Chemistry - Chemical Changes

combined matomic masses of useful products / combined atomic masses of products
(X 100)

This gives a measure of how much waste is produced in a reaction, the higher the atom economy the less waste is produced.

• find the atomic masses for each element in the periodic table
• add them all up
• use the equation to find the atom economy

The Thermit Reaction

• Aluminium + Iron (III) Oxide -> Aluminium Oxide + Iron
• 2Al + Fe2O3 -> Al2O3 + 2Fe
• Oxidised: Aluminum
• Reduced: Iron

*If we list every atom and ion in the equation, we can see what’s really changed

• We can see that the oxide ions have not really changed using an ionic equation
• Ionic equations are simple forms of balanced equations, but they only show the atoms and ions which have changed
• OIL RIG (oxidation is loss, of electrons, and reduction is gain)

Combustion/Oxidation, Displacement, Decompostion, Neutralisation

• Combustion = adding oxygen (this is the same as oxidation)
• Displacement = when the metals switch places
• Decompostion = when a compound separates into different things
• Neutralisation = acid + base -> salt + water

In an experiment that is repeated several times, the mean value can be found (after omitting anomalous results). Then the uncertainty can be found from the range of results used to find the mean.

E.g. 3.8, 4.0, 4.1, 3.9, 4.2
The mean = 4
The furthest two integers away from this are: 4.2 and 3.8
They are both 0.2 away from the mean so the uncertainty is +0.2

If the lowest is 3 away from the mean, and the highest is 7, then the uncertainty would be the largest (so 7).

Salts are produced when acids are neutralised by bases. If the base is insoluble, then the acid is neutralised when an excess of the base remains. This excess can be removed by filtration. If the base is soluble (an alkali) then an indicator must be used to determine when the acid (or alkali) has been neutralised. The method used to ensure this is done accurately is known titration.

Acids contain H+ ions
Alkalis contain OH- ions
H+ + OH- -> H2O

2NaOH + H2SO4 -> Na2SO4 + H20

• Fill a burette with 25cm3 of sulfuric aicd (using a beaker and a funnel)
• Rinse out a pipette using sodium hydroxide
• Using a pipette filler, fill a pipette with 25cm3 of sodium hydroxide
• Transfer all of the sodium hydroxide to a conical flask underneath the burette
• Add 4 drops methyl orange indicator to the alkali
• Carefully add the acid into the conical flask using the burette
• Stop when the methyl orange turns orange (Yellow->Orange->Red)
• Record the results in a table, the volume of acid used is called the titre
• Repeat twice more (the first neutralisation is a rough one, in order to determine the rough amount of alkali needed) - refilling the burette is not necessary
• During your repeats, add an amount of alkali close to the amount which neutralised it during the rough titration, and add the final bits drop by drop for accuracy