Can someone help me how to do this problem?

Question

What is the energy change for 2 H2 + )2 ----. 2 H20

H-H 436 kj/mol
H-O 467 jk/mol
O-O 146 kj/mol
O=O 498 kj/mol

can someone tell me how to go about doing this problem? the answer is suppose to be -498 kj/mol but im stuck on how to get it

Answer 1

You are given the bond energies of a set of possible bonds which may exist in the molecules involved in the reaction.
(Which I will re-write below...correctly),
2H2 + O2 --> 2H2O

You need to find the change in the bond energies from the reactants side of the reaction to the products side to find the total change in energy.

To determine the types of bonds between the atoms you will need to recall a little knowledge about Lewis structures and chemical bonding.

What type(s) of bonds are present in H2?
--There is only a single bond between the two Hydrogen atoms, H-H, but there are 2 moles of H2, so remember to multiply this value by two.

What type(s) of bonds are present in O2?
--This time there is a double bond between the Oxygen atoms, O=O.

What type(s) of bonds are present in H2O?
--Visualize the molecule...there are two, single, covalent bonds between the Hydrogen and Oxygen atoms, H-O, so multiply this by two...but then there are two water molecules in the equation, so multiply again by two (in the end, you will multiply by 4).

To find the change in energy, subtract the initial energy state from the final energy state (products minus reactants).
Subtracting the energy released (when a bond is formed) from the energy absorbed (when a bond is broken) will give you the net change.

Energy absorbed (to break the bonds on the reactants side):
(2 mol * 436 kJ /mol) + (1 mol * 498 kJ/mol) = 1370 kJ

Energy released (when a bond is formed on the products side):
(2 moles * 2 * 467 kJ/mol) = 1868 kJ

1370 kJ - 1868 kJ = -498 kJ
This reaction is exothermic, so the sign on the change in energy should be negative.

Answer 2

MrJ

There would be no negative bond energies, you simply have to understand the sign convention. Since the sign convention you are using defines an exothermic reaction as negative, you are measuring the heat absorbed in the reaction. Thus, you need to subtract the bond energies of the products from the bond energies of the reactants or:

(2*436+498)-4*467 = -498