1

Chapter 11 Gases

11.9

Gas Laws and Chemical Reactions

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

2

Gases in Equations

The volume or amount of a gas in a chemical

reaction can be calculated from

• STP conditions or the ideal gas law.

• Mole factors from the balanced equation.

3

STP and Gas Equations

What volume (L) of O2 gas is needed to completelyreact with 15.0 g of aluminum at STP?

4Al(s) + 3O2 (g) 2Al2O3(s)

Plan: g Al mol Al mol O2 L O2 (STP)

15.0 g Al x 1 mol Al x 3 mol O2 x 22.4 L (STP)

26.98 g Al 4 mol Al 1 mol O2

= 9.34 L O2 at STP

4

Ideal Gas Equation and Reactions

What volume (L) of Cl2 gas at 1.2 atm and 27°C is needed to completely react with 1.5 g aluminum?

2Al(s) + 3Cl2(g) 2AlCl3(s)

5

Ideal Gas Equation and Reactions

2Al(s) + 3Cl2 (g) 2AlCl3(s) 1.5 g ? L 1.2 atm, 300K

1. Calculate the moles of Cl2 needed.

1.5 g Al x 1 mol Al x 3 mol Cl2 = 0.083 mol Cl2 26.98 g Al 2 mol Al

2. Place the moles Cl2 in the ideal gas equation.

V = nRT =(0.083 mol Cl2)(0.0821 L• atm/mol K)(300 K) P 1.2 atm

= 1.7 L Cl2

6

What volume (L) of O2 at 24°C and 0.950 atm is needed to react with 28.0 g NH3?

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(g)

Learning Check

7

1. Calculate the moles of O2 needed.

28.0 g NH3 x 1 mol NH3 x 5 mol O2

17.03 g NH3 4 mol NH3

= 2.06 mol O2

2. Place the moles of O2 in the ideal gas equation.

V = nRT =(2.06 mol)(0.0821 L• atm/mol K)(297 K)

P 0.950 atm

= 52.9 L O2

Solution