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Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad

Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad

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Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad

REVIEW QUESTION

Explain why volume of a gas decreases on increasing pressure on it at constant temperature?
There are large empty spaces between the molecules. On increasing pressure on the gas, the distance between molecules decreases. So the volume of the gas decreases.

How does temperature effect vapour pressure of a liquid?
The vapour pressure of a liquid increases as the temperature increases. The molecules in the liquid are more energetic at higher temperatures, and more molecules can escape from the liquid phase into the gas phase.

Water boils at 120oC in a pressure cooker, why?
Pressure cooker has valve to controls the pressure. This valve exerts a pressure of 2 atm. Therefore, the valve does not allow water vapours to escape until the pressure inside the pot reaches 2 atm. Because vapour pressure of water becomes 2 atm when the temperature reaches 120oC. So water boils at 120oC in a pressure cooker.

Is evaporation a cooling process?
According to kinetic theory, the temperature is a measure of average kinetic energy of the molecules of a liquid. During evaporation, the escape of high energy molecules from the surface of a liquid, lowers the average kinetic energy of the remaining molecules and therefore, the temperature of the liquids falls down. Thus evaporation is a cooling process.

Can you make water boil at 70oC?
Yes, when the pressure of atmosphere is 1 atm, water boils at 100oC at sea level. So water boils at this height above sea level, when its vapour pressure is 34kPa at 70°C.

Express the pressure 400 mm Hg in kPA?
1 atm                =      760 mm     =      101.325 kPa       =      1.01325 x 105 Pa
760 mm     =      1.01325 kPa
1mm         =      1.01325/760
400 mm     =      1.01325/760 x 400                      =      0.533 kPa

Explain the effect on the volume of a gas by a change in the (a) pressure (b) temperature.
Effect of pressure change on the volume of gas:
There are large empty spaces between the molecules. On increasing pressure on the gas, the distance between molecules decreases. So the volume of the gas decreases.
Effect of temperature change on the volume of gas:
When pressure of the gas is kept constant and temperature is increased, the average kinetic energy of gas molecules increases. So, the molecules of the gas hit the wall of the container more frequently and energetically. This increases internal pressure. As a result, volume of the gas increases to restore constant pressure.
Explain the following properties of gases (a) Diffusion (b) Effusion
Diffusion:
·        The movement of molecules from higher concentration to lower is called diffusion.
·        Diffusion takes place in all directions.
·        For example the smell of rose or a scent spread due to diffusion.
Effusion:
·        The escape of gas molecules through the hole one after the other without collision is called effusion.
·        Effusion takes place through a hole of molecular size.
·        For example escape of gas molecules from punctured tyre.

Explain the following terms.
Evaporation:
·        Conversion of a liquid to a gas or vapour at all temperatures is called vaporization or evaporation.
·        Place some liquid (i.e. ether) in an open container and observe.
·        You will notice that the volume of the liquid gradually decreases and finally no more of the liquid is left.
·        This is because liquids constantly change into gas or vapours.
·        According to kinetic theory, the temperature is a measure of average kinetic energy of the molecules of a liquid. During evaporation, the escape of high energy molecules from the surface of a liquid, lowers the average kinetic energy of the remaining molecules and therefore, the temperature of the liquids falls down. Thus evaporation is a cooling process.
Vapour pressure:
·        The pressure exerted by the vapours of a liquid in equilibrium with its liquid is called vapour pressure.
·        In a closed container no molecules can escape into the outside air, when a partially filled container is sealed.
·        Some of the liquid molecules vapourize.
·        As the time passes, the number of molecules changing into vapours increases.
·        Some of these molecules because of their random motions will collide with the liquid surface.
·        Such molecules are recaptured by the molecules at the surface of the liquid. This process is called condensation.
·        These two opposing processes will continue. After some time, the number of molecules evaporating will become equal to the number of molecules condensing.
·        At this stage equilibrium is established between the liquid and its vapours.
Boiling point:
·        The temperature at which vapour pressure of a liquid becomes equal to the external or atmospheric pressure is called boiling point.
·        Vapour pressure of the liquid keeps on increasing with the increase in temperature.
·        At a certain temperature, the vapour pressure of the liquid becomes equal to the atmospheric pressure or external pressure.
·        At this stage liquid starts boiling.
Explain the effect of external pressure on boiling point.
·        Liquids boil when their vapour pressure is equal to the pressure exerted on the liquid by its surroundings.
·        The normal boiling point of water is 100°C.
·        In the mountains the atmospheric pressure is less than 1 atm, so water boils below 100°C.
·        Pressure cooker has valve to controls the pressure. This valve exerts a pressure of 2 atm. Therefore, the valve does not allow water vapours to escape until the pressure inside the pot reaches 2 atm. Because vapour pressure of water becomes 2 atm when the temperature reaches 120oC. So water boils at 120oC in a pressure cooker.

Differentiate between amorphous and crystalline solids.
·        The solids in which atoms, molecules or ions are arranged in a regular repeating three-dimensional well ordered pattern are called as crystalline solids.
·        Amorphous solids are those in which atoms, ions or molecules are not arranged in a definite pattern, rather these are randomly arranged.

What are allotropes? Explain the allotropic form of any two solids .
The different forms of an element in the same physical state and phase are called allotropic form or allotropes.
Diamond:
In diamond each carbon atom is bonded to four others, creating a rigid compact array. This makes diamond the hardest known substance.
Graphite:
In graphite carbon atoms are arranged in layers of hexagonal arrays. Weak bonds exist between the layers that allow them to slide over one another. This makes graphite soft.

Find the boiling point of each liquid when the atmospheric pressure is 1 atm.
Name of Liquid                      Boiling point
Chloroform                         61oC
Ethanol                              78 oC
Water                                100 oC
Ethanoic Acid                      119 oC

At what temperature ethanol will bon when the atmospheric pressure is 51 kPa.
At 101.3 kPa pressure, Ethanol boils      =      78°C
At 1 kPa pressure, Ethanol boils            =      78/101.3
At 51 kPa pressure, Ethanol boils          =      78/101.3 x 51
=      39.274°C
How can you make water to boil at 80°C?
The boiling point of water is 100°C when pressure is 760 mm Hg or one atm pressure.

At 100°C pressure of water   =      760 mm Hg
At 1°C pressure of water      =      760/100
At 80°C pressure of water    =      760/100 x 80
                                        =      608 mm Hg
So we can boil water at 80°C when pressure will be 608 mm Hg or 0.8 atm.

At what temperature chloroform will boil when the external pressure is 50 kPa.
When external pressure is 101.3 kPa than B.P of chloroform      =      61°C.
When external pressure is 1 kPa than B.P of chloroform            =      61/101.3
When external pressure is 50 kPa than B.P of chloroform          =      61/101.3 x 50
                                                                                        =      30.11°C
So, chloroform boils at 30.11°C when the external pressure is 50 kPa.

Can you boil chloroform at 0°C.
Yes. As temperature in °C can be changed into Kelvin scale.
Chloroform boils at 61°C = 273 + 61 = 334oK and 0 °C = 273oK

At 334oK temperature pressure on chloroform =      101.3 kPa
At 1oK temperature pressure on chloroform     =      101.3/334
At 273oK temperature pressure on chloroform  =      101.3/334 x 273
                                                                =      82.82kPa
Thus when pressure is reduce to 82.82 kPa, chloroform boils at 273oK (0oC).

Predict the boiling point of chloroform at 600 mm Hg.
Since 1 atm = 760 mm = 101.325 kPa .
When 760 mm Hg pressure then boiling point of chloroform       =      61oC.
When 1 mm Hg pressure then boiling point of chloroform          =      61/760
When 600 mm Hg pressure then boiling point of chloroform       =      61/760x600
                                                                                        =      48.16oC
Therefore boiling paint of chloroform is 48.1oC at 600 mm Hg.

The air in a perfectly elastic balloon occupies 885 cm3 during the fall when the temperature is 20°C. During the winter, the temperature on a particular day is -10°C, the balloon occupies 794.39 cm3. If the pressure remains constant show that the given data proves the volume temperature relation according to the Charles's Law
·        According to the Charles's law, ratio of volume to absolute temperature is constant for any set of conditions.
·        Convert 0°C temperature to Kelvin temperature by adding 273.
·        Find V/T for each set of conditions and compare.
Temperature (oC)
Volume
Temperature (K)
V/T
20
885
293
885/293 = 3.0204
-10
794.39
263
794.39/263 = 3.0204
The ratio V/T = 3.0204 is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by the Charles's law.
In the past, gas volume was used as a way to measure temperature using devices called gas thermometers. An experimenter obtains following data from as thermometer.
Volume
Temperature
2.7
0oC
3.7
100 oC
5.7
300 oC
Show that gas thermometer obtained results according to Charles's law
  • According to the Charles's law, ratio of volume to absolute temperature is constant for any set of conditions.
  • Convert 0°C temperature to Kelvin temperature by adding 273.
  • Find V/T for each set of conditions and compare
Temperature (oC)
Volume
Temperature (K)
V/T
0
2.7
273
2.7/273 = 0.0099
100
3.7
373
3.7/373 =  0.0099
300
5.7
573
5.7/573 = 0.0099

The ratio V/T = 0.0099 is fairly constant. Thus volume of the gas varies directly with the absolute temperature as stated by the Charles's law.

In automobile engine the gaseous fuel-air mixture enters the cylinder and is compressed by a moving piston before it is ignited. If the initial cylinder volume is 990 cm3. After the piston moves up, the volume is 90 cm3. The fuel-air mixture initially has a pressure of 1.0 atm  and final pressure 11.0 atm. Do you think this change occurs according to the Boyle's law?
·        According to the Boyle's law. product of pressure and volume is constant at any two sets of conditions.
·        Calculate P x V for the two sets of condition and compare. (1 dm = 1000 cm3)

P1 x V1 before change      =      1 x atm x 990/1000 dm3       =      0.99 atm.dm3

P2 x V2 after change                 =      11 atm x 90/1000 dm3         =      0.99 atm.dm3

So it is proved that          P1 x V1       =      P2 x V2
Thus the calculated result agrees with the pressure-volume relationship according to the Boyle's Law.

A sample of neon that is used in a neon sign has a volume of 1500 cm3 at a pressure of 636 torr. The volume of the gas after it is pumped into the glass tube of the sign is 1213.74 cm3 when it shows a pressure of 786 torr. Show that this data obeys Boyle's law
·        According to the Boyle's law, product of pressure and volume is constant at any two sets of conditions.
·        Calculate P x V for the two sets of condition and compare.
·        (1 dm = 1000 cm3) and (a atm = 760 torr)
P1 x V1 before change = 636/760 x atm x 1500/1000 dm3 = 0.8368 x atm x 1.5 dm3
                                                                                                            = 1.255 atm.dm3
P2 x V2 after change = 786/760 x atm x 1213.74/1000 dm3 = 1.0342 x atm x 1.21374 dm3
                                                                                                            = 1.255 atm.dm3

So it is proved that          P1 x V1       =      P2 x V2

Thus the calculated result agrees with the pressure-volume relationship according to the Boyle's Law.

Instrumentation changes as science progresses, comments on it?
·        If any instrumentation changes occur, the internal validity of the main conclusion is affected as alternative explanations are readily available.
·        The instrumentation, or if dropping out leads to relevant bias between groups, a whole class of alternative explanations is possible that account for the observed differences.
·        If any instrumentation changes occur, the internal validity of the main conclusion is affected, as alternative explanations are readily available

The water level in an aquarium decreases slowly even though the tank does not leak. What change of state is occurring?
·        The volume of the liquid gradually decreases and finally no more of the liquid is left.
·        This is because liquids constantly change into gas or vapours even when the temperature is less than the boiling point of a liquid.
·        In evaporation, some molecules in the liquid break away and enter the gas or vapour state.

What types of attractive forces do you expect between the molecules of HF and HCl?
Hydrogen fluoride (HF):
The H-bond in HF is so strong that the molecules remain associated through H-bonding even in the gaseous state.
Dipole-Dipole Forces in HCl:
The attractive forces· between positive end of one polar molecule and negative end of other polar molecule are known as dipole-dipole forces.

Name two substances that are solids at 25oC. Name two substances that are liquid at 25oC.
Solid at 25oC      =      Sodium Chloride (NaCl) and Silver Nitrate (AgNO3)
Liquid at 25oC     =      Water and Ethanol
Identify the process occurring in each of the following
Mothballs slowly disappear
Sublimation
A cold windshield becomes covered with ice when struck by Raindrops
Condensation
An autoclave is used to sterilize surgical equipment. It is far more effective to produce steam by autoclave than steam produced from boiling water in the open atmosphere, because it generates steam at a pressure of two atmospheres. Explain why an autoclave is such an efficient sterilization device.
When the pressure of atmosphere is 1 atm or 101.325 kPa water boils at 100oC at sea level. This is because at this temperature vapour pressure of water is 1 atm or 101.325 kPa.
When vapour pressure of water becomes 2 atm then the temperature reaches 120oC. So water boils at 120oC in a autoclave and produces more stream in less time. Due to high pressure autoclave is more effective to produce stream as compare to boiling water in the open atmosphere (1 atm)

THEORY

Freeze dried foods are light weight and can be conveniently re-constituted by adding water. How?
Freeze drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublimate directly from the solid phase to the gas phase.

Define Pressure. What is its unit?
Force exerted by a gas on unit area of a container is called its pressure. It unit is pascal (Pa).

Why gases are compressible?
Gases are compressible due to presence of large empty spaces between the gas molecules.
What do you know about the mobility of gases?
Gases have tendency to expand and fill the entire available space. So these can be transported through pipes over long distances.
What do you know about the density of gases?
Gases have very low densities under normal conditions. This is due to large spaced between gas molecules.
Define gas laws?
The relationship that expresses the influence of one variable on another with the two variable constant are called gas laws.
State and explain Boyle’s law?
Volume of a fixed amount of gas at a given temperature is inversely proportional to the applied pressure.
p              a      1/v
p              =      1/v x constant
p x v         =      constant

State and explain Charles’s law?
Volume of given mass of a gas varies directly with absolute temperature at constant pressure.
V              a      T
V              =      constant x T
V/T           =      constant

When you put nail polish remover on your palm, you feel a sensation of coldness.
The chemical nail polish remover is very volatile, it evaporates very quickly. To change from a liquid state, to a gaseous state takes energy. The warmth of your palm supplies the energy. The flow of heat is from your palm and into the liquid. This removes heat from your palm and has the sensation of being cool.

Does vapour pressure depend upon the nature of a liquid?
Yes it depends. The vapour pressure of a liquid decreases when inter molecular forces increases.

Define distillation.
A process called distillation is used to purify the liquids. It is a process in which a liquid is heated to vapourize it and the vapours are cooled to condense them back to the liquid in a different container.

What are allotropic forms of sulphur?
Rhombic Sulphur, Monoclinic Sulphur and Plastic Sulphur

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Download Class Notes: Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad
Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad
Download 9th Class Chemistry Notes (Unit # 5) for Federal Board (FBISE) Islamabad
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