Thermochemistry problems? 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!  :)

For the first question 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?

Water calculation:

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

= 1474.86 kJ
H is enthalpy.  

By definition H = U + PV (where U is internal energy, P is pressure and V is volume).  

For question 1, your work says:


  (250) x (4.184) x (26.26-24.85)
  
  = 1474.86 kJ


Because you failed to show units in your work, you are off by a factor of 1000.

But the main reason your answer is different from the desired answer is you have calculated the amount of heat released by 0.152 g reacting, whereas the desired answer comes from calculating the heat released by 3 moles of A reacting, considering change in enthalpy of the system to be negative when heat is released to the surroundings.  

For question 2:

Answer ii is wrong because the monoatomic forms of hydrogen and bromine are not the standard states of the elements.  Answer iii is wrong because the equation is written for 2 moles being formed rather than 1 mole.