Question

Can Someone Explain Hess's Law?

How do you find enthalpy by combining equations?

For Example,

ΔH for the reaction

IF5(g) → IF3(g) + F2(g)

is ____ kJ, give the data below.

IF(g) + F2(g) → IF3(g) ΔH = -390 kJ

IF(g) + 2F2(g) → IF5(g) ΔH = -745 kJ

Answer 1

`ΔH = "+355 kJ"`

Hess's Law states that the energy involved in a chemical process is the same whether the process takes place in one or in several steps.

You are given two equations:

`bb"(1)"color(white)(m) "IF(g)" + "F"_2"(g)" → "IF"_3"(g)"; color(white)(m)ΔH = "-390 kJ"`
`bb"(2)"color(white)(m) "IF(g)" + "2F"_2"(g)"color(white)(l) → "IF"_5"(g)"; ΔH = "-745 kJ"`

From these, you must devise a third equation (the target equation).

`bb"(3)"color(white)(m)"IF"_5"(g)" → "IF"_3"(g)" + "F"_2"(g)"; ΔH = ?`

The target equation has `"IF"_5"(g)"` on the left, so you reverse equation (2).

`bb"(4)"color(white)(m)"IF"_5"(g)"→ "IF(g)" + "2F"_2"(g)"; ΔH ="+745 kJ"`

When you reverse an equation, you change the sign of its `ΔH`.

Equation (4) has `"IF(g)"` on the right, and that is not in the target equation.

You need an equation with `"IF(g)"` on the left.

Just rewrite Equation (1).

`bb"(5)"color(white)(m) "IF(g)" + "F"_2"(g)" → "IF"_3"(g)"; color(white)(m)ΔH = "-390 kJ"`

Now, you add equations (4) and (5), cancelling species that appear on opposite sides of the reaction arrows.

When you add equations, you add their `ΔH` values.

This gives us the target equation (6):

`bb"(4)"color(white)(m)"IF"_5"(g)"→ color(red)(cancel(color(black)("IF(g)"))) + color(red)(cancel(color(black)(2)))"F"_2"(g)"; ΔH ="+745 kJ"`
`ul(bb"(5)"color(white)(m) color(red)(cancel(color(black)("IF(g)"))) + color(red)(cancel(color(black)("F"_2"(g)"))) → "IF"_3"(g)"; color(white)(ll)ΔH = color(white)(l)"-390 kJ")`
`bb"(6)"color(white)(m)"IF"_5"(g)" → "IF"_3"(g)" + "F"_2"(g)"; color(white)(m)ΔH = "+355 kJ"`