# Question #6a733

##### 2 Answers

+294kJ

#### Explanation:

We will use Hess' Law to work out

(Normally an enthalpy cycle would be drawn but I do not know how to do that on a laptop so here is an alternative)

Hess' Law states that there is an indirect route along with the direct route in a reaction. We can use this indirect route to find the enthalpy change of the direct route.

Route A is the

Route B is the

Route C is the

We need to find route C which is A+B

224+436= 660kJ

660 - 72 = 588kJ

588kJ is for 2HBr but we only want HBr so we divide 588 by 2.

**Warning! Long Answer.**

#### Explanation:

You get the answer by applying **Hess's Law**.

You are given three equations:

From these, you must devise the target equation

The target equation has **(2)** and divide it by 2.

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

**When you divide an equation by 2, you halve its #ΔH#**.

Equation **(4)** has

You need an equation with

Divide equation **(1)** by 2.

Equation **(5)** has

You need an equation with

Reverse equation **(3)** and divide it by 2.

Now, you add equations **(4), (5)** and **(6)**, 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 **(7)**: