Conall's Stuff: Chemistry

Acids, Bases and Salts

Acids : H2SO4, HCL, HNO3, CH3COOH
Bases : substance that reacts with an acid to create a salt. This is called neutralisation

Neutralisation : acid + base --> salt + water
Metal Oxides and hydroxides are usually bases (CuO, NaOH for example)
A base dissolved in water is called an alkali

Examples

2HCl + CuO --> CuCl2 + H2O

HNO3 + NaOH --> NaNO3 + H2O

Acids also react with metal carbonates:

acid + metal carbonate --> salt + water + carbon dioxide

Example

2HCl + Na2CO3 --> 2NaCl + H2O + CO2

Ionic and Covalent Bonds

Covalent bond - bond between atoms caused by sharing of electrons. Example Cl2. Each Cl atom has 17 electrons (2,8,7) so they each want to gain 1 electron for a complete outer shell. If they can't achieve this, they can do the next best thing and each share 1 of the other's electrons.

Ionic bond - bond between atoms due to mutual attraction. In this case, 1 atom wants to lose electrons to gain a complete outer shell, and another wants to gain them, for example Na (2,8,1) and Cl (2,8,7). Na gives its 1 outer electron to Cl, leaving Na with a positive charge, and Cl with a negative charge. These charged particles are known as ions. Opposite charges attract, bonding the Na and Cl ions together. Common ions:

Sulphate SO4 (2-)
Carbonate CO3 (2-)
Na+, K+, Fe 2+, Mg 2+, Cu 2+, Pb 2+
Cl-, I-, Br-

Testing for different ions

Positive Ions

If you have a mystery solution add a few drops of NaOH:

white ppt --> Ca(OH)2 so positive ion was Ca2+
blue ppt --> Cu(OH)2 so positive ion was Cu2+
green ppt --> Fe(OH)2 so positive ion was Fe2+
brown ppt --> Fe(OH)3 so positive ion was Fe3+
white ppt that dissolves on adding excess NaOH --> Al(OH)3 / Al(OH)4 so Al3+
if nothing, but on heating gives off amonia, then positive ion was NH4+

Note, test for amonia gas (NH3) is that it turns damp litmus paper blue (it is alkali).

If you have a mystery salt (not in solution) you can use a flame test:

yellow/orange flame --> Na+
lilac flame --> K+
blue/green flame --> Cu2+
brick red flame --> Ca2+

Halides (Cl-, Br-, I-)

If you have a mystery solution add dilute HN03, followed by AgNO3 solution:

white ppt --> AgCl, so was a Cl-
cream ppt --> AgBr, so was a Br-
yellow ppt --> AgI, so was an I-

Others
Carbonate,CO3 : add dilute acid (any, e.g. HCl) - CO2 (limewater milky) is given off
Sulphate, SO4 : add dilute HCl, then BaCl. If white ppt (BaSO4), then was a SO4 (2-)

Spectroscopy (studying pattern of light given off when heating a sample) can tell us which elements are present.

Electrolysis (splitting with electricity)

2 electrodes ( anode +ve, cathode -ve) in solution or molten substance
DC current appplied so electrons start flowing between the electrodes
+ve ions attracted to the cathode
-ve ions attracted to anode

What happens at the anode and cathode?

If using a solution of NaCl

there will be H+ and Na+ ions at the cathode
there will be OH- and Cl- ions at the anode
at the cathode, the rule is that the least reactive ion will discharge. So with H+ and Na+, H+ is less reactive (most ions are, apart from lithium, Li+) so H2 gas is discharged. (2H+ + 2e- --> H2)
at the anode, the simplest ion will discharge. In this case Cl- is the simplest, so it will discharge as Cl2 gas. (2Cl- --> Cl2 + 2e-)

If using molten NaCl

it is simpler as there are no H+ or OH- ions to worry about
at the cathode Na+ ions will create Na metal (Na+ + e- --> Na)
at the anode Cl- ions will discharge as Cl gas (2Cl- --> Cl2 + 2e-)

Titration (used to find out concentrations)

Usually between an acid and a base - a neutralisation
use a pipette to get exactly 25cm of alkali, and transfer it into a conical flask
add a few drops of indicator - phenolphthalein will be pink in alkali, turning colourless in acid
use a burette to deliver the acid until the liquid in the flask turns colourless.
repeat twice more being very careful as you reach the point of neutralisation.
if you know the molarity (moles/dm) of the alkali, you can use this with the volume measurements and the chemical equation to work out the molarity of the acid - see mole question 2 below for the calculation you would do.

Moles

1 mole is 6.023 x 10^23 (an unimaginably large number)
1 mole of atoms (or molecules) will have a mass (in grams) equal to the relative formula mass of that substance

e.g. MgO has a RFM of 24 + 16 = 40. So 1 mole of MgO will have a mass of 40g

Mole Question 1

How many moles of copper are in 6,000,000 atoms of copper?

1 mole Cu is approximately 6 x 10^23 atoms
we have                    6 x 10^6 atoms
therefore we have (6 x 10^6)
------------------
    (6 x 10^23) moles
which is 10^-17 moles

Mole Question 2

Calculate the molarity of an acetic acid solution if 34.57 mL of this solution are needed to neutralize 25.19 mL of 0.1025 M sodium hydroxide.

CH3COOH (aq) + NaOH (aq) --> Na+(aq) + CH3COOH-(aq) + H2O (l)

(Molarity = concentration in mol/dm)

we know 1 mole of acid will react with 1 mole NaOH
we know 1 dm of NaOH solution contains 0.1025 moles of NaOH
we have 25.19 mls of NaOH
by x-multiplication, we can calculate how many moles of NaOH we actually have:

1000.00 ml NaOH contains 0.1025 moles NaOH
25.19 ml NaOH contains x moles NaOH

we have 25.19 x 0.1025
--------------
1000 moles
we have 0.002581975 noles NaOH in our experiment

as ratio of acid : alkali = 1 : 1, we must have0.002581975 moles of acid, and that is contained in 34.57 ml.
by x-multiplication we can calculate the molarity:

    34.57 ml acid contains 0.002581975 moles acid
    1000.00 ml acid contains           x moles acid

    acid molarity = 1000 x 0.002581975
                    ------------------
                          34.57         mols/dm

                  = 0.0747 mols/dm (to 3 sig. fig.)

Reversible Reactions (example, Haber Process for Ammonia)

Reactants react to form products and products react to create the original reactants again

    A + B <--> C + D

In a closed system (reaction done 'in a box' so nothing can escape), the reaction will never end - as soon as C and D start to form, they react to form A and B again - ultimately an equilibrium will be reached; in other words a balance will be reached where A and B are being created at the same rate as C and D.

Some reversible reactions are useful in industry so we want to influence the reaction so that the products we want are created, rather than the reverse reaction which just takes us back to where we started. A good example is the Haber process, to create ammonia:

    N2 + 3H2 <--> 2NH3 (+ heat)

The forward reaction is exothermic - it creates heat as a by-product. Therefore the reverse reactrion must be endothermic, taking in the heat to split ammonia back into N2 and H2. We want to encourage the forward reaction and inhibit the reverse reaction. We can do this by varying temperature and pressure.

Pressure
Increasing pressure brings molecules closer together making them more likely to collide and react. Increasing pressure will have a greater effect on the side where there are more molecules. In this case it will favour the forward reaction as there are 4 molecules on the left of the equation vs 1 on the right. How much the pressure is increased is dictated by cost - creating huge pressures is expensive so a balance between cost and performance is struck. In this case, the pressure is usually around 200 atm.

Temperature
Increasing temperature will favour the reaction that needs heat - the endothermic reaction In this case, the reverse reaction is endothermic so theoretically we would want to do this reaction at a low temperature to inhibit the reverse reaction. However as with pressure there is a balance to be struck. The forward reaction will occur only very slowly in cold conditions, so some heat is required to get the reaction to happen at a reasonable rate.   In this case the temperature is usually around 450C.

Catalysts
Catalysts are often used to make the reaction happen quicker, but they affect both sides, so cannot influence equilibrium like pressure and temperature can - they just speed things up in both directions. In the Haber process iron is the catalyst.

Homologous Series (a group of similar compounds)

members of a homologous series

have the same general formula
have similar properties which change grandually as they increase in size (e.g. boiling points increase as the size of the compound increases)
The ones you need to know are

meth (1)
eth (2)
prop (3)
but (4)

examples are alkanes and alkenes, alchohols and carboxylic acids

Alkanes

general formula for alkane = CnH2n+2
methane CH4, ethane C2H6, propane, butane..

   H H
   | |
   H-C-C-H   Ethane
   | |
   H H

Alkenes

general formula for alkene = CnH2n
They all have a double bond
ethene C2H4 is example - note there is no methene as with only 1 C, there can be no double-bond

Alchohols

general formula for alchohol = CnH2n+1OH - basically an alkane with an H replaced by OH
examples, methanol (CH3OH), ethanol (C2H5OH)
draw them just like the alkane but with 1 H replaced by OH

Carboxylic acids

general formula CnH2n+1COOH - basically an alkane with an H replaced by COOH
drawing them is easy but remember the COOH has a C=O bond and an OH (hydroxide):

C=O
|
OH

Ethanoic acid : CH3COOH

      H OH
| |
    H-C-C=O
      |
      H

Hot Topics

Bunch 1
Pg 33 Fuel Cells
A fuel cell takes in fuel and oxygen and produces electricity. The hydrogen fuel cell takes in hydrogen and oxygen which react to give electricity and water (reverse electrolysis of water).

Good : 80% efficient, waste products is only water
Bad : expensive to create hydrogen, highly explosive, takes more space to store than petrol.

Pg 36 Cracking Hydrocarbons
turning long-chain hydrocarbons (gloopy liquids) into shorter molecules which are more useful
e.g. Diesel -- CRACK --> Petrol, Paraffin, Ethene.
The CRACK is thermal decomposition (heating) over a catalyst. In industry, the catalysts are Silicon dioxide and Aluminium Oxide. In the lab, use porcelain chips.

Know the lab experiment to crack paraffin.

Pg 37 Using Alkenes to make Polymers
The opposite of cracking is polymerisation - the joining together of monomers (alkenes) to produce long-chain molecules.

Polymerisation is used
(1)to create polythene (poly-ethene)
(2)to create polypropene (plastic containers)
(3)to create PVC (electric cables & pipes)
(4)to make PTFE (teflon, non-stick coating for pans)

Pg 58 Group 1 - Alkali metals
Pg 59 Group 7 - Halogens
Pg 60 Group 0 - Noble Gases
Pg 67 Relative formula mass & 2 formula mass calclutions
Pg 77 Titrations & more titrations
Pg 85 - 93

Bunch 2
Pg 14 Limestone & Thermal Decomposition
Limestone is mainly calcium ccarbonate. (CaCO3)
It (like all carbonates) decomposes to the oxide + CO2

Metal Carbonate --HEAT--> Metal Oxide + CO2
CaCO3 --HEAT--> CaO + CO2

Calcium Oxide reacts with water to give calcium hydroxide.
CaO + H2O --> Ca(OH)2

Remember the neutralisation reaction : "base + acid --> salt + water" and that metal oxides and metal hydroxides are bases.

So, CaO or Ca(OH)2 can be used to neutralise acidic soil. However, using Ca(OH)2 is quicker-reacting.

e.g. CaO + 2HCl --> CaCl2 + H2O
   Ca(OH)2 + 2HCl --> CaCl2 + 2H2O

Pg 19 HCl & Indigestion Tablets
Pg 20 Reactions of Acids
Pg 28 Fractional Distillation of Crude Oil
Pg 32 Bio Fuels
Pg 34 Measuring energy content of fuels
Pg 48 Prepping Insoluble SaltsPg 49 Flame Tests
Pg 50 Test for negative ions & spectrosopy
Pg 69 Calculating masses in reactions
Pg 74 Measuring amounts

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Subjects

Chemistry