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Module G - The Liquid State

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Module h: the liquid state

Module G: the liquid stateCHM 1101Oct. 23, 2014

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Liquids: An IntroductionIn a liquid, the atoms or molecules are usually separated by somewhat greater distances than in a solid. Movement of atoms or molecules gives a liquid its most distinctive property: the ability to flow.Covers the bottom and assumes the shape of its container.

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MixturesMixtures can be classified as either heterogeneous or homogeneousHeterogeneous mixture - mixing of components is visually non-uniform and has distinct regions of different composition e.g. sugar & salt, oil & water. Homogeneous mixture mixing is uniform, (at least to the naked eye) and has a constant composition throughout e.g. seawater (sodium chloride with water) and brass (copper with zinc). Homogeneous mixtures can be classified according to the size of their constituent particles as either solutions or colloids

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SolutionsA solution is a homogenous mixture of substances in which no settling occurs. Contain particles with diameters in the range 0.12 nm.Colloids, such as milk and fog, contain larger particles, with diameters in the range 2500 nm.

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Suspensions have larger particles than colloids but particles separate on standing. 4

SolutionsFor solutions in which a gas or solid is dissolved in a liquid, the dissolved substance is called the solute and the liquid is called the solvent.The ability of a solid to go into solution depends on the strength of attraction among particles.

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DissolutionSolids in liquids: the smaller the magnitude of the solute-solute interactions, the more readily dissolution occurs. Solubility is the mass of a substance that can dissolve in 100 g of water at a given temperature.

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DissolutionLiquids in liquids: When a liquid dissolves in another, it is said to be miscible. Polar liquids dissolve in polar solvents, e.g. ethanol, C2H5OH, will dissolve in water, H2O. Non-polar liquids are soluble in other nonpolar solvents, e.g. toulene and hexane are miscible.

Gases in liquids: some nonpolar gases (CO2 and O2) will dissolve in polar solvents such as water. Gases such as HF, HCl and CO2 will dissolve in water due to hydrogen bonding, ionization and other reactions.

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Water

Solutions and SolubilitySolubility of solids in liquids as temperature increases?

Solubility of liquid and gases as temperature increases?

The effect of pressure changes on solubility of solids and liquids in liquids?

Solubility of gases in solvents increases as partial pressures of gases increase. E.g. Carbonated water.

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Henrys Law12

Using Henrys LawAt 0C and an O2 pressure of 1.00 atm, the aqueous solubility of O2(g) is 48.9 mL O2 per litre. What is the molarity of O2 in a saturated water solution when the O2 is under its normal partial pressure in air, 0.2095 atm?13

Vapour pressureThe pressure exerted by a vapour in dynamic equilibrium with its liquid is called the vapour pressure.

Liquids with high vapour pressures at room temperature are said to be volatile, and those with very low vapour pressures are non-volatile.Liquid boils when its vapour pressure equals atmospheric pressure.Volatility is determined by intermolecular forces.

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(a) A liquid is allowed to evaporate into a closed container. Initially, only vaporization occurs.(b) Condensation begins. The rate at which molecules evaporate is greater than the rate at which they condense, and the number of molecules in the vapour state continues to increase.(c) The rate of condensation is equal to the rate of vaporization. The number of vapour molecules remains constant over time, as does the pressure exerted by this vapour.14

Vapour Pressure

(a) A mercury barometer. (b) The pressure exerted by the vapour in equilibrium with a liquid injected to the top of the mercury column depresses the mercury level. (c) Compared with (b), the vapour pressure is independent of the volume of liquid injected. (d) Compared with (c), the vapour pressure is independent of the volume of vapour present. (e) Vapour pressure increases with an increase in temperature15

Vapour pressureThe weaker the intermolecular forces, the more volatile the liquid (the higher its vapour pressure).

What can we say about the intermolecular forces?LiquidVapour Pressure (mm Hg)VolatilityDiethyl ether534VolatileAcetone231VolatileWater23.8Moderately volatileMercury0.0018Non-volatile

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Vapour Pressure

17Liquids with strong inter-molecular forces would have low vapour pressures. Vapour Pressure increases with T up to a maximum of B.P. of liquid.

Raoults LawIn the 1880s, the French chemist F. M. Raoult found that a dissolved solute (impurity) lowers the vapour pressure of the solvent.Raoults Law states: the partial pressure exerted by a solvents vapour above an ideal solution, PA, is the product of the mole fraction of solvent in the solution, XA and the vapour pressure of the pure solvent at the given temperature, PA

PA = XA PA

The vapour pressure of a volatile liquid is directly proportional the mole fraction of that liquid in an ideal mixture. Applies only to ideal solutions and to all volatile components of the solutions18

Ideal Mixture of Volatile LiquidsIdeal mixtures obey Raoults Law.Intermolecular forces before and after mixing are approximately the same.

The tendency of the molecules to escape is unchanged in an ideal mixture. 19

Ideal Mixture of Volatile Liquids20

Predicting Vapour Pressures of Ideal Mixtures of Volatile LiquidsExample 1The vapour pressures of pure benzene and pure toluene at 25C are 95.1 and 28.4 mmHg, respectively. A solution is prepared in which the mole fractions of benzene and toluene are both 0.500. What are the partial pressures of the benzene and toluene above this solution? What is the total vapour pressure?Pbenz = 47.6 mmHgPtol = 14.2 mmHgPtotal = 61.8 mmHg21

Predicting Vapour Pressures of Ideal Mixtures of Volatile Liquids22

Vapour Composition23Vapour in equilibrium with two volatile liquids will contain more molecules of the more volatile liquid. Example 3: What is the composition of the vapour in equilibrium with the benzene/toluene solution of Example 1?Need to find the mole fraction of each component in the vapour phase.

There is more benzene in the vapour phase than in the liquid phase.

Vapour Composition

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