Question

For the enthalpy equation what do the BE mean:
(picture soon)

Answer 1

\[ΔH=ΣBE(Reactants)+\Sigma BE(Products) \]

Answer 2

Bond enthalpy. But you also have a sign wrong. You are calculating the change in enthalpy of the system by adding all the bond enthalpies of the reactants (because you have to supply enough heat to break all those bonds) and subtracting all the bond enthalpies of the products (because each of those bonds will release heat when it forms).

For example, suppose you want to calculate dH for the formation of water from hydrogen and oxygen gas:
\[2{\rm H}_2(g) + {\rm O}_2(g) \rightarrow 2 {\rm H}_2{\rm O}(g)\]
You would add up the bond enthalpies of two H-H single bonds and the O=O double bond, because you're breaking those, and must supply heat to do so, and then subtract the enthalpy of four O-H single bonds, because heat will be released when you let those form.
\[\Delta H = 2(436 {\rm kJ/mol}) + 498 {\rm kJ/mol} - 4 (467 {\rm kJ/mol}) = - 498 {\rm kJ/mol}\]
The result is negative, meaning the change in enthalpy of the system is lowered. (Since energy is conserved, the enthalpy lost by the system must go somewhere -- in fact, it emerges as heat. That is, this energy releases a lot of heat, which the passengers in the airship Hindenberg found out to their great sorrow in 1936.)

Answer 3

The teacher gave us + because the heat lost is a negative so it ends up being a negative any way.

Answer 4

I've never seen bond enthalpies listed as negative numbers, but even if they were, you simply must have a difference in sign between the two quantities. If both are positive, or both negative, you'll get the wrong answer.