States of Matter-II Liquids Chemistry 1st years notes for kpk 2021
Q.2 i) Give at least two of the effects on our lives if water has weak hydrogen bonding among its molecules.
Effects of Weak Hydrogen Bonding:
- Water would not be a liquid at room temperature. It would be a gas. So we will have no liquid water to be used and no survival on earth.
- Large protein molecules in our body are stabilized due to strong hydrogen bonding. Fibres, hair, muscle proteins consist of long chains of amino acids. These chains are coiled around each other and form a spiral helix. The hydrogen bonds stabilize these spiral helixes. So the structure of the body would not be stable if the hydrogen bonding is not strong.
Q.2 ii) HF is a liquid at ordinary temperature while HCl is a gas.
HF has hydrogen bonding among its molecules due to the greater difference of electro-negativities between H and F atoms. Strong hydrogen bonding among its molecules makes HF liquid at ordinary temperature. On the other hand, due to lower electronegativity of Cl, it cannot undergo hydrogen bonding among its molecules. So the dipole-dipole forces among its molecules are not strong enough to keep it in liquid form. Hence HCl is a gas at ordinary temperature.
Q.2 iii) H2O has high boiling point than HF, although Fluorine is more electronegative than oxygen.
Boiling point of a substance depends upon the strength of attractive forces present among its molecules. Hydrogen bonds in HF are strong as compared to that of H2O because of greater electronegativity of fluorine than oxygen. But on average, one molecule of HF forms one hydrogen bond and one molecule of H2O forms two hydrogen bonds because it has two hydrogen atoms in its molecule. So water has a higher boiling point due to greater number of hydrogen bonds and a three-dimensional network of hydrogen bonding in its structure.
Q.2 iv) Water and Ethanol can mix easily in all proportions.
Water (H2O) and ethanol (C2H5OH) both have hydroxyl group (-OH) in their structure. So when ethanol and water are mixed, hydrogen bonding is formed between ethanol and water molecules and they become miscible in all proportions.
Q.2 v)Neon and Argon both are mono-atomic noble gases of the same group. Neon has boiling point of -248°C while Argon has -189°C. Why?
Boiling point of a substance is directly related to the strength of attractive forces present among its molecules. Neon and Argon both are non-polar gases and London dispersion forces which depend upon the size of molecules, are the only forces present in their molecules. Due to small size, neon molecules have weaker London dispersion forces while Argon has stronger London dispersion forces due to large size of its molecules. Hence neon has less boiling point a as compare to Argon.
Q.2 vi) Different liquids have different rates of evaporation even at the same temperature.
Different liquids have different rates of evaporation even at the same temperature because the strength of intermolecular forces is different in different liquids. Stronger the intermolecular forces, slower will be the rate of evaporation and vice versa.
For example, rate of evaporation of water is slower than that of ether, because water has strong hydrogen bonding and ether has weak London forces.
Q.2 vii) Earthenware vessels keep water cool even in hot summer days. Vacuum distillation can be used to avoid decomposition of a sensitive liquid.
Earthenware vessels have porous surface. Water molecules come on the surface through these pores and evaporate. These molecules take energy away from the filled water and water remains cool even in hot summer days.
The distillation carried out under reduced pressure is called vacuum distillation. High boiling sensitive liquids cannot be purified at normal temperature because they decompose before their boiling points are reached. For example, glycerine boils at 290oC at normal atmospheric pressure, but it decomposes at this temperature. Under reduced pressure i.e. vacuum, the boiling point of glycerine decreases to 210oC at 50 mmHg. So it can be distilled easily under vacuum.
Q.2 viii) A liquid boils at different temperatures at sea-level and at mountains.
Boiling point of a substance is the temperature at which vapour pressure of the liquid becomes equal to the external pressure. So the boiling point varies with external pressure. At sea-level, external pressure is high and more heat is required to equalize vapour pressure to the external pressure that is why boiling point is high. At mountains, external pressure is low and hence less heat is required to equalize the vapour pressure to the external pressure that is why the boiling point is low at mountains.
Q.2 ix) Evaporation of a liquid causes cooling.
The molecules of liquid are always in motion state. The energy of molecules is not equally distributed. When evaporation takes place, high energy molecules leave the liquid and low energy molecules are left behind. The temperature of the liquid falls and heat moves from surrounding to the liquid and then the temperature of the surrounding also falls. That is why evaporation causes cooling.
Q.2 x) Temperature of a liquid remains constant during boiling although heat is being supplied continuously.
When a liquid is heated, its temperature rises until the boiling point is reached. At boiling point the temperature becomes constant because the heat supplied at this temperature is used to break the inter molecular forces and convert the liquid into vapours.
Q.2 xi) Why water droplet is spherical?
Water has high surface tension because of hydrogen bonding. Liquid droplets tend to be in a state of minimum surface energy which is directly related to the surface area. The force, surface tension, which is trying to hold the droplet together, therefore tries to reduce the surface area of the droplet. Mathematically only a sphere has the smallest surface area for a given volume, compared to other geometric shapes. Hence, in the absence of external forces, liquid droplet tends to be spherical.
Q.3) Discuss the properties of liquids on the basis of Kinetic molecular theory.
a) What do you know about hydrogen bonding?
b) Give a-few applications of hydrogen bonding in daily life.
Properties of Liquids:
The properties of liquids on the basis of the kinetic molecular theory are discussed below.
1. Definite Volume:
Liquids have a definite volume. They do not fill the whole container like gases.
2. Indefinite Shape:
A liquid has no definite shape because intermolecular forces are not so strong to stop the layers from sliding over each other. Thus, it takes the shape of its container.
Miscible liquids diffuse into one another. This diffusion is very slow in comparison to that of gases.
Liquids are generally incompressible due to negligible intermolecular spaces. But at very high pressure they can be compressed slightly.
A liquid has the ability to flow. Due to this, a liquid can be poured from one container to another.
Liquids have the ability to evaporate at all temperatures.
Substances containing hydrogen, bonded to a strongly electronegative element such as F, O or N, in addition to covalent bond have an additional force of attraction between the hydrogen of one molecule and electronegative atom of another molecule. Such interaction between the molecules is known as hydrogen bonding.
The following figure shows the hydrogen bonding in water.
Properties of Hydrogen Bond:
- Hydrogen bond is stronger than dipole-dipole interaction but weaker than covalent bond. It is approximately twenty times weaker than covalent bond.
- Hydrogen bond is directional.
- Hydrogen bond results in the formation of long chains and network of molecules.
Applications of Hydrogen Bonding:
Hydrogen bonding indirectly plays an important role in our daily life. Few applications of hydrogen bonding are discussed below.
Soaps and detergents are made up of long non-polar hydrocarbon tail and a polar anion i.e. head. In water, the anionic head is stabilized by a hydrogen bond with H2O molecule. While the non-polar tail remains outside H2O because it is not soluble in water. Thus, hydrogen bonding helps in cleansing action of soaps and detergents.
In Biological Compounds:
Large protein molecules in a living organism are stabilized due to hydrogen bonding. Fibres, hair, muscle proteins consist of large chains of amino acids. These chains are coiled around each other and form a spiral helix. The hydrogen bonds stabilize these spiral helix.
DNA in Cells:
DNA (Deoxyribonucleic acid) has two spiral strands which are coiled about a common axis. They form a double helix ladder-type structure. The steps of these ladders are formed by hydrogen bonds.
Hydrogen Bonding in Paints & Dyes:
The adhesive nature of certain paints and dyes is also due to hydrogen bonding. Similarly, sticky action of glue and honey is also due to hydrogen bonding.
Food material such as carbohydrates e.g. glucose, fructose and sucrose are also stabilized due to hydrogen bonding. All these contain –OH groups which form hydrogen bonding.
Structure of Ice:
Due to hydrogen bonding, the structure of ice has empty spaces. That is why the density of ice is less than that of liquid water and it floats on water. Ice acts as an insulator and prevents the liquid below it to freeze, hence maintaining aquatic life in cold weather.
Q.4 a) What do you know about the dipole-dipole interactions?
“The attractive forces between the positive pole of one polar molecule and negative pole of another polar molecule are called dipole-dipole forces”.
A polar molecule is also called a dipole because a polar molecule has two poles, one is partially negative and the other is partially positive. It is due to the difference in electronegativity values of two bonded atoms. The more electronegative atom develops a partially negative charge on itself and a partially positive charge on the less electronegative atom. For example, HCl molecule is a dipole shown below.
Whenever these polar molecules are close to each other, attractive forces created between the opposite poles of two dipoles which are called dipole-dipole forces.
Dipole-dipole forces are significant forces in many compounds. Dipole-dipole interactions in HCl, HI, HBr, H2S and PH3 are shown in the figure below.
Significance of Dipole-Dipole Interactions:
Generally, stronger the dipole-dipole forces, greater the values of thermodynamic properties like melting point, boiling point, heat of vaporization and heat of sublimation etc.
How are these different from hydrogen bonding?
Difference b/w Dipole-dipole Interactions & hydrogen Bonding:
Hydrogen bonding itself is a type of dipole-dipole interaction. Differences between these inter molecular forces are given below.
Dipole-dipole interactions occur between all those molecules which have permanent poles but hydrogen bonding is specific to only those molecules in which hydrogen is combined with highly electromotive atom such as O, N or F.
Hydrogen bonding is stronger than dipole-dipole interactions.
For example HBr, HI and H2S have dipole-dipole interactions because of less electromagnetically difference. While HF, H2O etc. have hydrogen bonding due to greater electromagnetically difference b/w constituent atoms.
Q.5) Explain the factors affecting London forces.
Factors Affecting London Forces:
London forces are present in all type of molecules whether polar or non-polar but they are very significant for non-polar molecules like Cl2, H2 and noble gases. The strength of these forces depends upon the following factors.
- Size of Molecules
- Shape of Molecules
Size of Molecules:
The strength of London forces depends upon the size of atoms. The size of atoms increases down the group in a periodic table due to an increase in electrons of valence shells. These create stronger temporary dipoles causing increase in strength of London dispersion forces.
- In the noble gases group VIII A, Neon has smaller size than Xenon. Due to small size, neon molecules have weaker London dispersion forces. As a result neon molecules have lower boiling point (-245.9oC) while xenon has greater boiling point (-107.1oC).
- Due to small size, Fluorine molecules have weaker London dispersion forces while Iodine molecules have stronger London dispersion forces. As a result, fluorine has lower boiling point (-188.1oC) and is in the gaseous state while iodine has higher boiling point (184.4oC) and is in the solid state.
Shape of Molecules:
Shape of molecules significantly affects the electronic distribution which as a result affects the strength of London dispersion forces. Shape of molecules may be straight and long chain or branched and short chain. The straight-long chain molecules have greater London dispersion forces than the branched-short chain molecules, even if they have the same number of electrons.
n-Butane and Iso-Butane both have molecular formula C4H10 but have different structures as given below.
n-Butane is a straight and long-chain compound. It has a greater boiling point (272.5 K). While Iso-Butane is branched and short-chain compound. It has a lower boiling point (261.3 K).
Q.6 a) Define and explain the vapour pressure of a liquid.
The pressure exerted by the vapours in equilibrium with the liquid at a given temperature is called vapour pressure of a liquid.
If a liquid is present in closed container, the molecules of the liquid are converted into vapours by evaporation. After some time, vapours are converted in to liquid by condensation. In the beginning, rate of evaporation is greater than rate of condensation, but after some time both the rates become equal and a dynamic equilibrium is established as shown in the figure below.
Q.6 b) Discuss the factors, which affect vapour pressure.
Factors Affecting Vapour Pressure:
The factors which affect the vapour pressure are discussed below.
- Nature of Liquid
Nature of Liquid:
Different liquids have different vapour pressures because they have different intermolecular forces. The liquids with strong intermolecular forces have low vapour pressure than the liquids with weak intermolecular forces. For example, the vapour pressure of water is 24 mmHg at 25oC which is lower than ether which is 537 mmHg at the same temperature.
Vapour pressure of liquids increases with the increase in temperature and vice versa. When the temperature is increased, the kinetic energy of molecules increases, as a result, vapour pressure increases. For example, vapour pressure of water at 25oC is 24 mmHg, at 50oC is 93 mmHg, at 80oC is 355 mmHg and at 100oC is 760 mmHg.
Q.6 c) How is vapour pressure experimentally determined?
Measurement of Vapour Pressure:
There are two general methods for the measurement of vapour pressure.
1. Barometric Method
In this method, one meter (1000 m) long glass tube is filled with mercury. It is inverted in a dish of mercury. The mercury level in the tube falls. Until, it is stopped by atmospheric pressure. Its height is measured which is 760 mm and called the one atmospheric pressure as shown in figure.
Now a small amount of ether is placed at the lower end of mercury in the tube with the help of a dropper, whose vapour pressure is to be measured. The ether vapours will exert pressure and push the mercury downward. This fall of mercury level in the tube is measured. It gives the vapour pressure of liquid. For ether, it is 537 mmHg at 25oC as shown in figure.
2. Manometric Method:
It is an accurate method for the measurement of vapour pressure of a liquid. The apparatus consists of a flask with T-shaped glass tube. On end of the tube is connected to U-shaped Manometer and the other and is connected to a vacuum pump as shown in figure.
The liquid in the flask is frozen with the help of freezing mixture. The space above the liquid is evacuated using vacuum pump. The frozen liquid is melted again to release any entrapped air. This process is repeated for a number of times so that the entire air is removed.
Now close the vacuum pump end and open the manometric end. Set the thermostat at a particular temperature. On evaporation of the liquid, vapours exert pressure on mercury column in the left limb of the manometer. Mercury level in the right limb will be pushed up against the atmospheric pressure. After sometime equilibrium is established and no change in the mercury height is further observed. The difference in mercury height gives the vapour pressure of the liquid in the flask.
V.P (l) = P + Δh
Q.7 a) Define and explain the boiling point of a liquid.
The temperature at which vapour pressure of the liquid becomes equal to the atmospheric pressure or some other external pressure is called the boiling point.
When a liquid is heated, its vapour pressure increases. At a certain temperature, the vapour pressure becomes equal to the atmospheric pressure. At this stage, the liquid starts boiling. At the boiling point, the kinetic energy of the molecules of the liquid becomes maximum. At this point, any further heating will not increase the temperature. This heat will be used to break the intermolecular forces and convert the liquid into vapours. That is why the temperature of the liquid remains constant at this boiling point.
At the atmospheric pressure i.e. 760 mmHg, the boiling point of CS2 is 46.30oC, CCl4 is 76.50oC, C2H5OH is 78.26oC, C6H6 is 80.15oC, H2O is 100oC and CH3COOH is 118.50oC.
Q.7 b) How is the boiling point of a liquid dependent on the atmospheric pressure?
Effect of Boiling Point on External Pressure:
The liquid boils at a temperature where its vapour pressure becomes equal to the external pressure. So if the external pressure is reduced the liquid will boil at low temperature as it will require less heat to equalize it vapour pressure to the external pressure. And if external pressure is increased, the liquid will require more heat to equalize its vapour pressure to the external pressure and as a result, the boiling point will be high. The following table shows the effect of external pressure on water’s boiling point.
|Sea level||1 atm||373 K|
|Murree Hills||0.921 atm||371 K|
|Mount Everest||0.425 atm||345 K|
Q.8) What are Liquid-Crystals? Give their uses in daily life.
The semisolid substances which have properties in between the true solids (crystalline solids) and true liquids (clear liquid) are called Liquid Crystals.
They are either semisolids or turbid liquids. Because they show the
(i) Optical properties like crystalline solids and
(ii) Surface tension or viscosity like liquids
When a solid is melted, it is converted to liquid. However, many crystalline solids pass through a turbid liquid phase before finally converting in to clear liquid. This turbid phase is called liquid crystal. A liquid crystalline phase exists between two temperatures, a melting temperature and a clearing temperature.
Crystalline Solid ⇌ Liquid-Crystal ⇌ Clear Liquid
Uses of Liquid Crystals:
From 1888 to until about 30 years ago, Liquid crystals were limited to laboratories. But now these have many applications. Since liquid crystals are temperature substances, therefore they are used:
- Mainly in electrical and medical researches.
- To find the point of potential failure in electrical circuits.
- To find the blockage in veins and arteries by skin-thermography.
- For display of electrical devices (LCD=Liquid crystal display) such as digital watches, room thermometer, calculators and computers etc.
- As a screen in oscillographs and TV.
- As a solvent in liquid-solid chromatography.
Q.9) The boiling points and molar masses of hydrides of some elements of second period in the periodic table are given below:
Compound B.P. in K Molar mass in g/mol
CH4 109 16
NH3 240 17
H2O 373 18
Suggest reasons for the difference in their boiling points.
Methane (CH4) has the lowest boiling point (109 K) among three. The reason is that methane molecules are non-polar in nature and weak London forces are present among the molecules of methane. Weak forces of attraction between the molecules make it low boiling.
Ammonia (NH3) and water (H2O) both have hydrogen bonding among their molecules. But hydrogen bonding in H2O is stronger than that in NH3 because oxygen is more electronegative than nitrogen. Further, one molecule of NH3 can form one hydrogen bond because it has only one lone pair on nitrogen atom while H2O molecule can form two hydrogen bonds with neighboring molecules due to the presence of two lone pairs on oxygen atom. That is why H2O has higher boiling point (373K) than that of NH3 (240 K).
Q.10) Write short notes on the following.
b) Surface tension
“The resistance of a liquid to its flow is called viscosity”. In other words, the viscosity of a liquid is a measure of its internal resistance to flow.
Liquids have the ability to flow because molecules of the liquid can slide over each other. The velocity of flow of liquid nearer to the sides of the tube is less than the velocity of flow at the centre of the tube.
The resistance of flow of a liquid is due to internal friction among the layers of molecules. Liquids which flow very slowly like honey or glycerin, have high viscosities as compared to ether and water which have low viscosities.
Factors Affecting Viscosity of Liquid:
Viscosities of liquids differ from each other mainly due to the following factors:
- Molecular Size
- Molecular Shape
- Intermolecular Forces
b) Surface Tension:
Surface tension is defined as the tension of the surface film of a liquid caused by the attraction of the particles in the surface layer by the bulk of the liquid, which tends to minimize the surface area.
Liquid molecules have the property of cohesion. Cohesion is the tendency of liquid to cling together. Cohesive forces between water molecules are due to hydrogen bonding. The molecules at the surface of water are attracted by many molecules below it but not from above. The molecules at the surface, therefore feel a net attraction inward. This attraction creates surface tension.