Gasses

16 6 GasesGasesPrepared by

Dr Mohd Zul Helmi Rozaini

University of East Anglia

University Malaysia Terengganu

6.1 The states of matter6.1 The states of matter

Matter exists in only three states: _________ _________ _________

The most stable state of a particular substance can be changed by __________________________

6.2 Describing gases6.2 Describing gases

Gases ________________________ ____________________ This implies that the individual gas

atoms or molecules are free to move anywhere within their container

Therefore the forces between them ____________________


The defining characteristic of gases is the pressure they exert

The pressure (p) exerted by a gas is dependent on: ______________ ______________ ______________


Pressure (p) At any temperature above absolute zero,

atoms/molecules are always in motion

Pressure is the collective result of these collisions


The atmosphere exerts pressure on the Earths surface

The pressure of the

A measures the difference in pressures exerted by

6.2 Describing gases6.2 Describing gases 6.2 Describing gases6.2 Describing gases

The SI unit for pressure is pascal (Pa) 1 Pa = 1 N m-2

A number of non-SI units are used:

3 The gas laws -The atoms

The volume occupied by a gas changes in response to changes in pressure, temperature and amount of gas


Law Boyle investigated gases in a J-shaped tube

Determined


Law Determined the volume of

a gas is directly proportional to its

Law Gas volume is directly

proportional to the amount

6.2 Describing gases6.2 Describing gases 6.2 Describing gases6.2 Describing gases

The ideal gas equation All four variables (p, V, T and n) can be

related using a single constant This is known as the gas constant (R) In SI units, R = 8.314 J mol-1 K-1

pV = This is known as

4WE6.1: Calculation of gas pressureWE6.1: Calculation of gas pressure

A 1.00 x 103 L steel storage tank contains 88.5 kg of methane, CH4. If the temperature is 25C, what is the pressure of the inside tank?

WE6.2: PressureWE6.2: Pressure-- volume variationsvolume variations

A sample of helium gas is held at constant temperature inside a cylinder with a volume of 0.80 L when a piston exerts a pressure of 1.5 x 105 Pa. if the external pressure on the piston is increased to 2.1 x 105, what will the new volume be?

6.3 Molecular view of gases6.3 Molecular view of gases

As gases are

molecules,the most important energy component to consider is their kinetic energy, Ekinetic

The kinetic energy of an object is given by the equation:

Molecular speeds A molecular beam

apparatus measures the speeds of molecules in a gas


5 Speed and energy The energy of a molecule is related to its

speed At a given temperature, all


Average kinetic energy The most probable kinetic energy is not

the same as the average kinetic energy The average kinetic energy of gas

molecules depends on the temperature of the gas


6.4: Molecular kinetic energy6.4: Molecular kinetic energy

Determine the average molecular kinetic energy and molar kinetic energy of gaseous sulfur hexafluoride, SF6, at 150C


Ideal gases An ideal gas has the following two

characteristics: The volume occupied by the molecules of an

ideal gas is negligible compared with the volume of its container.

The energies generated by forces among ideal gas


How does an gas behave? Consider how changes in V, T or n affect the

pressure, p In an ideal gas each molecule is

independent of all others

Lets consider the effect of changing one property while holding the other properties constant

Increasing the amount of gas: Pressure is directly proportional to the

amount of gas This


Changing the : Pressure is inversely proportional to volume This agrees

6.3 Molecular view of gases6.3 Molecular view of gases Changing :

Pressure is directly proportional to temperature

This is in agreement with

6.3 view of gases6.3 view of gases

76.4 Additional gas properties6.4 Additional gas properties

Determination of molar mass The ideal gas equation can be combined

with n = m/M to find the molar mass of an unknown gas

pV = nRT can be used

This information can then be used


Gas The density of a gas varies significantly

with the conditions The ideal gas equation and n = m/M can

be combined and rearranged to obtain an :

WE6.5: Molar mass determinationWE6.5: Molar mass determination

Calcium carbide, CaC2 is a hard, grey-black solid with melting point of 2000C. It reacts vigorously with water to produce a gas and a solution containing OH- ions.

A 12.8 g sample of CaC2 was treated with excess water and resulting gas was collected in an evacuated 5.00 L glass bulb with a mass of 1254.49 g. the filled bulb had a mass of 1259.70 and the pressure of the gas inside was 1.00 x 105Pa when the temperature was 26.8C.

Calculate the molar mass of the gas and determine its formula. (Assume that the product gas is insoluble in water, and the vapor pressure of water is neglible in relation to the pressure of the product gas)


The equation reveals three features of gas density: The density of an ideal gas increases linearly

with pressure at fixed temperature

The density of an ideal gas increases linearly with molar mass at a given temperature and pressure

Gas density has a significant effect on interactions

8WE6.6: Gas densityWE6.6: Gas density

A hot air balloon will rise when the density of its air is 15% lower than that of the atmospheric air. Calculate the density of air at 298 K and 1.0 x 105 Pa (assume that dry air is 78% N2and 22% O2), and determine the minimum temperature of air that will cause a balloon to rise


Rates The temperature of a gas determines

the average speed of the gas molecules of which it is comprised To state this dependence quantitatively:

This is called the root-mean-square speed


Effusion The movement of molecules


9 Diffusion The movement of one type of molecule

6.4 Additional gas properties6.4 Additional gas properties 6.5 Gas mixtures6.5 Gas mixtures

Gas behaviour depends on the number of gas atoms or molecules but not on their identity

Daltons law of partial pressures In a mixture of gases in which no chemical

reaction occurs, each gas

6.5 Gas mixtures6.5 Gas mixtures

Describing gas mixtures Mole fraction (X)

The number of moles of a substance as a fraction of the number of moles of all substances in the mixture

The partial pressure of a component in a gas mixture is its mole fraction


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When referring to lower concentrations in a gas mixture, scientists use parts per million (ppm) or parts per billion (ppb)

Mole fractions is moles per mole ppm is moles per million moles ppb is moles per billion moles 1 ppm is a mole fraction of

WE6.7: Gas mixtureWE6.7: Gas mixture

Exactly 8.00 g of O2 and 2.00 g of He place in a 5.00 L tank at 298 K. Determine the total pressure of the mixture, and find the partial pressures and mole fractions of the two gases

WE6.8: Working with (WE6.8: Working with (ppmppm))

The exhaust gas from an average car contains 206 ppm of the pollutant nitrogen oxide, NO. If a car emits 0.125 m3 of exhaust gas at 1.00 x 105Pa and exactly 350 K, what mass of NO has been added to the atmosphere?

6.6 Gas stoichiometry6.6 Gas stoichiometry

The principles of stoichiometry apply equally to solids, liquids and gases

No matter what phase substances are in, their chemical behaviour can be described in molecular terms

For gases,

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WE6.9: Gas WE6.9: Gas stoichiometrystoichiometry WE6.5 described the synthesis of ethyne, C2H2, from calcium

carbide, CaC2. Modern industrial production of ethyne is based on a reaction of methane, CH4, under carefully controlled conditions. At temperature greater than 1600 K, two methane molecules rearrange to give three molecules of hydrogen and one molecule of ethyne

2CH4 (g) C2H2 (g)+ 3H2 (g)

A 50.0 L steel vessel, filled with CH4 to a pressure of 10.0x105 Pa at 298 K, is heated to exactly 1600 K to convert CH4 into C2H2. What is the maximum possible mass of C2H2that can be produced?What pressure does the reactor reach at 1600 K? Assume that both CH4 and C2H2 behave as ideal gases under the conditions of the reaction.

WE6.10: limiting WE6.10: limiting ragentsragents in a in a gas mixturegas mixture

Margarine cn be made from natural oils such as coconut oil by hydrogenation.

C57H104O6 (l) + 3H2 (g) -> C57H110O6 (s) An industrial hydrogenator with a volume of

2.50 x 102 L is charged with 12.0 kg of oil and 7.00 x 105 Pa of hydrogen,H2 at 473 K. The reaction produces the maximum amount of margarine. What is the final pressure of H2 and what mass of margarine will be produced? Assume that H2 behaves as ideal gas under the reaction condition.

6.6 Gas stoichiometry6.6 Gas stoichiometry 6.7 Intermolecular forces6.7 Intermolecular forces

The halogens Exist as diatomic

molecules Although they have similar

covalent bonding, bromine and

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When the average kinetic energy is large enough, molecules remain separated from and the substance is a gas

When intermolecular

6.7 Intermolecular forces6.7 Intermolecular forces 6.7 Intermolecular forces6.7 Intermolecular forces

Real gases The ideal gas model makes two

assumptions:

Neither of these assumptions is true for a real gas

How close do real gases come to ideal behaviour?

6.7 Intermolecular forces6.7 Intermolecular forces

The ideal gas model can still be used to discuss the properties of real gases, as long as conditions do not become too extreme


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The van der Waals equation A modified ideal gas equation that accounts

for attractive forces and molecular volumes

Adds two

Each correction term includes a constant that has a specific value for every gas

6.7 Intermolecular forces6.7 Intermolecular forces 6.7 Intermolecular forces6.7 Intermolecular forces

WE6.12:Van WE6.12:Van derder WaalsWaals Gasses such as methane are sold and

shipped in compresses gas cylinders. A typical cylinder has a volume of 15.0 L and when full, contains 62.0 mol of CH4. after prolonged use, 0.620 mol of CH4 remains in the cylinder.

Use the Van der Waals equation to calculate the pressures in the cylinder when full and after use, and compare the values to those obtained from the ideal gas equation. Assume a constant temperature of 27C


Melting and boiling points Can be used as indicators of the

strengths of intermolecular forces The boiling point is the temperature at

which the average kinetic energy of molecular motion balances the attractive energy of intermolecular attractions

When the pressure is 1.013 x 105 Pa, that temperature is the normal boiling point

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The conversion of a liquid into a gas is called vaporisation

Condensation is the reverse process At temperatures below the freezing point,

the molecules become locked in place and the liquid solidifies. When the pressure is , that temperature is the normal freezing point

Boiling


6.8 Types of intermolecular 6.8 Types of intermolecular forcesforces

There are three general types: Dispersion forces

The attractions between the negatively charged electron clouds and the positively charged nuclei of neighbouring molecules


Dispersion forces Exists because the electron clouds of

molecules can be distorted

Dispersion forces are the net attractive forces between

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The magnitude of dispersion forces depends on how easy it is to distort the electron cloud of a molecule

This ease


Dispersion forces increase in strength with the number or electrons


For molecules with comparable numbers of electrons, the shape of the molecule makes an


Dipolar forces


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Hydrogen bonds There are two requirements:

First, there must be an electron-deficient hydrogen atom that can be attracted to an electron pair

Hydrogen atoms in O-H, F-H and N-H bonds meet this requirement



Hydrogen bonds can form between different molecules

Molecules can form more than one hydrogen bond (eg glycine)

Hydrogen bonds can form within a molecule (eg salicylic acid)

Hydrogen bonding is particularly important in biochemical systems

Binary hydrogen compounds It is both the strength and number of

hydrogen bonds that a binary hydrogen compound can form which determines its boiling point


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SummarySummary

The three most important states of matter are solid, liquid and gas

Gases occupy all of the space in which they are contained

The pressure exerted by a gas is due to the collisions of rapidly moving gas atoms or molecules with the walls of the container

SummarySummary

Boyles Law, Charles Law and Avogadros Law describe the relationships between volume and pressure, volume and temperature, and volume and amount of a gas respectively

The combination of these laws gives the ideal gas equation, pV = nRT

SummarySummary

In order to determine the kinetic energy of a gas molecule, it is necessary to measure the speed with which it is moving

All gases have an identical molecular kinetic energy distribution

Real gases approximate ideal behaviour under certain conditions

SummarySummary

The movement of gas molecules can be described as either effusion or diffusion

Each gaseous component in a mixture of ideal gases exerts a partial pressure

The mole fraction of a substance equals the ratio of moles to the total number of moles

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SummarySummary

Intermolecular forces are partially responsible for the fact that real gases do not exactly obey the ideal gas laws

The van der Waals equation for a real gas makes corrections for the volume of the gas molecules and for the attractive force between them

Melting and boiling points give good indications of the strength of intermolecular forces

SummarySummary

There are three general types of intermolecular forces: Dispersion forces Dipolar forces Hydrogen bonds

Gasses

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