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IT’S A GAS…

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The Nature of Gases Gases have some interesting characteristics that have fascinated scientists for 300 years. The first gas to be studied was air & it was a long time before it was discovered that air was actually a mixture of particles rather than a single gas.

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The Nature of Gases But this realization did not make the study of gas behavior more difficult. Although air is a mixture of several different gases, it behaves much the same as any single gas. Regardless of their chemical identity, gases tend to exhibit similar physical behaviors

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The Nature of Gases Gas particles can be monatomic (Ne), diatomic (N2), or polyatomic (CH4) – but they all have these characteristics in common: 1) Gases have mass. 2) Gases are compressible. 3) Gases fill their containers. 4) Gases diffuse 5) Gases exert pressure. 6) Pressure is dependent on Temp.

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**Kinetic Molecular Theory**

There is a theory that modern day chemist’s use to explain the behaviors and characteristics of gases - the Kinetic Molecular Theory of Matter. The word kinetic refers to motion. The word molecular refers to molecules

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**Kinetic Molecular Theory**

The theory states that the tiny particles in all forms of matter are in constant motion. This theory is used to explain the behaviors common among gases There are 3 basic assumptions(ideas) of the KMT as it applies to gases.

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**KMT Assumption #1 A gas is composed of small hard particles.**

The particles have an insignificant volume and are relatively far apart from one another. There is empty space between particles. No attractive or repulsive forces between particles.

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KMT Assumption #2 The particles in a gas move in constant random motion. Particles move in straight paths and are completely independent of each other Particles path is only changed by colliding with another particle or the sides of its container.

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KMT Assumption #3 All collisions a gas particle undergoes are perfectly elastic. No energy is lost from one particle to another, and the total kinetic energy remains constant.

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Gases have mass. Gases seem to be weightless, but they are classified as matter, which means they have mass. The density of a gas – the mass per unit of volume – is much less than the density of a liquid or solid, however.

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Gases “R” squeezable If you squeeze a gas, its volume can be reduced considerably A gases low density allows for there to be a lot of empty space between gas molecules.

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**This empty space can be compressed simply by adding pressure.**

We can use this ability of a gas to do work for us. Think of shocks on a car. You really are riding on a pillow of air. A bump in the road compresses the gas in the shocks until the bump’s energy is absorbed.

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**Gases fill their containers**

Gases expand until they take up as much room as they possibly can. Gases spread out to fill containers until the concentration of gases is uniform throughout the entire space.

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Gases exert pressure Gas particles exert pressure by colliding with objects in their path. The sum of all of the collisions makes up the pressure the gas exerts.

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**Pressure depends on Temp**

The higher the temperature of a gas -the higher the pressure that the gas exerts The reverse of that is true as well, as the temperature of a gas decreases – the pressure decreases.

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Today’s temp: 35°F Pressure Gauge

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Today’s temp: 85°F Pressure Gauge

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Gas variables In order to describe a gas sample completely and then make predictions about its behavior under changed conditions, it is important to deal with the values of: 1) amount of gas 2) volume 3) temperature 4) pressure

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Gas Laws Studies of the behavior of gases played a major role in the development of physical sciences in the 7th and 8th centuries. The Kinetic Molecular theory marked a significant achievement in understanding the behavior of gases. Observations have become mathematical laws which we can use to predict outcomes.

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Boyle’s Law Robert Boyle was among the first to note the relationship between pressure and volume of a gas. He measured the volume of air at different pressures, and observed a pattern of behavior which led to his mathematical law. During his experiments Temperature and amount of gas weren’t allowed to change

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**As the pressure increases**

Volume decreases

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**How does Pressure and Volume of gases relate graphically?**

PV = k Temperature, # of particles remain constant Pressure

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**Eg: A gas has a volume of 3.0 L at 2 atm. What is its volume at 4 atm?**

Boyle’s Mathematical Law: What if we had a change in conditions? since PV = k P1V1 = P2V2 Eg: A gas has a volume of 3.0 L at 2 atm. What is its volume at 4 atm?

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Charles’s Law Jacques Charles determined the relationship between temperature and volume of a gas. He measured the volume of air at different temperatures, and observed a pattern of behavior which led to his mathematical law. During his experiments pressure of the system and amount of gas were held constant.

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**Volume of balloon at room temperature**

Volume of balloon at 5°C

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**How does Temperature and Volume of gases relate graphically?**

V/T = k Pressure, # of particles remain constant Temp

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**V1 V2 = T1 T2 Charles’s Mathematical Law:**

What if we had a change in conditions? since V/T = k V1 V2 T T2 = Eg: A gas has a volume of 3.0 L at 127°C. What is its volume at 227 °C?

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Gay Lussac’s Law Old man Lussac determined the relationship between temperature and pressure of a gas. He measured the temperature of air at different pressures, and observed a pattern of behavior which led to his mathematical law. During his experiments volume of the system and amount of gas were held constant.

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**Think of a tire... Car before a trip Pressure Gauge Let’s get on**

the road Dude!

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Think of a tire... Car after a long trip Pressure Gauge WHEW!

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**How does Pressure and Temperature of gases relate graphically?**

P/T = k Volume, # of particles remain constant Temp

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**P1 P2 = T1 T2 Lussac’s Mathematical Law:**

What if we had a change in conditions? since P/T = k P P2 T T2 = Eg: A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C?

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**Boyle’s P V T Charles’ V T P Gay-Lussac’s P T V**

Summary LAW RELAT-IONSHIP CON-STANT Boyle’s P V P1V1 = P2V2 T Charles’ V T V1/T1 = V2/T2 P Gay-Lussac’s P T P1/T1 = P2/T2 V

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Properties of Gases.

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