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Molecular Composition of Gases

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Molecular Composition of Gases

Chapter 14

(page 440)

Why is it important to know the volume - mass relationship of gases, the Idea Gas law, and the stoichiometry of gases??

- Kinetic molecular theory
- Brownian motion
- Pressure
- Pascal
- Barometer
- Diffusion
- Boltzmann’s constant

Properties of gases

No interaction between atoms or molecules, except during collisions

Straight trajectory until a collision occurs

Mostly empty space

Gases consist of widely separated atoms or molecules in constant, random motion

Properties of gases

- Gases have a unique set of physical properties:
- Gases are translucent or transparent.
- Gases have very low densities when compared to liquids or solids.
- Gases are highly compressible compared to liquids and solids.
- Gases can expand or contract to fill any container.

Mostly empty space

These can be explained by the kinetic molecular theory

Gases consist of atoms or molecules with a lot of space in between,

that are in constant, random motion

kinetic molecular theory:the theory that explains the observed thermal and physical properties of matter in terms of the average behavior of a collection of atoms and molecules.

Properties of gases

Evidence for the atomic / molecular nature of matter:

If the liquid and gas are both made from the same molecules (H2O), you can explain the “disappearance” by assuming that the molecules are much more spread out in the gas phase.

Brownian motion

Brownian motion can be seen by magnifying diluted milk and observing tiny fat globules getting knocked around by the surrounding water molecules

- What Brownian motion tells us:
- Matter consists of discrete particles (molecules or atoms)
- Molecules (or atoms) are in constant, vigorous motion as a result of temperature

Brownian motion

Brownian motion provides a peek into the microscopic world of atoms to see details that are normally hidden by the law of averages, and the enormous number of incredibly small atoms.

- In the early 1800’s Joseph Gay-Lussac studied gas volume relationships involving a chemical reaction between hydrogen and oxygen and observed that 2 L of hydrogen would react with 1 L of oxygen to form 2 L of water vapor at constant temperature and pressure
- Hydrogen gas + oxygen gas ---- water vapor
2 L 1 L ---- 2 L

2 volumes 1 volume ---- 2 volumes

Take a new sheet of paper and fold it into three sections

Write your name, the title of the chapter and the number

On the first section from the sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

- In Gay-Lussac experiment it was found that the reaction took place in a 2:1:2 relationship between hydrogen, oxygen and water vapor
- If you had 600 L H2, 300 L O2you would get 600 L H2O formed
- With hydrogen and chlorine combining then:
- Hydrogen gas + chlorine gas ---- hydrogen chloride gas
1 L 1 L ---- 2 L

1 volumes 1 volume ---- 2 volumes

- The relationship that he found between gas volumes is now known to be a law
- Gay-Lussac’s law of combining volumes of gases states – at constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers

- Remember from Dalton’s atomic theory that atoms are indivisible
- Also remember that Dalton believed that one atom of one reactant always combined with one atom of the other reactant (which caused questions when forming water vapor)
- Gay-Lussac disproved the second theory of Dalton, but a scientist by the name of Avogadro formulated an explanation of the problem
- Avogadro reasoned that molecules contained more than one atom and formulated his theory which later became a law
- Avogadro’s law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

- Remember that one mole is equal to the atomic mass of that atom
- Well, one mole of a molecule or a compound is equal to the combined molecular masses of the atoms
- Avogadro suggested that one mole of an element, molecule or an compound contained the same number of particles which is 6.022 x 1023 particles
- So 1 mole of H2 (2.015g) contained the same number of particles as 1 mole O2 (32g) at standard pressure (1 atm) and temperature (273 K) STP
- Standard volume at STP is 22.414 L

- Write a detailed three dollar summary of what you learned (a paragraph, with a topic sentence and three supporting sentences)
- Turn to page 468 and complete # 1 – 4 then turn them in
- Honors chemistry homework
- Page 468 # 7 - 16

- Molar Volume
- Ideal gas

Mostly empty space

No interaction between gas atoms or molecules except in collisions

Straight trajectories until collision occurs

Gases consist of widely separated atoms or molecules in constant, random motion

Gas pressure is increased by

more frequent and/or harder collisions

You can affect the gas pressure by changing:

1. the density

More molecules means more impacts and a higher pressure.

2. the volume of the container

With less space to move around, there are more collisions and a higher pressure.

You can affect the gas pressure by changing:

1. the density

More molecules means more impacts and a higher pressure.

2. the volume of the container

With less space to move around, there are more collisions and a higher pressure.

3. the temperature

With more kinetic energy, the molecules move faster. The collisions are harder and more frequent.

Boyle’s law: V versus P

Robert Boyle’s experiment:

- Mercury (Hg) was poured down a tube shaped like the letter “J.”
- - The tube was closed on the lower end.
- The gas inside the tube took up space (volume).
- The temperature and number of gas molecules inside the tube stayed constant.
- - Boyle observed the change in pressure in mmHg as a function of volume. He then graphed the relationship between pressure and volume.

Boyle’s law: V versus P

Pressure versus volume

- Temperature
- Number of moles

constant

Pressure versus volume

- Temperature
- Number of moles

constant

Boyle’s law: V versus P

Boyle’s law: V versus P

Boyle’s law: V versus P

If a weather balloon is released on the ground with a volume of 3.0 m3

and a pressure of 1.00 atm, how large will it get when it reaches an

altitude of 100,000 ft, where the pressure is 0.0100 atm?

On the second section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Boyle’s law: V versus P

If a weather balloon is released on the ground with a volume of 3.0 m3 and a pressure of 1.00 atm, how large will it get when it reaches an altitude of 100,000 ft, where the pressure is 0.0100 atm?

Asked:Volume of the balloon when it reaches 100,000 ft

Given:

Relationships:

Solve:

Answer:The balloon will have a volume of 300 m3.

Charles’s law: V versus T

The volume increases as the temperature increases

Volume versus temperature

- Pressure
- Number of moles

constant

Charles’s law: V versus T

Charles’s law: V versus T

Doubling the Kelvin temperature will double the volume of a gas

Kelvin temperatures simplify the V versus T relationship

Charles’s law: V versus T

Charles’s law: V vs. T

If you inflate a balloon to a size of 8.0 L inside where the temperature is 23oC, what will be the new size of the balloon when you go outside where it is 3oC?

Charles’s law: V vs. T

If you inflate a balloon to a size of 8.0 L inside where the temperature is 23oC, what will be the new size of the balloon when you go outside where it is 3oC?

Asked:Volume of the balloon when the temperature drops to 3oC

Given:

Relationships:

Solve:

Combined gas law

Imagine you were to hitch a ride on a high-altitude research balloon that reaches and altitude of 100,000 ft. At sea level, where the pressure is 1.00 atm and the temperature is 20oC, you’ll need 18 m3 of helium to fill the balloon. What will be the new volume of the gas when you reach altitude, where the pressure is 0.0100 atm and the temperature is –50oC?

Imagine you were to hitch a ride on a high-altitude research balloon that reaches and altitude of 100,000 ft. At sea level, where the pressure is 1.00 atm and the temperature is 20oC, you’ll need 18 m3 of helium to fill the balloon. What will be the new volume of the gas when you reach altitude, where the pressure is 0.0100 atm and the temperature is –50oC?

Asked:Volume of the balloon when it reaches 100,000 ft

Given:

Relationships:

Solve:

On the third section of that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

Avogadro’s law: V versus moles

Two equal volumes of hydrogen react with one volume of oxygen to produce two volumes of water vapor.

Gas volumes act like moles because the same size container has the same number of molecules (at the same temperature and pressure).

Moles versus volume

- Temperature
- Pressure

constant

Avogadro’s law: V versus moles

Moles versus volume

- Temperature
- Pressure

constant

Avogadro’s law: V versus moles

Moles versus volume

- Temperature
- Pressure

constant

Avogadro’s law: V versus moles

Twice the volume, twice the number of moleculesassuming the temperature and pressure are constant

Restrictions

The ideal gas law

Combining the previous gas laws, we obtain the ideal gas law

In reality, the ideal gas law is an approximation which is accurate for many gases over a wide range of conditions.

The ideal gas law is not accurate at very high density or at very low temperature.

The ideal gas law

R is the only constant

The universal gas constant

The ideal gas law

Calculating the universal gas constant using various units

Watch out for the units!

On the first section of back side on that sheet of paper, please write six things that you learned from your notes so far that could appear on your test.

The ideal gas law

Limitations of the ideal gas law

In an ideal gas, we assume that:

1. individual gas molecules take up no space

2. gas molecules do not interact with each other

For very small volumes or very low temperatures, gas atoms and molecules are very close together, and van der Waals attractions are no longer negligible

PV = nRT

PV = nRT

What would be the pressure inside of a 50.0 L tank containing 1,252 g of helium at 20oC?

PV = nRT

What would be the pressure inside of a 50.0 L tank containing 1,252 g of helium at 20oC?

Asked:Tank pressure

Given:

Relationships:

Solve:

PV = nRT

What would be the pressure inside of a 50.0 L tank containing 1,252 g of helium at 20oC?

Asked:Tank pressure

Given:

Relationships:

Solve:

What would be the new volume of a bubble in a bread dough once it goes from a room temperature (20oC) volume of 0.050 cm3 to a 191oC oven?

What would be the new volume of a bubble in a bread dough once it goes from a room temperature (20oC) volume of 0.050 cm3 to a 191oC oven?

Asked:Find the volume of a bubble under changing temperature conditions

Given:

Relationships:

Solve:Pressure and moles stay constant, so they cancel out.

What would be the new volume of a bubble in a bread dough once it goes from a room temperature (20oC) volume of 0.050 cm3 to a 191oC oven?

Asked:Find the volume of a bubble under changing temperature conditions

Given:

Relationships:

Solve:Pressure and moles stay constant, so they cancel out.

Boyle’s law

Charles’s law

Avogadro’s law

Combined gas law

Ideal gas law

R = universal gas law

- Write a three dollar summary of what you learned (a paragraph, with a topic sentence and three supporting sentences)
- Turn to page 468 and complete # 5 – 6 then turn them in
- Honors chemistry Homework:
- Page 468 # 17 - 26

- Molar Volume
- Ideal gas

- N/A

Ideal gas law

We can now solve stoichiometry problems involving gases

R = universal gas law

We can now solve stoichiometry problems involving gases

and

solids

solutions

other gases

Steps for solving stoichiometry problems

If you burn a 125 g candle made of paraffin wax, the peak temperature of the flame is about 1,400oC. Assuming the carbon dioxide produced is at that temperature, what volume of CO2 is produced at a pressure of 790 mmHg? The combustion reaction is:

2C20H42 + 61O2→ 40CO2 + 42H2O

If you burn a 125 g candle made of paraffin wax, the peak temperature of the flame is about 1,400oC. Assuming the carbon dioxide produced is at that temperature, what volume of CO2 is produced at a pressure of 790 mmHg? The combustion reaction is:

2C20H42 + 61O2→ 40CO2 + 42H2O

Asked:Volume of CO2 produced

Given:Paraffin: mass of 125 g

CO2: P = 790 mmHg, T = 1,400oC

Relationships:Molar mass of paraffin = 282.6 g/mole

Mole ratio: 2 moles C20H42 ~ 40 moles CO2

PV = nRT

Asked:Volume of CO2 produced

Given:Paraffin: mass of 125 g; CO2: P = 790 mmHg, T = 1,400oC

Relationships:Molar mass of paraffin = 282.6 g/mole

Mole ratio: 2 moles C20H42 ~ 40 moles CO2

PV = nRT

Solve:

0.442 moles C20H42

125 g C20H42

Asked:Volume of CO2 produced

Given:Paraffin: mass of 125 g; CO2: P = 790 mmHg, T = 1,400oC

Relationships:Molar mass of paraffin = 282.6 g/mole

Mole ratio: 2 moles C20H42 ~ 40 moles CO2

PV = nRT

Solve:

0.442 moles C20H42

8.85 moles CO2

Asked:Volume of CO2 produced

Given:Paraffin: mass of 125 g; CO2: P = 790 mmHg, T = 1,400oC

Relationships:Molar mass of paraffin = 282.6 g/mole

Mole ratio: 2 moles C20H42 ~ 40 moles CO2

PV = nRT

Solve:

8.85 moles CO2

Asked:Volume of CO2 produced

Given:Paraffin: mass of 125 g; CO2: P = 790 mmHg, T = 1,400oC

Relationships:Molar mass of paraffin = 282.6 g/mole

Mole ratio: 2 moles C20H42 ~ 40 moles CO2

PV = nRT

Solve:

8.85 moles CO2

1,170 L CO2

A common reaction occurs when an acid reacts with a metal to produce hydrogen gas. Consider a reaction in which 0.050 L of 1.25 M hydrochloric acid reacts with excess magnesium:

If the gas produced is captured in a 1.00 L container that starts out empty, what will be the pressure at the end of the reaction when the gas has cooled to 22oC?

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

A common reaction occurs when an acid reacts with a metal to produce hydrogen gas. Consider a reaction in which 0.050 L of 1.25 M hydrochloric acid reacts with excess magnesium:

If the gas produced is captured in a 1.00 L container that starts out empty, what will be the pressure at the end of the reaction when the gas has cooled to 22oC?

Mg(s) + 2HCl(aq) → H2(g) + MgCl2(aq)

Asked:Pressure of H2 produced

Given:HCl: V = 0.050 L, Molarity = 1.25 moles/L

H2: V = 1.00 L, T = 22oC

Relationships:Mole ratio: 2 moles HCl ~ 1 mole H2

PV = nRT

Volume

Asked:Pressure of H2 produced

Given:HCl: V = 0.050 L, Molarity = 1.25 moles/L

H2: V = 1.00 L, T = 22oC

Relationships:Mole ratio: 2 moles HCl ~ 1 mole H2

PV = nRT

Solve:

0.050 L HCl

1.25 M HCl

0.0625 moles HCl

Asked:Pressure of H2 produced

Given:HCl: V = 0.050 L, Molarity = 1.25 moles/L

H2: V = 1.00 L, T = 22oC

Relationships:Mole ratio: 2 moles HCl ~ 1 mole H2

PV = nRT

Solve:

0.0625 moles HCl

0.0313 moles H2

Asked:Pressure of H2 produced

Given:HCl: V = 0.050 L, Molarity = 1.25 moles/L

H2: V = 1.00 L, T = 22oC

Relationships:Mole ratio: 2 moles HCl ~ 1 mole H2

PV = nRT

Solve:

0.0313 moles H2

0.756 atm H2

What volume of butane gas is needed at room temperature (23oC) and typical pressure (0.954 atm) to produce 85.0 L of carbon dioxide at 825oC and a pressure of 1.04 atm? The reaction is:

2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)

What volume of butane gas is needed at room temperature (23oC) and typical pressure (0.954 atm) to produce 85.0 L of carbon dioxide at 825oC and a pressure of 1.04 atm? The reaction is:

2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)

Asked:Volume of C4H10 needed to produce 85.0 L of CO2

Given:C4H10: P = 0.984 atm, T = 23oC

CO2: P = 1.04 atm, T = 825oC, V = 85.0 L

Relationships:Mole ratio: 2 moles C4H10 ~ 8 moles CO2

Asked:Volume of C4H10 needed to produce 85.0 L of CO2

Given:C4H10: P = 0.984 atm, T = 23oC

CO2: P = 1.04 atm, T = 825oC, V = 85.0 L

Relationships:Mole ratio: 2 moles C4H10 ~ 8 moles CO2

Solve:

C4H10

CO2

Asked:Volume of C4H10 needed to produce 85.0 L of CO2

Given:C4H10: P = 0.984 atm, T = 23oC

CO2: P = 1.04 atm, T = 825oC, V = 85.0 L

Relationships:Mole ratio: 2 moles C4H10 ~ 8 moles CO2

Solve:

C4H10

CO2

Use the mole ratio to substitute for nCO2

Asked:Volume of C4H10 needed to produce 85.0 L of CO2

Given:C4H10: P = 0.984 atm, T = 23oC

CO2: P = 1.04 atm, T = 825oC, V = 85.0 L

Relationships:Mole ratio: 2 moles C4H10 ~ 8 moles CO2

Solve:

Asked:Volume of C4H10 needed to produce 85.0 L of CO2

Given:C4H10: P = 0.984 atm, T = 23oC

CO2: P = 1.04 atm, T = 825oC, V = 85.0 L

Relationships:Mole ratio: 2 moles C4H10 ~ 8 moles CO2

Solve:

m R T

m = mass

M = molar mass

M =

PV

M P

D = density

D =

RT

- Write a three dollar summary of what you learned (a paragraph, with a topic sentence and three supporting sentences)
- Turn to page 469 and complete # 32 then turn it in.
- Honors chemistry Homework:
- Page 471 # 73 - 81

- Homework requirement: Learn all terms and concepts covered on this topic.
- Make sure you have all assignments between page 224 and 227completed and turned in by your test date.