Predict whether the following reactions will be exothermic or endothermic
A.] N2 (g) + 3H2 (g) --> 2NH3 (g)
B.] S (g) + O2 (g) --> SO2 (g)
C.] 2 H2O (g) --> 2 H2 (g) + O2 (g)
D.] 2 F (g) --> F2 (g)
Bond ________ ∆H˚ (kJ/mol)
H-H _________ 432
N(triple)N ____942
O=O ________494
F-F __________155
H-N _________ 386
H-O _________459
S=O _________ 522
Please help! I have no idea how to determine if a reaction is endothermic or exothermic!
delta Hrxn = (DHreactants)-(DHproducts)
If DHrxn is + it is endothermic.
If DHrxn is - it is exothermic.
To determine whether a reaction is endothermic or exothermic, you can use the concept of bond energies. Bond energy refers to the amount of energy required to break a particular bond, and it is represented by the values given in the table you provided.
Here's how you can determine the nature of the reaction for each of the given reactions:
A.] N2 (g) + 3H2 (g) --> 2NH3 (g)
In this reaction, you need to determine the sum of the bond energies in the reactants and the products. To break the bonds in the reactants, you will need to input energy. Then, to form the bonds in the products, energy will be released.
Bond energy required to break N≡N bond: 2 * 942 kJ/mol
Bond energy required to break H-H bond: 6 * 432 kJ/mol
Bond energy required to form N-H bonds: 6 * 386 kJ/mol
Now, you can calculate the overall energy change in the reaction by summing up the bond energies of the breaking bonds and subtracting the bond energies of the forming bonds:
Energy required to break bonds - Energy released from forming bonds
= (2 * 942 kJ/mol) + (6 * 432 kJ/mol) - (6 * 386 kJ/mol)
= 1884 kJ/mol + 2592 kJ/mol - 2316 kJ/mol
= 2160 kJ/mol
Since the energy change is positive (+2160 kJ/mol), it means that energy is absorbed from the surroundings, making this reaction endothermic.
B.] S (g) + O2 (g) --> SO2 (g)
By applying the same logic as above, you can calculate the energy change for this reaction as well:
Energy required to break S-S bond: 522 kJ/mol
Energy required to break O=O bond: 1 * 494 kJ/mol
Energy required to form S=O bond: 1 * 522 kJ/mol
Energy required to break bonds - Energy released from forming bonds
= (1 * 522 kJ/mol) + (1 * 494 kJ/mol) - (1 * 522 kJ/mol)
= 522 kJ/mol + 494 kJ/mol - 522 kJ/mol
= 494 kJ/mol
Since the energy change is negative (-494 kJ/mol), it means that energy is released, making this reaction exothermic.
Following the same steps, you can determine the nature of the other two reactions:
C.] 2 H2O (g) --> 2 H2 (g) + O2 (g)
Energy required to break H-O bonds: 4 * 459 kJ/mol
Energy required to form H-H bonds: 4 * 432 kJ/mol
Energy required to form O=O bonds: 1 * 494 kJ/mol
Energy required to break bonds - Energy released from forming bonds
= (4 * 459 kJ/mol) - (4 * 432 kJ/mol) + (1 * 494 kJ/mol)
= 1836 kJ/mol - 1728 kJ/mol + 494 kJ/mol
= 602 kJ/mol
Since the energy change is positive (+602 kJ/mol), it means that energy is absorbed, making this reaction endothermic.
D.] 2 F (g) --> F2 (g)
Energy required to break F-F bonds: 1 * 155 kJ/mol
Energy required to form F-F bonds: 1 * 155 kJ/mol
Energy required to break bonds - Energy released from forming bonds
= (1 * 155 kJ/mol) - (1 * 155 kJ/mol)
= 155 kJ/mol - 155 kJ/mol
= 0 kJ/mol
Since the energy change is zero, it means that there is no net energy change, making this reaction neither endothermic nor exothermic.
In summary:
A.] N2 (g) + 3H2 (g) --> 2NH3 (g) - Endothermic
B.] S (g) + O2 (g) --> SO2 (g) - Exothermic
C.] 2 H2O (g) --> 2 H2 (g) + O2 (g) - Endothermic
D.] 2 F (g) --> F2 (g) - Neither endothermic nor exothermic