Thermochemical equation

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A Thermochemical Equation is a balanced stoichiometric chemical equation that includes the enthalpy change, H H. In the form of a variable, the thermochemical equation will look like this:

A B -> C

? H = (Ã, Â ±) #

Where {A, B, C} is the usual agent of chemical equations with coefficients and "(Ã, Â ±) #" is a positive or negative numerical value, usually with kJ.

Video Thermochemical equation

Understanding Aspects of Thermochemical Equations

Enthalpy (H) is the transfer of energy in a reaction (for that chemical reaction in the form of heat) and? H is an enthalpy change. ? H is a state function. Being a state function means? H is independent of the process between the initial and final states. In other words, it does not matter what step we take from the initial reactant to the final product - H will always be the same. ? Hrxn, or enthalpy change of a reaction, has the same value? H as in thermochemical equations, but in kJ/mol units is that it is an enthalpy change per mole of a certain substance in the equation. Values? H was determined experimentally under standard conditions 1atm and 25Ã, Â ° C (298.15K).

As discussed earlier, H H can have a positive or negative sign. A positive sign means the system uses heat and endotherms. A negative value means that heat is generated and the system is exothermic.

Endotermik: A B Panas -> C,? H & gt; 0

Eksotermis: A B -> C Panas,? H & lt; 0

Since enthalpy is a function of state, the H given for a particular reaction is only true for the right reaction. Physical state (reactant or product) is important, as does molar concentration.

Problem H depends on the physical state and molar concentration means the thermochemical equation must be stoichiometrically correct. If one equation agent is changed through multiplication, then all agents must be changed proportionally, including? H. (See Manipulating Thermochemical Equations, below.)

The nature of thermochemical multiplication is largely due to the First Law of Thermodynamics, which says that energy can not be created or destroyed, a concept known as energy conservation. This applies on a physical or molecular scale.

Maps Thermochemical equation

Manipulating thermochemical equations

Multiplication coefficient

The thermochemical equation can be changed, as mentioned above, by multiplying it by any numerical coefficient. All agents must be multiplied, including? H. Using the thermochemical equations of the variables as above, we get the following example.

A B -> C

? H = (Ã, Â ±) #

Assume that one needs to multiply A by two to use the thermochemical equation (in addition, below). One then has to multiply all the agents in the reaction with the same coefficients, such as:

2A 2B -> 2C

2? H = 2 [(Ã, Â ±) #]

This is once again logical when the First Law of Thermodynamics is considered. Twice as many products are produced, so that twice as much heat is eliminated or released.

It should also be noted that the division coefficients work in the same way.

Hess's Law: The Addition of Thermochemical Equations

Hess's law states that the amount of energy change of all thermochemical equations included in the overall reaction equals the overall energy change. Because? H is a state function and does not depend on how the reactants become products, we can use some steps (in the form of some thermochemical equations) to find? H of the whole reaction.

Contoh:

Reaksi (1) C _{(graphite, s)} O _{2 (g)/dd>}

This reaction takes place through two steps (a series of reactions):

C _{(graphite, s)} Ã,½O _{2 (g)} -> CO _(g)

? H = -110.5 kJ

-> CO _{2 (g)}

CO _(g)

? H = -283.0 kJ

We want to add these two reactions together to get Reaction (1) so we can find? H, so we check to make sure that the agent in the reaction sequence is not present in (1) cancel each other. In this example, CO _(g) is not in (1) and cancel. We added a joint reaction sequence.

C _{(graphite, s)} Ã,½O _{2 (g)} Ã,½O _{2 (g)} -> CO _{2 (g)}

Yours

C ->

To find out? H, we add? H of the two equations in the reaction sequence:

(- 110.5 kJ) (-283.0 kJ) = (-393.5 kJ) =? H Reactions (1)

Some things to keep in mind

• If you have to reverse the reaction to cancel, sign? H should also be reversed.
• If you have to multiply the agent to cancel it, all other agents and? H should also be multiplied by that number.
• Generally? The H values ​​given in the table are below 1atm and 25Ã, Â ° C (298.15 K), so beware of what conditions your reactions are under.

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Where Search for Value? H

Values? H has been determined experimentally and is available in tabular form. Most general chemistry textbooks have appendices including common H values. There are several online tables available. For more information there is software offered with Active Thermochemical Tables (ATcT), available online.

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See also

• Chemicals
• Thermochemistry
• Chemical Reaction
• Enthalpy

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References

Atkins, Peter, and Loretta Jones. 2005. Chemical Principle, Quest for Insight (3rd ed.). W. H. Freeman and Co., New York, NY.

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External links

• General chemical information index: http://chemistry.about.com/library/blazlist4.htm
• Step by step more help on Hess's Law: http://members.aol.com/profchm/hess.html

Source of the article : Wikipedia