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### when to change sign of enthalpies

Posted: Sun Feb 09, 2020 11:52 pm
Hi, i'm looking at question 4 on the Pizza Rolls review sheet. When do I know to change the sign of the enthalpies given?

### Re: when to change sign of enthalpies

Posted: Sun Feb 09, 2020 11:54 pm
If you mean question 4a, you would flip the sign if u flip the chemical equation.

### Re: when to change sign of enthalpies

Posted: Sun Feb 09, 2020 11:55 pm
For a Hessâ€™s law equation anything you do to the chemical equation you need to do to the Delta H as well

### Re: when to change sign of enthalpies

Posted: Sun Feb 09, 2020 11:58 pm
If you are using the reverse reaction, you would flip the sign of delta H.

### Re: when to change sign of enthalpies

Posted: Sun Feb 09, 2020 11:58 pm
If you mean 4A, we change the sign of enthalpies when we reverse the equation. For example, we reversed equation 2 because we wanted Vodka on the product side since we have it on the product side in the overall recipe. We then multiplied it by two since we want 2 Vodka on the product side to match the 2 Vodka in the overall recipe. I hope that makes sense!

### Re: when to change sign of enthalpies

Posted: Mon Feb 10, 2020 12:34 am
Flipping the sign accounts for flipping the equation because we have changed from an endo to exo or exo to endo rxn.

### Re: when to change sign of enthalpies

Posted: Mon Feb 10, 2020 1:17 am
By flipping the equation it would mean that you are just taking the reverse reaction instead right? So then basically the reactants are on the products side in order to cancel things out?

### Re: when to change sign of enthalpies

Posted: Mon Feb 10, 2020 2:52 am
Yes, taking the reverse reaction allows the products and reactants to switch sides so that things can get canceled out as needed.