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### Chapter 8.55, Hess's Law

Posted: Wed Jan 11, 2017 8:05 pm
I am having trouble understanding why the O2 in the 2Al+O2-- Al2O3 equation, is multiplied by 3/2. I know we are trying to cancel it out to get the equation needed, but nothing in that equation was multiplied, just the O2. Could someone explain?

### Re: Chapter 8.55, Hess's Law

Posted: Wed Jan 11, 2017 8:27 pm
The 3/2 multiplier comes from balancing the equation. Before attempting any Hess's Law problems, it is important to make sure the equation is balanced. In this case, we had 2 oxygens compared to 3 on the right hand side. Thus the 3/2 multiplier.

### Re: Chapter 8.55, Hess's Law

Posted: Wed Jan 11, 2017 8:29 pm
In order for the equation to be balanced, there needs to be the same amount of moles of an element on both the reactant and product side of the equation. In order for the equation to be balanced in regards to the O2, you need to multiply the whole reaction by 3/2 first. Hope that this helps!

### Re: Chapter 8.55, Hess's Law

Posted: Wed Jan 11, 2017 9:25 pm
Would we also then multiply ΔH by 3/2, or will that remain constant?

### Re: Chapter 8.55, Hess's Law

Posted: Wed Jan 11, 2017 10:30 pm
When we are multiplying the O2 reactant by 3/2 to balance this chemical reaction, there is no need to multiply the corresponding ΔH by 3/2. Balancing the equation is a separate idea from the multiplying the entire equation and its corresponding ΔH technique in Hess's Law problems.