Beautiful Chemistry

This is too good to leave out. Plenty of people study creative arts with the sciences. Good science here and excellent video. Worth watching till the end.

 

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Moles – Percentage Purity and Yield

The yield is the amount of product you obtain from a reaction. Suppose we own a factory that makes fertilizers. We will want the highest yield possible, for the lowest cost.

If we are making medical drugs then the yield will still be important, but the purity of the product may be even more important. This is because the impurities may harm the people using the drugs.

 Finding the percentage yield

The formula for percentage yield is:

Aspirin is made from salicylic acid. 1 mole of salicylic acid gives 1 mole of aspirin. The chemical formula for salicylic acid is C7H6O3 and the chemical formula for aspirin is C9H8O4.

In an experiment, 100.0 g of salicylic acid yielded 121.2 g of aspirin. What was the percent yield?

Solution:

1. Calculate the Mr (RMM = relative molecular mass) of the substances.

  • Ar : C = 12, H = 1, O = 16
  • So, Mr : salicylic acid = 138, aspirin = 180.

2. Convert  grams to moles for salicylic acid

  • 138 g of salicylic acid = 1 mole
  • So, 100 g = 100 ÷ 138 mole = 0.725 moles

3. Work out the calculated mass of the aspirin.

  • 1 mole of salicylic acid gives 1 mole of aspirin
  • So, 0.725 moles gives 0.725 moles of aspirin
  • 0.725 moles of aspirin = 0.725 × 180 g = 130.5 g
  • So, the calculated mass of the reaction is 130.5 g

4. Calculate the percent yield.

  • The actual mass obtained is 121.2 g
  • So, the percent yield = 121.2 ÷ 130.5 × 100% = 92.9%

Finding Percentage Purity

When we make something in a chemical reaction, and separate it from the final mixture, it will still have small amounts of other substances mixed with it. It will be impure.

The formula for percentage purity is:

Example:

The aspirin from the above experiment was not pure. 121.2 g of solid was obtained, but analysis showed that only 109.2g of it was aspirin. Calculate the percentage purity of the product.

Solution:

Percentage purity = 109.2 ÷ 121.2 × 100% = 90.0%


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Redox Reactions

Redox (reduction-oxidation) reactions are a family of reactions that are concerned with the transfer of electrons between reactants. Redox reactions are a matched pair – you don’t have an oxidation reaction without a reduction reaction happening at the same time. Oxidation means loss of electrons while reduction means gain of electrons. Each reaction by itself is called a “half-reaction”, simply because we need two half-reactions to form a whole reaction.  We write them out like this.

Mixing  magnesium powder and copper oxide together – a displacement reaction.

The copper ion has gained 2 electrons, and is reduced, the Mg atom has lost 2 electrons and is oxidised

The OILRIG rule: Oxidation is loss, reduction is gain of electrons.

Revision Tips: Periodic Table

This is NOT a substitute for learning, just a few little hints and tips.

Groups go DOWN (similar properties), periods go ACROSS (properties change from metal to non-metal)

You should be able to draw an electron configuration up to a proton number of about 20

Magnesium 2,8,2,  Group 2 period 2,

Calcium 2,8,8,2,  Group 2 period 3

Remember:

Shell 1 2 electrons

Shell 2 8 electrons

Shell 3 8 electrons

Shell 4 18 electrons

Group 1 metals

  • From Li to Fr reactivity increases down the group 1 electron in outermost shell. Form 1+ ions
  • Monatomic
  • Form ionic compounds, white solids that dissolve in water to form colourless solutions
  • Highly reactive with water, oxygen and the halogens, kept under paraffin
  • Atoms get bigger down the group so electron further away from nucleus
  • Good conductors of heat and electricity
  • Often very soft can be cut with a knife
  • Float on water – low density
  • Low mp and bp

(Group II metals (Mg, Ca)  have 2 electrons to give away so in general are less reactive than their corresponding group I metal)

Halogens Group VII (7)

  • Ionic bonding with metals e.g NaCl, 1ions formed
  • Covalent molecular bonding, diatomic e.g Br2 so low mp and bp increasing down group
  • 1 vacancy in outermost shell
  • Reactivity decreases down the group because vacancy further away from the nucleus – more difficult to fill it. Cl displaces Br displaces I
  • React with metals to form metal salts Example?
  • Do I remember the test for a chloride? If not, look it up now.
  • Chlorine gas will displace bromine from sodium bromide (Cl more reactive than Br) – green gas turns to brown gas

Noble Gases Group 0 or VIII (8)

  • Don’t do much at all. No free electrons in outermost shells
  • All monatomic
  • Uses: He balloons (very light), Ar in filament lamps (because very unreactive – prevents contamination of the filament), Ne in signs (glows bright green),Xe in flash photography (stroboscopes).
  • He/Ne used in lasers (glows red)

Transition Metals – identify where they are in the PT

  • frequently colourful compounds (salts) – used in stained glass windows
  • high mp and bp.
  • high density – none float like Na on water
  • hard, tough and strong, malleable and ductile
  • good conductors of heat and electricity
  • crystalline, hence shiny.
  • often alloyed to get the best of both worlds (Ti is alloyed for lightness, strength and corrosion resistance), bronze = Cu alloyed with Sn (tin), harder than Cu alone.
  • mainly vary in their inner electrons so often quite similar chemically.
  • uses for iron (steelmaking), copper (electrical wiring and water pipes) and zinc (anti-corrosion coatings)
  • catalysts – metals like palladium, gold and platinum used often to increase the surface area for reactions to occur. Iron catalyses the Haber Process for ammonia manufacture.
  • A few to remember – potassium permanganate (Mn in oxidation state VII) KMnO4 is purple, potassium dichromate (the Cr is in oxidation state VI)  K2Cr2O7  is orange. Both are strong oxidising agents and on reduction their colours change. permanganate becomes colourless, dichromate turns from orange to green.