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**Unit 5: Gases – Temperature & Pressure Relationships: Gay-Lussac’s Law**

11.08 & 09 Ms. Boon Chemistry

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**Catalyst Agenda What is air pressure? How do we measure air pressure?**

Objectives: I can define Standard temperature and pressure. I can state Gay-Lussac’s Law and use it to solve problems involving pressure and temperature. Catalyst What is air pressure? How do we measure air pressure? Agenda Catalyst Soda Can Crush Pressure and Temperature STP Exit Slip HW: Complete today’s practice problems. Pp. 445 #3, 9, 10, 24, 25

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Homework Review The name of the gas law relating pressure and volume is called Boyle’s Law. Boyle’s law states that pressure and volume are inversely proportional, if the temperature is kept constant. This means: if pressure increases, volume decreases. If pressure decreases, volume increases. If volume increases, pressure decreases. If volume decreases, pressure increases.

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Homework Review The name of the gas law relating volume and absolute temperature is Charles’s law. Charles’s Law states that volume and absolute temperature are directly proportional, if the pressure is kept constant. This means: If temperature increases, volume increases. If temperature decreases, volume decreases. If volume increases, temperature increases. If volume decreases, temperature decreases.

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Homework Review The name of the gas law relating pressure and pressure and absolute temperature is Gay- Lussac’s law. Gay-Lussac’s law states that pressure and absolute temperature are directly proportional, if the volume is kept constant. This means: If pressure increases, temperature increases. If pressure decreases, temperature decreases. If temperature increases, pressure increases. If temperature decreases, pressure increases.

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Homework Review Avogadro’s law expresses the direct relationship between amount of gas molecules and the volume of gas, if temperature and pressure are kept constant. This means: If the number of gas particles increases, the volume increases. If the number of gas particles decreases, the volume decreases.

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Soda Can Crush The demonstration shows the relationship between ___________ and ____________. As temperature increases, pressure ___________________. As temperature decreases, pressure ___________.

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Soda Can Crush Summary of Procedure: (1) A small amount of water (~15 mL) is poured into an empty soda can. (2) The can is heated until steam comes out and the water boils for about 30 seconds. (3) Then the can is quickly plunged upside down into ice cold water. Hypothesis: What do you think will happen to the can when it is put into the ice cold water? Why?

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**Soda Can Crush: Explanation**

Describe what happened using the kinetic molecular theory of gases! When we put the can on the hot plate, the temperature of the gas inside the can increases. But the air pressure inside and outside the can remain the same because the can is open. When we flip the can over and dunk it in the icy water, the temperature of the gas decreases. This causes a decrease in pressure. When the pressure outside the can is more than inside, the can crushes.

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**Gay-Lussac’s Law: Temperature and Pressure Relationships**

In 1802, Joseph Gay-Lussac discovered that gas pressure and Kelvin Temperature are directly proportional at constant volume. For example, if the absolute temperature (temperature expressed in Kelvin) is doubled, the pressure is doubled. If the pressure is cut in half, the absolute temperature is also cut in half.

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When gas volume is held constant, the following equation expresses the pressure and temperature relationship: If any three of the variables in the above equation are known, then the unknown fourth variable can be calculated.

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**Example 1: P1 = 100 mm Hg, T1 = 50 K, P2 = ??, T2 = 100 K**

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**Example 2: At 273K, the pressure of a sample of nitrogen is 1000 atm**

Example 2: At 273K, the pressure of a sample of nitrogen is 1000 atm. What will the temperature be at 250 atm?

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**Example 3: At 500K, a 5 L sample of oxygen has a pressure of 760 torr**

Example 3: At 500K, a 5 L sample of oxygen has a pressure of 760 torr. If the temperature doubles, what will the new pressure be?

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**Pressure and Temperature Unit Conversions**

Example Pressure problem (#7) 320 mm Hg = ________ atm

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**Standard Temperature and Pressure:**

STP stands for Standard Temperature and pressure. It is a term used in Chemistry to indicate that an experiment or measurement was recorded when the temperature was 0° C and the air pressure was 1 atm. STP is a number (or CONDITION) that we can use in calculations that does not change. If you see STP (standard temperature and pressure) you should immediately think: 0° C (or 273K) and 1 atm (or 760 torr or 760 mm Hg)

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**STP = 0° C (or 273K) and 1 atm (or 760 torr or 760 mm Hg)**

Example: A chemist has a 10 L sample of fluorine gas at standard temperature and pressure. If the volume is kept constant and the temperature is increased to 300K. What is the new pressure?

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Exit Slip 4c At constant volume, as temperature increases, pressure __________. Pressure and temperature are _________ proportional. T1 = 300K, P1 = 1000 mm Hg, T2 = 900 K, P2= ___? A 10 mL sample of hydrogen gas exerts a pressure of 150 torr at a temperature of 300K. When the pressure changes to 50 torr, what is the new temperature? What is standard temperature and pressure in Kelvin and atmospheres? What is the new temperature when a sample of gas at standard temperature and pressure is decreased to 0.5 atm?

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