[dsmith]

Question 1:

A compound A reacts according to the following hypothetical equation and has a molecular weight of 48.36 g/mol.

3 A (s) +B (aq) ––> 2 C (aq) ∆H° = ?

A sample of A, weighing 0.152 g reacts in a flask containing 250.00 g of water and the water temperature increases from 24.85 °C to a temperature of 26.26 °C. Calculate ∆H° for the reaction as written in the equation.

Answer: -1408 kJ

Question 2:

For which of the following reactions is ∆H° = ∆H°f, the heat of formation?

i. C (s) + 2 F2(g) ––> CF4 (g) ∆H° = – 221.0 kJ
ii. H(g) + Br (g) ––> HBr (g) ∆H° = –366.2 kJ
iii. 2 C(s) + H2(g) + 3 Cl2(g) ––> 2 CHCl3 (g) ∆H° = –268.2

Answer: i only

Also, could someone explain exactly what delta H means in terms of Thermochemistry. I would like to have a better understanding of it.

Thank you very much! 

[dsmith]

I attempted to use the q=m x s x (delta) T for the water and I attempted to use stochiometry for substance A but I always receive the wrong answer. Could someone help me on where to start with these types of problems?

Also, I am preparing for an upcoming Chemistry exam. I completed a practice worksheet consisting of 20 questions. These were the only questions that I couldn't figure out.

[Ciubba]

Are you sure you wrote iii correctly? It looks like ΔH=ΔHF to me... Does it specify if C(s) is graphite?

[Borek]

I attempted to use the q=m x s x (delta) T for the water and I attempted to use stochiometry for substance A but I always receive the wrong answer.

Sounds like a correct approach, you must be doing some math mistake. I got -1409 kJ.

Heat of formation is that of formation of ONE mole of substance from elements in their STANDARD state.

[dsmith]

Could you please show me your calculations. I got -1474 kJ for the water. After that I try to use stoichiometry for substance A.

Water calculation:

(250) x (4.184) x (26.26-24.85)

= 1474.86 kJ

[dsmith]

Also for the second question, I suspect that I need the chart for heat formation. It was on a review sheet that didn't come with the chart. I thought their was some other way to caculate the answer.

[Borek]

(250) x (4.184) x (26.26-24.85)

= 1474.86 kJ

These are not kJ and this is not yet a final result. This is amount of heat produced from the reaction of 0.152 g of A.

Also for the second question, I suspect that I need the chart for heat formation. It was on a review sheet that didn't come with the chart. I thought their was some other way to caculate the answer.

All you need is a definition I listed.

[dsmith]

I am still a little confused. So the answer I received is in Joules? I understand since the specific heat of substances are measured in joules. But how would I arrive to 1409 kJ from 1474 J?What other steps is required to arrive at the correct answer?

[Borek]

How many moles of A?