Chemistry Learning Objectives ( Monday, 26th May 2013)
key aspects:

activation energy
In Chemistry, we speak of an exothermic and
endothermic reaction. The exothermic
reaction shows a decrease of energy during it
and the product get less energy than the
reactants had. The endothermic reaction
shows a increase of energy during it and the
product get more energy than the reactants had. The difference is
always the reaction enthalpy of it’s, what is anyway logically, reaction.
The rate of reaction is defined as concentration change per time.
Before we can get a chemical reaction, the particles of reactants have to crash together and collide. The
collision theory says, that the more collisions between particles in a given time are, the faster the
reaction is. But not all collisions between restant particles will result in an action. As you can see in the
Maxwell-Boltzmann-distribution, at any given temperatures, the energy is spread between the particles.
Only a certain number of molecules will have enough energy to react and start a reaction. The higher
the energy of colliding particles, the greater the chances of effective collisions, i.e. ones that result in a
reaction. Only more or less 20% of the molecules can react succesfully.

How and why can the rate of reaction be affected?
 by heating: 10°C can double the rate of many reactions, because when you heat particles, they will
get more energy and so they will be more collides than before. ( e.g. fridge slows down reactions)
 by pressure: you can increase the concentration of gas and so more reactions ( see concentrate)
 by concentrate: more ions in the same volume increase the rate of reaction like you can see in the
collision theory. ( e.g. increase the concentration of gas)
 by surface: small pieces of solids, especially powders, react faster than larger pieces, because only
iron atoms on the surface can react, if you cute e.g. a nail up, more iron atoms are exposed to react.
 by catalyse: catalyst is a substance which speeds up a chemical reaction and at end of reaction, the
catalyst remains chemically unchanged because they search an alternative route and make it easier
for particles to react. There are two types of catalysis …
o homogeneous catalysis: reactants and catalysts are in the same state  alternative route
o heterogeneous catalysis: reactants and catalysts are in another state  weakening bonds
 http://www.youtube.com/watch?v=rmtFp-SV0tY
During combustion or fuel in a car engine toxic side products are built: nitrogen oxide (NOx), carbon
monoxide (CO) and unburnt hydrocarbons like C8H8. On the surface of the Particle in the catalystic
converter by a temperature of 900-1300°C reactions take place like …
2 NO  N2 O2
2 CO + O2  2 CO2
CH4 + 2 CO2  CO2 + 2 H2O

drawing (skeletal formula) and naming alkanes and alkenes
Drawing (skeletal formula)
Hydrocarbons (Kohlenwasserstoffe) are separated in Alkanes, Alkenes, Alkines and Aromaties.
 Alkanes have only one single bond (crude oil contains more or less 90% of Alkanes!)
 Alkenes have one or more double bonds.
A formula has different isomers, so different structures. (e.g. C6 H14  C1 C2 C3 …)
Rules of naming Alkanes and Alkenes
1.
2.
3.
4.
Find the largest continuous chain and name it.
Mark the branches and name them (ending –yl)
Indicate the position of the branches using the lowest numbers possible.
More than one of the same branche: 2=di; 3=tri; 4=tetra

properties and reactions of alkanes
 The alkanes are saturated (gesättigte) hydrocarbons.
 Miscibility: At the end of an experiment we realize that heptane (alkane) only mix with vegetable oil
and not with diesel. This because of the vdW-forces that both, vegetable oil and heptane, have. But
heptane has no hydrogen bonds and no dipoles, and diesel no vdW-forces, so they can’t react.
 Reaction with oxygen: By reaction with oxygen, alkanes leave some side products like unburned
alkanes as soot if there isn’t enough oxygen. The correlation (Zusammenhang) between the chain
length and ease of ignition (Entzündbarkeit) is that it’s only the vapour above the liquid that burns so
shorter alkanes are more volatile, have more vapour and are easier to ignite.
 Reaction with halogens: By combustion, beside reacting with oxygen, alkanes will also react with
halogens and this reaction will only start after an input of energy but only in sunlight. This is a called
substitution reaction. Mechanism of reaction is … (with equation: CH4 + Cl2  CH3Cl + HCl)
 Initiation step: breaks the Cl-Cl bond, so the heterolytic fission and build free radicals.
 Propagation step: free radicals attack the methane molecules to create a new molecule and
free radical that can then carry on the sequence of reactions. (CH4 + .Cl  .CH3 + HCl)
 Termination step: When ever two free radicals meet, they react to form a molecule and the
termination step “mops up” free radicals and eventually the reaction will stop.
 Viscosity: Is much more intensive by heptane than by paraffin oil e.g. This is because the paraffin oil
has a longer chain, more vdW-forces and it’s harder to pass each other  higher boiling point!

oil refinery process
 Fractional distillitation: The main source of petrol is the gasoline fraction obtained from the primary
distillation of crude oil in a fractioning column. Usually this fraction accounts for 15-30% of the crude
oil (it’s used too for motor fuels and to manufacture organic chemicals. It is a mixture of liquids with
between five and ten carbon atoms, which boils in the range 20-180°.
 Cracking: There are two basic processes for splitting alkane chains: using heat, which is called
thermal cracking; and using catalysts, which is called catalytic cracking. Sufficient energy must be
supplied to break the very strong C-C and C-H bonds; most is done by cat cracking. We usually
have 30-40 carbon atoms.
 Reforming: Straight-chain octane alkanes undergo a chemical change to turn them into new,
smooth-burning compounds like branched alkanes, ring (cyclical) alkanes or arenes (aromatic
compounds). There are several reforming processes using heat, pressure and various catalysts.