### Delta H

Posted:

**Fri Feb 08, 2019 11:09 am**What does this refer to, and what’s the equation for it?

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=41641

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Posted: **Fri Feb 08, 2019 11:09 am**

What does this refer to, and what’s the equation for it?

Posted: **Fri Feb 08, 2019 11:19 am**

Delta H is the change in enthalpy. H = U + PV is one of the equations.

Posted: **Fri Feb 08, 2019 11:26 am**

Delta H is enthalpy of the system. When pressure is constant, Delta H can tell you if the system is absorbing or losing energy in the form of heat. A negative H is the loss of heat while a positive H is the absorption of heat.

The equation of the enthalpy of the system is the summation of H of the products subtracting the summation of H of the reactants.

To do this method you may need to know all the standard enthalpies of formation of each substance.

deltaH = delta H (products) - delta H (reactants)

Or

When change in enthalpy is reliant on work and internal energy:

deltaH = (q + w) + PdeltaV

The equation of the enthalpy of the system is the summation of H of the products subtracting the summation of H of the reactants.

To do this method you may need to know all the standard enthalpies of formation of each substance.

deltaH = delta H (products) - delta H (reactants)

Or

When change in enthalpy is reliant on work and internal energy:

deltaH = (q + w) + PdeltaV

Posted: **Fri Feb 08, 2019 12:29 pm**

It's also important to note that enthalpy is additive because of Hess's Law. It is a state function, so the change in enthalpy (Delta H) at each step of a multistep reaction can be added to get the total enthalpy change.

Posted: **Fri Feb 08, 2019 12:39 pm**

You can also use bond enthalpies to determine delta H:

Delta H = H bonds broken (reactants - H bonds formed (products)

But this is not always accurate, as it uses average enthalpy values for the bonds.

Delta H = H bonds broken (reactants - H bonds formed (products)

But this is not always accurate, as it uses average enthalpy values for the bonds.

Posted: **Sat Mar 16, 2019 1:23 pm**

Because it is a state function, it means that you can add/subtract two different delta H’s correct?

Posted: **Sat Mar 16, 2019 1:26 pm**

Ashley P 4I wrote:Because it is a state function, it means that you can add/subtract two different delta H’s correct?

Yes, if you're asked to find the total enthalpy change you can add/subtract enthalpy values from multiple reactions

Posted: **Sat Mar 16, 2019 1:28 pm**

Milena Aragon 2B wrote:Ashley P 4I wrote:Because it is a state function, it means that you can add/subtract two different delta H’s correct?

Yes, if you're asked to find the total enthalpy change you can add/subtract enthalpy values from multiple reactions

Thank you so much!