Terms in this set (160)

Because they are unsaturated hydrocarbon compounds, alkenes are much more reactive than alkanes. In addition to combustion, the double bonds in alkenes can be broken to facilitate the addition of hydrogen and many other species.

Combustion of Alkenes

Like all other hydrocarbons, alkenes burn in oxygen to produce carbon dioxide and water vapor. Alkenes show a greater tendency than alkanes to undergo incomplete combustion, forming much carbon monoxide and carbon rather than carbon dioxide.

The combustion reaction of alkenes is not as important as that of alkanes. Unlike alkanes, alkenes are not used as fuels. Instead, because of their reactivity, alkenes are used as the starting point in the manufacture of many important chemicals.

An example of the combustion of an alkene is:

C2H4(g) + 3O2(g) ------------> 2CO2(g) + 2H2O(g)

Addition Reactions of Alkenes

Most of the reactions that alkenes undergo involve the breakage of their double bond and the addition of another species. These are addition reactions.

Addition of Hydrogen

Hydrogen can be added to an alkene to form the corresponding alkane. This reaction occurs in the presence of a palladium, platinum or nickel catalyst. Heat is required for this reaction when a nickel catalyst is used.

Hydrogenation, as this reaction is called, is an important process in the food industry when nickel is used as a catalyst. Via hydrogenation, less valuable(commercially), unsaturated, edible oils, such as those present in sunflower seed oil and peanut oil are converted to more valuable, saturated, edible fats such as margarines.

An example of the hydrogenation of an alkene is:

CH2=CH2(g) + H2(g) ------------> CH3-CH3(g)

Addition of Halogens

Halogens react with alkenes forming halogenoalkanes. This reaction can occur without catalysis.

The addition of liquid bromine to an alkene serves as a characteristic test. If liquid bromine, which is a brown liquid, is added to an alkene without light or heat, a colorless solution is formed. This is because the brown liquid bromine reacts with the alkene to form a bromoalkane which is colorless.

An example of this reaction is:

CH2=CH2(g) + Br2(l) -------------> BrCH2CH2Br(l)

Bromine water also reacts and is decolorised by alkenes. However, different products are formed in this reaction. A bromoalcohol and hydrogen bromide are the products formed when bromine water reacts with alkenes.

An example of this reaction is:

CH2=CH2(g) + Br2(aq) + H2O(l) ------------> BrCH2CH2OH(aq) + HBr(aq)

Addition of Hydrogen Halides

Hydrogen chloride, hydrogen bromide and hydrogen iodide all react with alkenes to form halogenoalkanes.

An example of the addition of a hydrogen halide to an alkene is:

CH2=CH2(g) + HBr(g) ------------> CH3CH2Br(l)

Oxidation of Alkenes

Alkenes are oxidized by alkaline potassium manganate(VII), a weak oxidizing agent. When alkenes are added to the purple alkaline potassium manganate(VII), there is a color change to brown as the alkenes are being oxidized. The product formed as a result of this oxidation is a diol. This reaction can be used to test for carbon-carbon double bonds.

An example of the oxidation of an alkene is:

CH2=CH2(g) (+ KMnO4, OH-) ------------> CH2OH-CH2OH(l)

Polymerization

Polymerization of alkenes occurs when many alkene molecules add together to form one large molecule called a polymer.

The polymerization of alkenes is important in the formation of plastics, rope, food boxes, bowls and buckets among many others

An example of a polymerization reaction is:

nCH2=CH2(g) ------------> -[-CH2-CH2-]-n(s)

Alkenes are much more reactive than alkanes because of their unsaturated nature. The breakage of their double bond allows many reactions, other than combustion, to occur in alkenes. Examples of these are addition reactions, oxidation and polymerization
Metals

Most elements are metals. On the periodic table, metals are separated from nonmetals by a zig-zag line stepping through carbon, phosphorus, selenium, iodine and radon. These elements and those to the right of them are nonmetals. Elements just to the left of the line may be termed metalloids or semimetals and have properties intermediate between those of the metals and nonmetals. The physical and chemical properties of the metals and nonmetals may be used to tell them apart.

Metal Physical Properties
lustrous (shiny) good conductors of heat and electricity high melting point high density (heavy for their size) malleable (can be hammered)
ductile (can be drawn into wires) usually solid at room temperature (an exception is mercury) opaque as a thin sheet (can't see through metals) metals are sonorous or make a bell-like sound when struck

Metal Chemical Properties
have 1-3 electrons in the outer shell of each metal atom corrode easily (e.g., damaged by oxidation such as tarnish or rust) lose electrons easily
form oxides that are basic have lower electronegativities are good reducing agents

Nonmetals

Nonmetal Physical Properties
not lustrous (dull appearance) poor conductors of heat and electricity nonductile solids
brittle solids may be solids, liquids or gases at room temperature transparent as a thin sheet
nonmetals are not sonorous

Nonmetal Chemical Properties
usually have 4-8 electrons in their outer shell
readily gain or share valence electrons form oxides that are acidic have higher electronegativities are good oxidizing agents
Formaldehyde is a toxic organic molecule with molecular formula CH2O. Draw the Lewis structure of formaldehyde.

Solution:

Step 1: Find the total number of valence electrons.

Carbon has 4 valence electrons
Hydrogen has 1 valence electrons
Oxygen has 6 valence electrons

Total valence electrons = 1 carbon (4) + 2 hydrogen (2 x 1) + 1 oxygen (6)
Total valence electrons = 12

Step 2: Find the number of electrons needed to make the atoms "happy"

Carbon needs 8 valence electrons
Hydrogen needs 2 valence electrons
Oxygen needs 8 valence electrons

Total valence electrons to be "happy" = 1 carbon (8) + 2 hydrogen (2 x 2) + 1 oxygen (8)
Total valence electrons to be "happy" = 20

Step 3: Determine the number of bonds in the molecule.

number of bonds = (Step 2 - Step 1)/2
number of bonds = (20 - 12)/2
number of bonds = 8/2
number of bonds = 4

Step 4: Choose a central atom.

Hydrogen is the least electronegative of the elements, but hydrogen is rarely the central atom in a molecule. The next lowest electronegative atom is carbon.

Step 5: Draw a skeletal structure.

Connect the other three atoms to the central carbon atoms. Since there are 4 bonds in the molecule, one of the three atoms will bond with a double bond. Oxygen is the only choice in this case, since hydrogen only has one electron to share.

Step 6: Place electrons around outside atoms.

There are 12 valence atoms total. Eight of these electrons are tied up in bonds. The remaining four complete the octet around the oxygen atom.

Each atom in the molecule has a complete outer shell full of electrons. There are no electrons left over and the structure is complete. The finished structure appears in the picture at the beginning of the example.
1. Physical nature of reactant - state of matter ie. liquid, solid, gas - surface area/particle size
- faster reaction rates occur btwn liquid-state than solid-state reactants
- small particle size/ with greater surface area also increases reaction rate

2. Reactant concentration - w/ increased concentration comes more molecules of that reactant present in the reaction mixture therefore more collisions occur btwn reactants

3. Reactant temperature - Increase in temp of system = increase in average kinetic energy of reacting molecules causing more collisions to take place and increasing reaction rate

4. Presence of catalysts - A catalyst is a substance that increases reaction rate without being consumed in the chemical reaction - catalysts enhance reaction rates by providing alternative reaction pathways that have lower activation energies then the original uncatalysed pathway
-Reactions need a certain amount of energy in order to happen. If they don't have it, reaction probably can't happen. A catalyst lowers the amount of energy needed so that a reaction can happen more easily. A catalyst is about energy. It doesn't have to be another moelcule. If you fill a room with hydrogen gas (H2) and oxygen gas (O2), very little will happen. If you light a match in that room (or just produce a spark), most of the hydrogen and oxygen will combine to create water molecules (H2O). It is an explosive reaction.


(and inhibitors - Inhibitors Slow reactions Down
Working in exactly the opposite way as catalysts. Inhibitors slow the rate of reaction. Sometimes they even stop the reaction completely. Inhibitor can make the reaction slower and more controllable. Without inhibitors, some reactions could keep going and going and going. If they did, all of the molecules would be used up. I.e When you are watching television, you have no reason to keep breaking down sugars at the same rates you would if you were working out.
;