17. Calculate H for the following reaction using Hess law
H2 (g) + Br2 (g) → 2 HBr (g)
Experimental information
2 H+ → H2 H -436.4 kJ
2 Br- → Br2 H -192.5 kJ
2 H+ + 2 Br- →2 HBr H -144.8 kJ
To calculate ΔH for the given reaction using Hess's Law, you need to manipulate the given equations in order to cancel out the necessary reactants and products. Here is how you can do it step-by-step:
Step 1: Multiply the first equation by 2 to match the stoichiometry of the target equation.
2(H2) + 2(Br2) → 4(HBr)
ΔH1 = (2)(-436.4 kJ) = -872.8 kJ
Step 2: Reverse the second equation to get Br2 on the reactant side.
Br2 → 2Br-
ΔH2 = -(-192.5 kJ) = 192.5 kJ
Step 3: Reverse the third equation and multiply it by 2 to match the stoichiometry.
2(HBr) → 2(H+) + 2(Br-)
ΔH3 = (2)(-(-144.8 kJ)) = -289.6 kJ
Step 4: Add all the modified equations together to get the target equation.
2(H2) + 2(Br2) + 2(HBr) → 4(HBr) + 2(H+) + 2(Br-)
Combine the enthalpy changes:
ΔHtotal = ΔH1 + ΔH2 + ΔH3
ΔHtotal = -872.8 kJ + 192.5 kJ -289.6 kJ
ΔHtotal = -969.9 kJ
Therefore, ΔH for the given reaction is -969.9 kJ.
To calculate ΔH for the given reaction using Hess's Law, you need to manipulate the given equations and their corresponding enthalpy changes (ΔH) to cancel out the reactants and products not involved in the desired reaction.
First, reverse the second equation to get Br2 on the reactant side. Since the enthalpy change also reverses, the equation and the ΔH value become:
Br2 → 2 Br- ΔH = +192.5 kJ
Next, multiply the first equation by 2 to balance the number of hydrogen atoms:
2 H2 → 4 H+ ΔH = -2 * -436.4 kJ = +872.8 kJ
Now, add these two equations together to get the desired reaction:
2 H2 + Br2 → 4 H+ + 2 Br- ΔH = +192.5 kJ + 872.8 kJ = +1065.3 kJ
Since the desired reaction is double the combined equation, the ΔH value must also be multiplied by 2:
2 H2 + Br2 → 2 HBr ΔH = 2 * +1065.3 kJ = +2130.6 kJ
Therefore, the ΔH for the given reaction using Hess's Law is +2130.6 kJ.
There is something inherently wrong with these equations. That's the major reason I didn't answer when you had this posted a few days ago.
You simply can't get H2 from 2 H^+ and you can't get Br2 from 2Br^-
IF you could get two H^+ to combine (and I doubt that will ever happen) you would have H2^2+ Same scenario with Br^- + Br^-.