# What is the standard enthalpy change for the reaction?

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Given the standard enthalpy changes for the following two reactions:

(1) Zn(s) + Cl2(g)ZnCl2(s)...... ΔH° = -415.0 kJ

(2) Pb(s) + Cl2(g)PbCl2(s)......ΔH° = -359.4 kJ

what is the standard enthalpy change for the reaction:

(3) Zn(s) + PbCl2(s)ZnCl2(s) + Pb(s)......ΔH° = ?

Given the standard enthalpy changes for the following two reactions:

(1) Zn(s) + Cl2(g)ZnCl2(s)...... ΔH° = -415.0 kJ

(2) Pb(s) + Cl2(g)PbCl2(s)......ΔH° = -359.4 kJ

what is the standard enthalpy change for the reaction:

(3) Zn(s) + PbCl2(s)ZnCl2(s) + Pb(s)......ΔH° = ?

##### 2 Answers

#### Answer:

#### Explanation:

**What you have**

You have two equations:

From these, you must devise the target equation:

**What you must do**

The target equation has **(1)**.

Equation **(4)** has

You need an equation with

Reverse Equation (2).

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

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 the target equation **(6)**: