CHEM1010 Week 6 Colligative Properties notes PDF

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Colligative Properties • Colligative properties • Types of bonds in organic molecules • Typical molecules found in the environment • Polymers • Humans and the environment

COLLIGATIVE PROPERTIES Some more definitions related to solutions: 

Electrolyte – a substance that dissolves in water to yield a solution that conducts electricity. They are essential for the transmission of electrical impulses e.g. nerve impulses, muscle contractions, batteries.

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Strong electrolytes dissociate completely into cations and anions – NaCl, KCl

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Weak electrolytes exist only partly as ions – HC2H3O2 (acetic acid)

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Nonelectrolyte – Substances that do not produce ions when dissolved in water – glucose (C6H12O6).

We only consider strong electrolytes and non-electrolytes when talking about colligative properties • Colligative properties – are properties of solutions that depend specifically on the number of dissolved particles in a solution e.g. freezing point depression, boiling point elevation and osmotic pressure.

VAPOUR PRESSURE DEPRESSION: Adding a non-volatile solute to a pure solvent lowers the vapour pressure of a solvent. Result: The solvent in a solution evaporates more slowly than the pure solvent would.    

To vaporise, solvent molecules must be at the surface of the liquid. Solute molecules decrease the surface area available for solvent molecules. The rate of solvent vaporisation decreases. The rate of solvent condensation remains the same.

FREEZING POINT DEPRESSION & BOILING POINT ELEVATION:

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Boiling point elevation (∆Tb) and freezing point depression are both a direct result of vapour pressure lowering. (∆Tf) – phenomenon used to keep e.g. water (l) Salt (NaCl &/or CaCl2) on roads keep roads wet, not icy. Antifreeze (ethylene glycol) prevents car radiators from freezing at low temperatures

∆Tb) – phenomenon used to boil liquids at a higher temperature: • Salt (NaCl) in water when cooking: the water boils at a higher temperature which effects the way the pasta is cooked.

Boiling point elevation (∆Tb) and freezing point depression are both a direct result of vapour pressure lowering.  

Depends on the concentration of the solute in molality (m, 𝑚 𝑚 𝑚𝑚𝑚 𝑚 𝑚𝑚𝑚𝑚−1). kb and kf are called the boiling point elevation constant and freezing point depression constant, respectively. ∆T = mk ∆T = mk bbff

Solvent

Boiling Point (°C at 1 atm)

Kb Freezing Point (Cm–1) (°C at 1 atm)

Kf (Cm– 1)

water

100.0

0.512

0.0

1.86

hydrogen acetate

118.1

3.07

16.6

3.9

benzene

80.1

2.53

5.5

5.12

chloroform

61.26

3.63

–63.5

4.68

nitrobenzene

210.9

5.24

5.67

8.1

Example: 1 m sucrose and 1 m ethylene glycol solutions boil at the same temperature

FREEZING POINT CALCS: • Ethylene glycol [CH2(OH)CH2(OH)] is used as an antifreeze. It’s water soluble and relatively non-volatile (BP 197°C). Calculate the freezing point of a solution containing 11.04 moles of ethylene glycol in 2075 g of water.

BOILING POINT CALCs: Ethylene glycol [CH2(OH)CH2(OH)] is used as a viscosity modifier in pharma and personal care products. Calculate the boiling point of a solution containing 7.75 moles of glycerol in 1895 g of water.

DIFFUSION: 

A concentration gradient occurs when the concentration of particles is higher in one area than another. In passive transport, particles will diffuse down a concentration gradient, from areas of higher concentration to areas of lower concentration, until they are evenly spaced.

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Molecules are in continuous motion from thermal energy randomly move about. Over time, this "random walk" leads to uniform distribution of different particles.

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Rate is dependent on temperature and pressure Diffusion can refer to the movement of solutes and solvents.

OSMOTIC PRESSURE: 

Osmosis – is the net flow of solvent through a semipermeable membrane separating an aqueous solution from pure water

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Semipermeable membrane – water molecules can pass through, but solute molecules cannot

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Osmotic pressure – the pressure that must be applied to the solution side in order to prevent the flow of water through the semipermeable membrane.

Acetic acid + egg 1. Dissolution of CaCO3, leaving a translucent, semi permeable membrane 2. Swelling of egg (150%) due to osmosis–net flow of water into egg through the membrane. • Also protons (H+) 3. Denaturing of proteins to make it ‘bouncy’–covalent disulphide bonds between cysteine residues in protein chains.   

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Osmosis – is an important phenomenon in biological systems. Cells are surrounded by a semipermeable membrane The cell membrane modulates the movement of water (and solutes)

THE CURIOUS CAPACITY OF CARBON: Carbon always forms four bonds in stable organic molecules – no lone pair around it. Carbon can exist in several shapes: tetrahedral, trigonal planar, linear

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§ Carbon forms single, double and triple bonds to itself and other elements. § More than one arrangement of atoms maybe possible for a given molecular formula.

ORGANIC COMPOUNDS: 

An organic compound is any of a large class of chemical compounds in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen.

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§ Organic molecules also possess other structural features" Heteroatoms - atoms other than carbon or hydrogen. • Double and triple bonds.

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§ These structural features distinguish one organic molecule from another. They determine the geometry, physical properties and reactivity of the molecule

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§ The bound atoms comprise what is called a functional group.

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§ Functional groups are groups of one or more atoms attached to a molecule which gives distinctive chemical properties. Hydrocarbons:

SOIL: COMPOSITION Soil is a finely divided, heterogeneous, porous material made up of mineral & organic matter where the pore spaces are filled with air &/or water. Healthy soil has ~composition:

ORGANIC MATTER:

WHAT IS A POLYMER?

POLYMERS (MACROMOLECULES) Molecules of high molecular mass made by bonding repeating units called monomers.

Example:

STRUCTURE: Polymers can exist with various skeletal structures – linear, branched or network polymers. • Implications on structural properties?

PLASTICS:

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REVOLUTIONARY POLYMERS? 1955 – LIFE MAGAZINE "The objects flying through the air in this picture would take [the housewife] 40 hours to clean.“ This industry has had implications on: Employment (more jobs, more money, more disposable things to buy) Materials – polymers are built to function e.g. kevlar Attitudes to waste (do you know how to fix things?) Environment (well...) Issues with plastics: The Case of Bisphenol A § BPA is the monomer for polycarbonate. § Plastics containing Bisphenol A (BPA) leach BPA. § BPA mimics estrogen and interacts with proteins meant to interact with estrogen.

OCEANS:

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SOLID WASTE: PLASTICS Plastic litter in the environment can sorb and spread contaminants: Endogenous chemicals – Pthalates: Softeners Bisphenol A (BPA): Hardeners PAHS from manufacturing process

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Sorbed contaminants – Polyaromatic Hydrocarbons (PAHs): Oil spills Polychlorinated Bisphenols (PCBs): Coolant fluids Dichlorodiphenyltrichloroethane (DDT): Insecticides Polybrominated dithenyl ethers (PBDEs): Flame retardants Dioxins: Industrial processes Heavy metals: Ship hulls, smelters

Almost all consumer plastics can be recycled – differ in difficulty and cost.

Recycling PET – the world’s most ubiquitous plastic waste 

Bottles are first separated washed, crushed, flaked, dried à REUSED

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The cleaned material can then be melted, extruded into pellets and treated in various ways to make it safe for reuse: o Industry: moulded into food containers of various types; resins for 3D printing. o Clothing industry: the flakes can be spun into thread or yarn for the manufacture of clothing that requires strength and durability e.g. coats or accessories that do not come in direct contact with the skin.

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Can also convert the recycled PET into polyester fibres. This is done through total glycolysis to convert it to bis(2-hydroxyethyl)terephthalate 𝑚𝑚6𝑚 𝑚 4(𝑚𝑚𝑚𝑚2𝑚𝑚𝑚𝑚2𝑚𝑚𝑚𝑚2𝑚𝑚𝑚𝑚)2 by the reaction:

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Plastic recycling is more challenging because of low density and low value (cf metals and glass), must be separated (will phase separate) and limited lifetime (the same piece of plastic can only be recycled about 2– 3 times before its quality decreases to the point where it can no longer be used).

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HDPE – is HDPE thermoplastic; it becomes liquid MP ~130 °C Recycling can occur without significant degradation. Liquid HDPE can easily be extruded or injection moulded into new products e.g. downcycled into plastic lumber, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery (e.g. rulers) and other durable plastic products and is usually in demand Other chemical pathways Improvements in instrumentation and automation improve the process. New plant-derived plastic (polylactic acid, PLA) allow for “indefinite recycling” &/or biodegradability. However, there may not be facilities for this in every city and most end up in landfill. Thermal depolymerisation -

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Reversal of the polymerisation reaction at high heat, then fractionation. Plastic pyrolysis can convert most plastics into diesel, carbon, naphtha blend stocks, and wax: http://www.plastic2oil.com/; https://resourcerecycling.com/plastics/2019/04/17/commercial-plastics- to-fuel-plantreceives-financing/ Tar made from shredded plastic has been incorporated into roads in India – improves durability and thermal resistance due to increased flexibility. http://www.ijprt.org.tw/files/sample/V3N1%285%29.pdf SUMMARY: ENVIROCHEM Our aim: to get you to understand some fundamental chemistry (and physics) in order to help you save/manage the environment. Chemistry has revolutionised the way we live – from plastic (POLYMERISATION) to fertilisers (REDOX CHEMISTRY) to buffers (EQUILIBRIUM) to cars (COMBUSTION ENGINES) – how do we reconcile progress with looking after the environment?...