Gas Laws

Opening thoughts… Have you ever: Seen a hot air balloon? Had a soda bottle spray all over you? Baked (or eaten) a nice, fluffy cake? These are all examples of gases at work!

Properties of Gases You can predict the behavior of gases based on the following properties: Pressure Volume Amount (moles) Temperature Lets review each of these briefly…

1.Pressure 2.Volume 3.Amount (moles) 4.Temperature You can predict the behavior of gases based on the following properties:

Pressure Pressure is defined as the force the gas exerts on a given area of the container in which it is contained. The SI unit for pressure is the Pascal, Pa. If you’ve ever inflated a tire, you’ve probably made a pressure measurement in pounds (force) per square inch (area).

Volume Volume is the three-dimensional space inside the container holding the gas. The SI unit for volume is the cubic meter, m 3. A more common and convenient unit is the liter, L. Think of a 2-liter bottle of soda to get an idea of how big a liter is. (OK, how big two of them are…)

Amount (moles) Amount of substance is tricky. The SI unit for amount of substance is the mole, mol. Since we can’t count molecules, we can convert measured mass (in kg) to the number of moles, n, using the molecular or formula weight of the gas. By definition, one mole of a substance contains approximately x particles of the substance. You can understand why we use mass and moles!

Temperature Temperature is the measurement of heat…or how fast the particles are moving. Gases, at room temperature, have a lower boiling point than things that are liquid or solid at the same temperature. Remember : Not all substance freeze, melt or evaporate at the same temperature. Water will freeze at zero degrees Celsius. However Alcohol will not freeze at this temperature.

Why does this happen? Relate answers to: pressure volume amount temperature

How do they all relate? Some relationships of gases may be easy to predict. Some are more subtle. Now that we understand the factors that affect the behavior of gases, we will study how those factors interact. Let’s go!

Boyle’s Law This law is named for Charles Boyle, who studied the relationship between pressure, p, and volume, V, in the mid-1600s. Boyle determined that for the same amount of a gas at constant temperature, results in an inverse relationship : when one goes up, the other comes down. P 1 V 1 = P 2 V 2 Remember: Boy s P lay V ideo games pressure volume

Gay-Lussac’s Law This law defines a direct relationship: With the same amount of gas he found that as the pressure increases the temperature also increases. If the temperature decreases than the pressure also decreases. P 1 /T 1 = P 2 /T 2 Remember: Gay – Lussac used T P on his neighbors yard

Charles’ Law This law is named for Jacques Charles, who studied the relationship volume, V, and temperature, T, around the turn of the 19 th century. This defines a direct relationship: With the same amount of gas he found that as the volume increases the temperature also increases. If the temperature decreases than the volume also decreases. V 1 /T 1 = V 2 /T 2 Remember: Charl ie Brown was a T V Show volume temperature

M INI -D EMOS I NVESTIGATING G AS L AWS Purpose: To understand gas laws based on their relationships to temperature, pressure and volume. FYI- You have not been given any previous information regarding gas laws. This activity is to get you familiar with two of the gas laws and their relation to the above variables.

Marshmallow Syringe In a moment, I will place a marshmallow in a syringe and tape the end to completely seal the hole. I will then push and pull the syringe and you will observe what will happen to the marshmallow. Write the following sentences in your science notebook and make a prediction: When I push the syringe, the marshmallow ____________________. When I pull the syringe, the marshmallow _____________________. Circle the variables you are dealing with: temperature pressure volume Complete this: As ___________________increases, __________________decreases. The relationship between these variables is ____________________(direct, inverse)

Marshmallow Demo Notes As they play, ask them to think about what is happening and which variables are involved. Allow them to discuss with partners. After 5 minutes, ask students, “Which variable is held constant?” Hopefully students know that temperature is constant. “So we’re working with volume and pressure. When we pull out the plunger, are we increasing or decreasing the pressure inside the syringe?” Students should know that we are decreasing the pressure, but if some students are confused, ask another student to explain to the class why the pressure is decreasing. Remind them it is a closed system, so the amount of matter inside the syringe remains the same. “Let’s ignore the marshmallow for a second. As we decrease the pressure, what happens to the volume inside the syringe?” Students will know we are increasing the volume. “Does that mean the gas molecules are taking up more volume? Are the gas molecules getting bigger?” Hopefully students will remember that there is empty space between the gas molecules and that gas molecules take the shape of their container. (If necessary, remind them through questioning.) “So when the volume is increased, it just means that there is more space in between the molecules, since the number of molecules stays the same.” “Now, the marshmallow. Why is it growing?” Students might say that the marshmallow particles are separating. Ask, “Would the same thing happen if we had put a rock in there?” Hopefully this questioning will get students to realize that there are air molecules in the marshmallow that are taking up more volume as the pressure is decreased.

Microwave Popcorn Have you ever popped a bag of popcorn before? What do you see? What do you think makes the popcorn pop? Popcorn pops when the moisture inside boils and expands, bursting the kernel open.

Microwave Popcorn Key Concept - When the temperature of a gas is increased, its volume will increase.

Final Questions… The final question is intended to be completed individually (i.e. On your OWN!) After doing some research, answer the following questions: Which law do you think was demonstrated in question one? Which law was demonstrated in question two?

Charles’ Law Two things happen when you microwave the soap: the soap gets heated, which makes it softer, and the air and water trapped inside the soap gets heated, causing the water to vaporize and the air to expand. These expanding gases cause the soap to expand and become a foam (popping popcorn is a similar process). Microwaving Ivory™ soap changes its appearance does not cause a chemical reaction, only a physical change. Charles' Law states the volume of a gas increases with its temperature. The microwaves add energy to the soap, water, and air molecules, causing them to move faster and further away from each other making the soap puff up. Other brands of soap don't contain as much air and just melt in the microwave.

Boyle’s Law-Math Practice Problem: A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume? Solution: Let’s find out!

Boyle’s Law-Math Practice A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume? The first step to solving gas law problems should be converting all temperatures to absolute temperatures. This is the most common place mistakes are made in this type of homework problem. T K = °C T i = initial temperature = 27 °C T i K = T i K = 300 K T f = final temperature = 77 °C T f K = T f K = 350 K

A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume? The next step is to use Charles' law to find the final volume. Charles' law is expressed as: V i /T i = V f /T f where V i and T i is the initial volume and temperature V f and T f is the final volume and temperature

A 600 mL sample of nitrogen is heated from 27 °C to 77 °C at constant pressure. What is the final volume? Solve the equation for V f : V f = V i T f /T i Enter the known values and solve for V f. V f = (600 mL)(350 K)/(300 K) V f = 700 mL Answer: The final volume after heating will be 700 mL.