IGCSE CHEMISTRY 0620 LESSON 4 EXPLANATION

Lesson 4

PART 1

1.1 The particulate nature of matter


• Show an understanding of the random motion of particles in a suspension (sometimes
known as Brownian motion) as evidence for the kinetic particle (atoms, molecules or ions)
model of matter

Brownian Motion and Diffusion

Brownian Motion

• Brownian motion is defined as the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller, often invisible particles.
• The observation of Brownian motion proves the correctness of the kinetic particle theory.
  

• Describe and explain diffusion

Diffusion

• This is the process by which different gases or different liquids mix and is due to the random motion of their particles.
• Diffusing particles move from an area of high concentration to an area of low concentration.
• Eventually the concentration of particles is even as they spread out to occupy all of the available space.
• Diffusion happens on its own and no energy input is required although it occurs faster at higher temperatures.
  

• Describe and explain Brownian motion in terms of random molecular bombardment 

• State evidence for Brownian motion

• An example of Brownian motion is the observed jerky and erratic motion of smoke particles as they are hit by the unseen molecules in the air which can be seen under a microscope.
• In 1905, physicist Albert Einstein explained that pollen grains in water were being moved by individual water molecules.
• In all cases, larger and visible particles are caused to move by the random bombardment of smaller, invisible particles.

• Describe and explain dependence of rate of diffusion on molecular mass

• Diffusion occurs much faster in gases than in liquids as gaseous particles move much quicker than liquid particles.
• At the same temperature, different gases do not diffuse at the same rate.
• This is due to the difference in their relative molecular masses.
• Lighter gas particles can travel faster and hence further, therefore: the lower its relative mass, the faster a gas will diffuse.
   
  
  
  NH₃ molecules have less mass than the HCl molecule, so diffuse faster, hence the product (a white cloud of NH₄Cl) forms closer to the end where the HCl is
  
  
  
  
  
  
  Lesson 4
  PART 2
  
  

2 Experimental techniques
• Name appropriate apparatus for the measurement of time, temperature, mass and volume, including burettes, pipettes and measuring cylinders

0. 
   
   Time
   

   • Time can be measured using a stopwatch or stopclock which are usually accurate to one or two decimal places.
   • The units of time normally used are seconds or minutes although other units may be used for extremely slow reactions (e.g. rusting).
   • 1 minute = 60 seconds.

   Temperature
   

   • Temperature is measured using a thermometer which can normally give readings to the nearest degree. Digital thermometers are available which are more accurate.
   • The units of temperature are degrees Celsius (ºC).

   Mass
   

   • Mass is measured using a digital balance which normally gives readings to two decimal places. These must be tared (set to zero) before use.
   • The standard unit of mass is kilograms (kg) but in chemistry grams (g) are used most often.
   • 1 kilogram = 1000 grams.

   Volume-liquids
   

   • The volume of a liquid can be determined using several types of apparatus, depending on the level of accuracy needed.
   • For approximate volumes where accuracy isn´t an important factor, measuring cylinders are used. These are graduated (have a scale so can be used to measure) and are available in 25 cm³, 50 cm³, 100 cm³ and 250 cm³.
   • Pipettes are the most accurate way of measuring a fixed volume of liquid, usually 10 cm³ or 25 cm³.
   • Burettes are the most accurate way of measuring a variable volume of liquid between 0 cm³ and 50  cm³ (e.g. in a titration).

 

Volume-gases

• The volume of a gas sometimes needs to be measured and is done by collecting it in a graduated measuring apparatus.
• A gas syringe is usually the apparatus used.
• A graduated cylinder inverted in water may also be used, provided the gas isn’t water soluble.
• If the gas happens to be heavier than air and is coloured, the cylinder can be used upright.

2.2.2 Methods of purification

• Describe and explain methods of purification by the use of a suitable solvent, filtration, crystallisation and distillation (including use of a fractionating column). (Refer to the fractional distillation of petroleum in section 14.2 and products of fermentation in section 14.6.)

• Suggest suitable purification techniques, given information about the substances involved

The choice of the method of separation depends on the nature of the substances being separated. All methods rely on there being a difference of some sort, usually in a physical property such as b.p., between the substances being separated.
Mixtures of solids

• Differences in density, magnetic properties, sublimation and solubility can be used.
• For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities.

Mixtures of liquids

• You can separate immiscible liquids with a separating funnel or by decanting (pouring carefully).
• Examples include when an organic product is formed in aqueous conditions.

Filtration

• Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g. sand from a mixture of sand and water). Centrifugation can also be used for this mixture.
• Filter paper is placed in a filter funnel above another beaker.
• Mixture of insoluble solid and liquid is poured into the filter funnel.
• Filter paper will only allow small liquid particles to pass through as the filtrate.
• Solid particles are too large to pass through the filter paper so will stay behind as a residue.

 

 

Crystallisation

• Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (II) sulphate in water).
• The solution is heated, allowing the solvent to evaporate to leave a saturated solution behind.
• Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form on the glass rod.
• The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow.
• Crystals are collected by filtering the solution.
• They are then washed with cold, distilled water to remove impurities and allowed to dry.

 

Simple Distillation

• Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water) or a pure liquid from a mixture of liquids.
• The solution is heated and pure water evaporates producing a vapour which rises through the neck of the round-bottomed flask.
• The vapour passes through the condenser, where it cools and condenses, turning into pure liquid H₂O that is collected in a beaker.
• After all the water is evaporated from the solution, only the solid solute will be left behind.

 

Fractional distillation

• Used to separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two).
• The solution is heated to the temperature of the substance with the lowest boiling point.
• This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker.
• All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture.
• For water and ethanol: ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker.
• When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated.

 

11.2 Air

• Describe the separation of oxygen and nitrogen from liquid air by fractional distillation

• The air is first filtered to remove dust, and then cooled in stages until it reaches –200°C.
• At this temperature the air is in the liquid state.
• Water vapour and carbon dioxide freeze at higher temperatures and are removed using absorbent filters.
• The Noble gases are still in the gaseous state at -200ºC, leaving a mixture of liquid nitrogen and oxygen.
• The liquefied mixture is passed into the bottom of a fractionating column.
• Note that the column is warmer at the bottom than it is at the top.
• Oxygen liquefies at -183°C and nitrogen liquefies at -196°C.
• Nitrogen has a lower boiling point than oxygen so it vaporises first and is collected as it rises in the gaseous state to the top of the column.
• The liquid O₂ is then removed from the bottom of the column.

• • The air is first filtered to remove dust, and then cooled in stages until it reaches –200°C.
  • At this temperature the air is in the liquid state.
  • Water vapour and carbon dioxide freeze at higher temperatures and are removed using absorbent filters.
  • The Noble gases are still in the gaseous state at -200ºC, leaving a mixture of liquid nitrogen and oxygen.
  • The liquefied mixture is passed into the bottom of a fractionating column.
  • Note that the column is warmer at the bottom than it is at the top.
  • Oxygen liquefies at -183°C and nitrogen liquefies at -196°C.
  • Nitrogen has a lower boiling point than oxygen so it vaporises first and is collected as it rises in the gaseous state to the top of the column.
  • The liquid O₂ is then removed from the bottom of the column.
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• State the adverse effect of common air pollutants on buildings and on health and discuss why these pollutants are of global concern
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  Carbon monoxide
  

  • Sources: incomplete combustion of fossil fuels e.g: incomplete combustion of gasoline:
    • C₈H₁₈ + 9O₂ → 5CO + 2CO₂ + 9H₂O

  • Adverse effects: poisonous, combining with hemoglobin in blood and prevents it from carrying oxygen.

  Sulfur dioxide
  

  • Sources: combustion of fuels, natural gas and sulfide ores e.g: zinc blende (ZnS) in the extraction of zinc:
    • 2ZnS + 3O₂ → 2ZnO + 2SO₂

  • Adverse effects: acid rain which causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms. Pollutes crops and water supplies, irritates lungs, throats and eyes.

  Oxides of nitrogen
  

  • Sources: reaction of nitrogen with oxygen in car engines and high temperature furnaces and as a product of bacterial action in soil.
  • Adverse effects: acid rain with similar effects as SO₂ as well as producing photochemical smog and breathing difficulties, in particular for people suffering from asthma.

  Compounds of lead
  

  • Sources: old water pipes, old paints, petrol in some kinds of racing cars and from very old engines.
  • Adverse effects: causes significant damage to the central nervous system, young infants are particularly susceptible to lead poisoning.
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• • Complete and incomplete combustion of hydrocarbons produce different products.
  • Complete combustion occurs in excess oxygen and produces CO₂ and H₂O.
  • Incomplete combustion occurs oxygen deficient conditions and produces CO, H₂O and sometimes carbon.