaction sequence together.
C(graphite, s) + ½O2(g) + ½O2(g) → CO2(g)
Or C(graphite, s) + O2(g) → CO2(g), Reaction (1)
To figure out ΔH, we add the ΔH of the two equations in the reaction sequence
(-110.5kJ) + (-283.0kJ) = (-393.5kJ) = ΔH of Reaction (1)
Some things to remember • If you have to reverse a reaction to get things to cancel, the sign of ΔH must also be reversed.
• If you have to multiply an agent to get it to cancel, all other agents and ΔH must also be multiplied by that number.
• Generally ΔH values given in tables are under 1atm and 25°C (298K), so be aware of what conditions your reaction is under.
Where to Find Values of ΔH Values of ΔH have been experimentally determined and are available in table form. Most general chemistry textbooks have appendixes including common ΔH values. There are several online tables available (for a good list of resources see www.library.adelaide.edu.au/guide/sci/chemistry/therm.html, link below). For more extensive information there is software offered with Active Thermochemical Tables (ATcT), available online.
Links Practice exercises: http://www.psigate.ac.uk/newsite/reference/chemlecs/thermoequationscs.html
General chemistry information index: http://chemistry.about.com/library/blazlist4.htm
Further step by step help on Hess’s Law: http://members.aol.com/profchm/hess.html
Resource for ΔH tables: http://www.library.adelaide.edu.au/guide/sci/chemistry/therm.html
Other Wikipedia Articles Chemistry Chemistry (derived from the Arabic word kimia, alchemy, where al is Arabic for the) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. ...
Thermochemistry Thermochemistry is the application of thermodynamics to chemistry. ...
Chemical Reaction A chemical reaction is a process that results in the interconversion of Chemical substances . ...
Enthalpy Enthalpy (symbolized H, also called heat content) is the sum of the internal energy of matter and the product of its volume multiplied by the pressure. ...
Sources Atkins, Peter and Loretta Jones. 2005. Chemical Principles, the Quest for Insight (3rd edition). W. H. Freeman and Co., New York, NY. | 
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