GEN CHEM QUESTION- help plz

[JustwantDDS]

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Which represents the Ea of the reverse rxn?
The answer is the blue one, but I don't know why

The rxn is endothermic b.c the products have more energy than the reactants
but wouldnt the Ea have to get over the whole hump? like the red line?


2nd question:

Bond Average Bond Energy
c---o (triple bond) 1075 kj/mol
c=o 728 kj/mol
c-cl 326
Cl-Cl 243

Calculate the heat of rxn for:

CO+ Cl2---> COCl2

Ok so I know

H rxn = BDE reactants + BDE products
but when I looked at the solution I got confused

This is what it says:


[ (-728 kj/mol)(1mol) + (-326 kj/mol)(2mol) ] + [ 243 kj mol (1mol) + (1075 kj/mol) (1 mol) ]

first off why are the reactants BDE negative?
and why are we using 1075, for c---o????

 

[allstaryankee25]

1. the activation energy of the REVERSE reaction is the blue arrow because it indicates the amount of energy required to pass over the hump to favor the production of reactants instead of products. Think about a ball on a hill - for the reverse reaction, you have to roll it up from right to left till it can roll down by itself - that's the Ea, shown by the blue arrow.

Ea of the forward reaction is designated by the red arrow. Same analogy, only your rolling the ball up from left to right this time.


2. The bond dissociation energy equation is tricky because it assumes you know that breaking bonds requires an input of energy (+) while forming bonds releases energy (-). This is conceptually challenging, so if it's tough to see why, just memorize this rule.

So anyways, the equation should be written as:
deltaH(rxn) = energy required - energy released.

In this problem, we're asked for deltaH(rxn) for
CO+ Cl2---> COCl2

The product, COCl2, has 1 C=O bond (728) and 2 C-Cl bonds (2 x 326). Since these bonds are FORMED, we make them both NEGATIVE (releasing energy)

On the other hand, the reactants bonds are being BROKEN, so we make their values POSITIVE (input of energy). The C---O triple bond has a value of +1075 while the Cl2 bond has a value of +243.

Now add 'em together, keeping the signs you've already figured out to get:
deltaH(rxn)= (1075 + 243) - (728 + 2(326)) = -62kj/mol.

 

[mashinka]

So the answer is the blue line because the Ea(reverse) is equal to the potential E. of the activated complex (which is the peak) - p.E of the products. So if you draw a line across where the PE of the products are, that gives you the answer, which happens to be the blue line.
Does that make sense?
For the second one...my notes say that Heat of Rxn = (H bonds broken) + (H bonds formed).
So in the reaction, we are getting a CO triple bond broken and Cl bond broken in order to form 2 C-Cl bonds and 1 CO double bonds.
So breaking bonds requires an input of energy, so a positive value of E is absorbed. And the formation of bonds released E, thus E is negative.
So that should get us
[ 243 kj mol (1mol) + (1075 kj/mol) (1 mol) ] + [ (-728 kj/mol)(1mol) + (-326 kj/mol)(2mol) ] = -62

Make sense? Let me know if you're still confused.

 

[JustwantDDS]

🙂 Thanks for the help guys

such a stupid question to not understand... got it though!