Can heat of formation be the heat of reaction?

[axp107]

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I thought I had this down cold.. so I never really bothered studying Hess' Law and some of this enthalpy stuff.

Well

Delta H(reaction) = Delta H of formation(products) - Delta H formation(reactants)

What if you only have 1 product? such as...

The standard enthalpy of formation for liquid water is:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

We only have 1 product here... so is -285.8kj/mol the Heat of formation of H20 (the product) or the whole reaction... b/c I normally thought they only included the enthalpy of the WHOLE reaction next to the equation.

 

[BrokenGlass]

I thought I had this down cold.. so I never really bothered studying Hess' Law and some of this enthalpy stuff.

Well

Delta H(reaction) = Delta H of formation(products) - Delta H formation(reactants)

What if you only have 1 product? such as...

The standard enthalpy of formation for liquid water is:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

We only have 1 product here... so is -285.8kj/mol the Heat of formation of H20 (the product) or the whole reaction... b/c I normally thought they only included the enthalpy of the WHOLE reaction next to the equation.

H2 and 02 are elements in their most stable state, so their enthalpies of formation are zero. -285.8kj/mol is the enthalpy of formation of H2O which is also the enthalpy change for the entire reaction. Formation is a type of reaction where reactants are elements in the their most stable state.

 

[axp107]

Thats what I thought.. expcept with that info alone the question asks:

Which of the following could be the standard enthalpy of formation of WATER VAPOR:

a) -480 kj/mol
b) -285
c) -241
d) +224

 

[BrokenGlass]

Thats what I thought.. expcept with that info alone the question asks:

Which of the following could be the standard enthalpy of formation of WATER VAPOR:

a) -480 kj/mol
b) -285
c) -241
d) +224

-285.8kj/mol is enthalpy of formation of LIQUID water, right? To go from liquid to gas we need to add energy (say in the form of heat), so enthalpy of formation of water vapor has to be greater than -285.8. So I would say the answer is d, + 224. Water has a high heat capacity, so it would take a lot of energy to first heat it, and then to vaporize it.

Just my 2 cents.

 

[axp107]

-285.8kj/mol is enthalpy of formation of LIQUID water, right? To go from liquid to gas we need to add energy (say in the form of heat), so enthalpy of formation of water vapor has to be greater than -285.8. So I would say the answer is d, + 224. Water has a high heat capacity, so it would take a lot of energy to first heat it, and then to vaporize it.

Just my 2 cents.

That was exactly my thought process.. BUT the answer is:

- 241 haha... apparently energy is added BUT it can't be ENDOTHERMIC

 

[BrokenGlass]

That was exactly my thought process.. BUT the answer is:

- 241 haha... apparently energy is added BUT it can't be ENDOTHERMIC

Water formation part is exothermic. The conversion of water to steam is endothermic. The sum is exothermic because the magnitude of the exothermic part is greater than the magnitude of the endothermic part. I don't have the exact problem statement, and often the correct answer choice is impossible to obtain without the entire problem statement.

It would help if people posted the problem statement as is. It would also help if people would post the explanations provided by their answer keys. This way we could all benefit a lot more.

 

[axp107]

The standard enthalpy of formation for liquid water is:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

Which of the following could be the standard enthalpy of formation of water vapor?

a) -480 kj/mol
b) -285
c) -241
d) +224

ANSWER:

c) Condensation must occur to form liquid water. Condensation is an exothermic process, so the formation of liquid water should be more exothermic than the formation of water vapor. The standard enthalpy of formation of water vapor will not be an endothermic process, so D is wrong.

 

[BrokenGlass]

The standard enthalpy of formation for liquid water is:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

Which of the following could be the standard enthalpy of formation of water vapor?

a) -480 kj/mol
b) -285
c) -241
d) +224

ANSWER:

c) Condensation must occur to form liquid water. Condensation is an exothermic process, so the formation of liquid water should be more exothermic than the formation of water vapor. The standard enthalpy of formation of water vapor will not be an endothermic process, so D is wrong.

So the point of this problem is to compare the following 2 reactions:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

H2(g) + 0.5 O2(g) --> H2O (g) Delta Hf = ?

Since bonds beween water molecules are stronger in liquid phase than in gas phase, more energy is released when liquid water is formed as compared to when water vapor is formed. So our answer has to be less exothermic than -285.8kj/mol. In other words,
at this point we have narrowed the answer down to C or D.

Now both of these reactions are oxidation reactions and oxidation reactions always release energy, i.e. they are exothermic to the best of my knowledge. In oxidation reaction energy may need to be added to get the reaction going, but on the whole, energy is released. So answer choice D is ruled out.

 

[lifeistough]

The standard enthalpy of formation for liquid water is:

H2(g) + 0.5 O2(g) --> H2O (l) Delta Hf = -285.8kj/mol

Which of the following could be the standard enthalpy of formation of water vapor?

a) -480 kj/mol
b) -285
c) -241
d) +224

ANSWER:

c) Condensation must occur to form liquid water. Condensation is an exothermic process, so the formation of liquid water should be more exothermic than the formation of water vapor. The standard enthalpy of formation of water vapor will not be an endothermic process, so D is wrong.

This has been helpful. I tried to look at this conceptually. Here is what I was thinking (even if I never knew anything about H(f), etc.):

The reactants are gases and what do I know gases? Well, I know that ideal gases are high in temperature and volume, but I'm interested in knowing that they are high temp reactants. Okay, now what? Well, the product is a liquid, so what do I know about liquids? I know that liquid molecules do not have the same kinetic energy (temp) as gases, since the motion in a lake is "wavy."

Okay, so something high temp is going to low temp. That's fair. If it is giving up heat to go lower in temp then heat must be exothermic. And finally, what was the problem asking for? It wants me make a conjecture on what the value of delta H for a vapor molecule. Basically, instead of H2 gases and O2 gases to liquid state, drop it down to H2O vapor phase. Well, it is still losing heat so the answer is an energy level decrease that is not so low as water, but a little above it. The only answer is C.

On the a CBT practice, if you get stuck, begin asking yourself questions and what you know. You may be surprised.