11.8 & 11.9 Three Gas Laws & Combined Gas Law

If we place a balloon in liquid nitrogen it shrinks: How Volume Varies With Temperature So, gases shrink if cooled. Conversely, if we heat a gas it expands (as in a hot air balloon). Let’s take a closer look at temperature before we try to find the exact relationship of V vs. T.

If a volume vs. temperature graph is plotted for gases, most lines can be interpolated so that when volume is 0 the temperature is -273  C. Naturally, gases don’t really reach a 0 volume, but the spaces between molecules approach 0. At this point all molecular movement stops. –273  C is known as “absolute zero” (no E K ) The Kelvin Temperature Scale

Lord Kelvin suggested that a reasonable temperature scale should start at a true zero value. He kept the convenient units of  C, but started at absolute zero. Thus, K =  C  C = ? K: K=  C+273 = = 335 K Notice that kelvin is represented as K not  K.

What is the approximate temperature for absolute zero in degrees Celsius and kelvin? Calculate the missing temperatures 0  C = _______ K100  C = _______ K 100 K = _______  C – 30  C= _______ K Kelvin Practice Absolute zero is – 273  C or 0 K

Charles’ Law The volume (L) of a gas is directly proportional to the temperature (K) if the pressure and amount (n) of gas remain constant.  Directly proportional: if one goes up, the other goes up. V = K x T( K is some constant) V = K T V 1 = V 2 T 1 T 2

Charles’s Law Graph

1. A sample of gas occupies 3.5 L at 300 K. What volume will it occupy at 200 K? 2. If a 1 L balloon is heated from 22°C to 100°C, what will its new volume be? Examples

Boyle’s Law At a constant temperature and amount (n) of gas, pressure (kPa) and volume (L) are inversely related  Inversely proportional: if one goes up, the other goes down P x V = K ( K is some constant) P 1 x V 1 = P 2 x V 2

Boyle’s Law Graph

Boyle’s Law Application A bicycle pump is a good example of Boyle’s law. A bicycle pump is a good example of Boyle’s law. As the volume of the air trapped in the pump is reduced, its pressure goes up, and air is forced into the tire. As the volume of the air trapped in the pump is reduced, its pressure goes up, and air is forced into the tire.

If a balloon is filled with helium gas that has an initial pressure of 95 kPa and volume of 3.2L is brought up to a higher altitude where the pressure is 75 kPa, what volume of the balloon will the gas occupy? Note, moles and temperature are constant Example

Gay Lussac’s Law The temperature (K) and the pressure (kPa) of a gas are directly related at constant volume and amount (n) of gas. P = K x T( K is some constant) P = K T P 1 = P 2 T 1 T 2

Gay-Lussac’s Law Graph

A sample of gas is stored in a steel container at -115 degrees celsius at a pressure of 39.9 kPa. If the pressure reaches 60.8 kPa, what is the final temperature in kelvin? Example

Putting the pieces together The Combined Gas Law Deals with the situation where only the number of molecules stays constant. P 1 x V 1 = P 2 x V 2 T 1 T 2 Lets us figure out one thing when two of the others change.

A sample of carbon dioxide occupies a volume of 25.0L when the pressure is 125 kPa and the temperature is 25 °C. Calculate the volume occupied by the gasw if the same quantity of carbon dioxide is at STP. Example

HOMEWORK Read pp. 547 – 550, Answer the following questions:  p. 553 #1-7  p. 562 #2-15