# Enthalpy Change

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Could anyone explain the formula for enthalpy change, because my chemistry teacher has us really confused, we even noticed that shes just understanding what she is explaining when she does the explaining, but I got some info from the net so I just wanted to know if someone could check it for me

Enthalpy change= enthalpy of products - enthalpy of reactants

the enthalpy energies are supposed to be negative or positive?

And is there any relation here with the enthalpy diagrams?
thanks

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The enthalpy change can be negative or positive, depending on if it's an exothermic or endothermic rxn. If it's endothermic, that means more energy is being absorbed than released, hence in the enthalpy diagram, the products are less stable than reactants: ____ -----, and the value for enthalpy change is positive since the enthalpy is going from lesser energy to more energy. In an exothermic rxn, the enthalpy diagram looks like this: ----- ____ and the products are more stable than reactants, so enthalpy change value is negative, since more energy is released than absorbed.

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[quote name='__inthemaking' post='16880' date='May 20 2008, 04:05 AM']The enthalpy change can be negative or positive, depending on if it's an exothermic or endothermic rxn. If it's endothermic, that means more energy is being absorbed than released, hence in the enthalpy diagram, the products are less stable than reactants: ____ -----, and the value for enthalpy change is positive since the enthalpy is going from lesser energy to more energy. In an exothermic rxn, the enthalpy diagram looks like this: ----- ____ and the products are more stable than reactants, so enthalpy change value is negative, since more energy is released than absorbed.[/quote]

Just wanted to add to the info above.. 'Enthalpy' (H) is a fancy word for energy; i.e 'enthalpy of reactants' means the total amount of energy stored in the electrones of the reactants (Hr).

Enthalpy change (delta H) = Hp - Hr ((where Hp is the enthalpy of products and Hr is the enthalpy of reactants)).

- In exothermic reactions, the total amount of energy stored in the reactants (Hr) is higher than that stored in the products (Hp). Logically, delta H would be -ve.
- In endothermic reactions, the the total amount of energy stored in the reactants (Hr) is lower than that stored in the products (Hp). Logically, delta H would be +ve.

Since the reactants in an exothermic reaction possess more energy, they are not that stable compared to products, and vice versa for endothermic reactions. As far as the enthalpy level diagram is concerned, the post above explains the methodology very well . Don't forget to label the axes (X: progress of reaction, Y: Enthalpy (KJ/mol)).

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[quote name='BIO-AQUA' post='17033' date='May 22 2008, 08:53 PM']Just wanted to add to the info above.. 'Enthalpy' (H) is a fancy word for energy; i.e 'enthalpy of reactants' means the total amount of energy stored in the electrones of the reactants (Hr).

Enthalpy change (delta H) = Hp - Hr ((where Hp is the enthalpy of products and Hr is the enthalpy of reactants)).

- In exothermic reactions, the total amount of energy stored in the reactants (Hr) is higher than that stored in the products (Hp). Logically, delta H would be -ve.
- In endothermic reactions, the the total amount of energy stored in the reactants (Hr) is lower than that stored in the products (Hp). Logically, delta H would be +ve.

Since the reactants in an exothermic reaction possess more energy, they are not that stable compared to products, and vice versa for endothermic reactions. As far as the enthalpy level diagram is concerned, the post above explains the methodology very well . Don't forget to label the axes (X: progress of reaction, Y: Enthalpy (KJ/mol)).[/quote]

In order to supplement this information, I am attaching two pictures that should aid in your comprehension. Enjoy!

Endothermic reaction:

[url="http://www.bbc.co.uk/scotland/education/bitesize/higher/img/chemistry/calculations_1/pe_diags/fig10.gif"]http://www.bbc.co.uk/scotland/education/bi...diags/fig10.gif[/url]

Exothermic reaction:

[url="http://www.bbc.co.uk/scotland/education/bitesize/higher/img/chemistry/calculations_1/pe_diags/fig03.gif"]http://www.bbc.co.uk/scotland/education/bi...diags/fig03.gif[/url]

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