chem notes o level

Chapter 1: Experimental Chemistry

How are physical quantities measured? Time -SI unit: s (seconds) -60 seconds = 1 minute -Measured with stopwatch Temperature -SI unit: K (kelvin) -Temperature in kelvin = Temperature in celsius + 273 -Measured with thermometer Length -SI unit: m (metre) -1 kilometre = 1000 metres -Measured with measuring tape, ruler Mass -SI unit: kg (kilogram) -1 kilogram = 1000 grams -Measured with beam, electronic balance Volume -SI unit: m^3 (cubic metre) -1 cubic metre = 100 millilitres -Measured with pipette (accurate fixed volume up to around 25 cm^3), volumetric flask (big volume up to around 250 cm^3), measuring cylinder (1 d.p. around 20-50 cm^3), burette (2 d.p. 50 cm^3) -Volume of gases are measured with a gas syringe Solubility of gases -Gases can dissolve in water -Insoluble gases: H2, N2, CH4 -Slightly soluble gases: O2 and CO2 -Very soluble gases: SO2, Cl2, NH3 How are gases collected? Water displacement -Gas is bubbled through water and collected in a gas jar -The gas can only be insoluble or slightly soluble in water -Density does not affect this method, but note that density of water is 1 g/cm^3 -e.g. hydrogen, oxygen, carbon dioxide

Downward delivery -Gas is collected in a gas jar and sinks down if denser than air -Solubility in water does not affect this method -Must be denser than air -The Mr of air is 29 -e.g. chlorine, hydrogen chloride, sulfur dioxide Upward delivery -Gas is collected in a gas jar and floats up if less dense than air -Solubility in water does not affect this method -Must be less dense than air -The Mr of air is 29 -e.g. ammonia

Calculating the density of gases -Relative atomic mass (Ar): number of protons and neutrons in an atom -The relative atomic mass of air is around 29 -Relative molecular mass (Mr): number of protons and neutrons of all atoms in a molecule -Density = mass / volume Methods for drying gases Concentrated sulfuric acid -Most gases can be dried -Gases which react with sulfuric acid cannot be used -The moist gas must be bubbled through the sulfuric acid Quicklime (calcium oxide) -Most basic and neutral gases can be used, such as ammonia -Gases which react to calcium oxide cannot be used -The gas must be passed through the calcium oxide -Calcium oxide readily absorbs moisture from the air, so it must be freshly heated before use Fused calcium chloride (fused means dried) -Hydrogen, nitrogen, carbon dioxide is regularly used -Gases which react with calcium chloride cannot be used -The gas must be passed through the calcium chloride -Calcium chloride readily absorbs moisture from the air, so it must be freshly heated before use How are substances in mixtures separated? What is a mixture? -A mixture consists of two or more substances that are not chemically combined -The physical properties of a substance determines the separation techniques used

Seperating solid-solid mixtures Using magnetic attraction -Magnetic substances can be separated from non magnetic substances -Magnetic substances: iron, cobalt, nickel, some alloys -Waste in a landfill is also sorted this way Sieving -A sieve is used to separate solids with different particle sizes -Large particles can’t pass through the smaller pores while small particles can Using suitable solvents -Solids of different solubility in the solvent can be separated -Solvent: liquid that dissolves solids -Solute: solid that dissolves in the solvent -A solute dissolves in a solvent Sublimation -Sublimation is used to separate solids where one substance changes from the solid to the gaseous state directly -Substances that sublime: naphtalene, ammonium chloride, iodine Separating solid-liquid mixtures FIltration -Seperates an insoluble solid from a liquid -An insoluble solid is one that is unable to dissolve into the liquid it is placed in -Filtrate: liquid that passes through the filter -Residue: solid that remains on the filter

Evaporation to dryness -Seperates a dissolved solid from its solvent -The mixture is heated until all the solvent has vaporised Crystallisation -Seperates a pure solid from its saturated solution -A saturated solution is one in which no more solute can be dissolved -A solution is heated until it is saturated

-It is then cooled gradually until crystals start to form -The crystals are then filtered from the solvent, washed with distilled water and then dried on filter paper Simple distillation -Seperates a pure solvent (liquid) from its solution -Simple distillation can only be used when there is a significant difference in boiling point Miscible and immiscible liquids Miscible liquids -Miscible liquids are liquids which form a uniform (homogeneous) solution when mixed together Immiscible liquids -Immiscible liquids are liquids which form a non-uniform (heterogenous) mixture when mixed together Seperating liquid-liquid mixtures Seperating funnel -A separating funnel is used to separate immiscible liquids -When a heterogenous mixture is left to settle into different layers in a separating funnel, each layer can be removed by opening the tap at the bottom of the flask and collecting it in separate flasks or beakers -The layers are separated by density, with the densest liquid at the bottom and the lest dense liquid at the top

Chromatography -Chromatography can be used to separate miscible liquids -A drop of a homogeneous solution is placed at the edge of a piece of paper, and indicated by a line drawn with a pencil as the start -It is then dipped into a solvent, keeping the solvent just above the start level -As the solvent is absorbed by the paper, the solution is separated into pure components -They are separated by solubility in the solvent, with the most soluble at the top and the lest soluble at the bottom -The furthest point the solvent reache is called the solvent front -The Rf value of a substance is the ratio of the distance moved by a substance and the distance moved by the solvent until it reaches the solvent front

Fractional distillation -When two liquids are miscible, the separating funnel cannot be used to separate them, and the difference in boiling point of the two liquids may be too small for simple distillation to work -Thus, a fractionating column is added to the distillation set up to separate the two liquids -The fractionating column contains small glass beads or other small solid objects to provide a larger surface area for vapours to condense onto

How can the purity of substances be determined? -Mixtures melt or boil at a range of temperatures, thus the greater amount of impurities in a substances, the larger the change in boiling or melting points -Impurities will lower the melting point and raise the boiling point -Other than chromatography, melting or boiling point data can be used to determine the purity of substances