0620 · 11.5Alkenes

Alkanes and alkenes

0:00Two hydrocarbons can look similar but behave very differently when tested with aqueous bromine. An alkane leaves the orange colour unchanged, while an alkene makes it disappear. That difference comes from the bonds inside their molecules. By the end of this video, you will understand the test, the manufacture of alkenes, and how their characteristic reactions work. A hydrocarbon is a compound containing hydrogen and carbon only. Carbon forms four covalent bonds, while hydrogen forms one.

0:26A covalent bond involves a shared pair of electrons between atoms. These ideas help us understand both the starting molecules and the products they form. Look first at ethane, which is an alkane. Its two carbon atoms share a single covalent bond, and each also bonds to three hydrogen atoms. Every carbon therefore has four bonds in total. Alkanes are saturated hydrocarbons because their carbon atoms are joined only by single bonds.

0:52Ethene has a different arrangement. Its two carbon atoms share a double covalent bond, and each bonds to two hydrogen atoms. The double bond contains two shared pairs of electrons, rather than one. When checking carbon’s four bonds, count that double connection as two. Alkenes are unsaturated hydrocarbons, and their bonding includes a carbon to carbon double covalent bond. Unsaturated means that the molecule contains at least one carbon to carbon double bond, so other atoms can add across it.

1:18Compared with ethane, ethene has fewer hydrogen atoms attached to the same number of carbons. The important identifying feature is the double bond, not simply the number of hydrogens.

Making alkenes by cracking

1:29Alkenes can be manufactured by cracking larger alkane molecules. Cracking means breaking these larger molecules into smaller molecules using a high temperature and a catalyst. Some bonds break, and the atoms rearrange into different products. Those products can include smaller alkanes, alkenes and hydrogen. The high temperature provides energy for the process. A catalyst increases the reaction rate without being used up overall. Remember both conditions when describing this method of manufacture.

1:56Simply separating a mixture into fractions does not produce new molecules, whereas cracking changes the substances chemically. There are two important reasons for cracking. Demand for short-chain fuels, such as petrol, is greater than the supply obtained directly from crude oil. Breaking larger molecules helps produce more useful smaller ones. Cracking also provides alkenes needed to make polymers, which you will study in the video on polymers. One possible cracking reaction turns decane into octane and ethene.

2:24Decane contains ten carbon atoms and twenty-two hydrogen atoms. Octane contains eight carbons and eighteen hydrogens, while ethene contains two carbons and four hydrogens. The products are a smaller alkane and an alkene. Check that the atoms have been conserved. Eight carbon atoms in octane plus two in ethene gives ten altogether. Eighteen hydrogen atoms plus four gives twenty-two. Cracking changes how atoms are connected, but it does not create or destroy them.

2:52Hydrogen can also be formed during cracking. In the second equation shown, one decane molecule breaks into five ethene molecules and one hydrogen molecule. Five ethene molecules hold ten carbon atoms and twenty hydrogen atoms between them. The remaining two hydrogen atoms form the hydrogen molecule, giving twenty-two hydrogens in total. Do not assume that every cracking equation must show hydrogen as a product. The first decane example forms octane and ethene without producing hydrogen.

3:20Larger molecules can break in different ways, so cracking can produce a mixture of substances. Any proposed equation must still have the same total number of each atom on both sides.

Testing with aqueous bromine

3:29To distinguish saturated and unsaturated hydrocarbons, add aqueous bromine to a sample and shake. Aqueous bromine means bromine dissolved in water, and it is also called bromine water. With an alkene, its colour changes from orange to colourless. An alkane leaves the aqueous bromine orange under the usual test conditions. The alkene reacts with the bromine, so the coloured reagent is used up. That is why the colour disappears. An alkane does not undergo this reaction under the usual test conditions.

3:58The observation therefore gives evidence of unsaturation when comparing these hydrocarbons. Describe the result precisely as orange to colourless. Saying that the liquid becomes clear is not enough, because a transparent liquid can still be coloured. Also keep the observation separate from the conclusion. The colour change is what you see, while the presence of an unsaturated hydrocarbon is what you infer. This test can appear in practical questions, including the practical paper or the alternative to practical paper.

4:26You may need to describe the method, state the observation, and identify the type of hydrocarbon. Give the reagent as aqueous bromine, not just an unspecified coloured solution. Then connect the result to saturation or unsaturation.

Addition reactions of alkenes

4:39This next part is for the Extended paper. In an addition reaction, only one product is formed. Atoms from another reactant join the alkene across its carbon to carbon double bond. The connection between those carbons becomes a single bond, allowing each to form a new bond. The two carbon atoms do not separate during this process. Their connection changes from double to single, while additional atoms attach to them.

5:01Each carbon still has four bonds in the finished product. This bond-counting rule lets you work out the structure instead of guessing it. Only one product means that the reactants combine to form one product substance. It does not mean that an entire reaction mixture contains just one molecule. Many molecules can react to make many copies of the same product. No separate small molecule is produced alongside it in these addition reactions.

5:25Ethene plus bromine gives one, two dibromoethane. The carbon to carbon double bond becomes a single bond, and a bromine atom attaches to each carbon. All the atoms from both reactants appear in the product. This is the addition reaction used to explain the aqueous bromine test. Follow the displayed formula carefully. Each carbon now bonds to the other carbon, two hydrogen atoms and one bromine atom. That gives four single bonds around each carbon.

5:51A common mistake is adding bromine atoms while leaving the double bond unchanged, which would give each carbon too many bonds. Ethene also undergoes addition with hydrogen in the presence of a nickel catalyst. The word equation is ethene plus hydrogen gives ethane. Each carbon gains one hydrogen atom as the double bond becomes single. The product is now a saturated hydrocarbon because it has no carbon to carbon double bond.

6:15This reaction is called hydrogenation because hydrogen is added. Nickel helps the reaction occur faster, but it is not part of the product molecule. If an exam asks you to state the catalyst, name nickel. Merely writing that a catalyst is used does not identify the required substance. The third reaction adds steam to ethene in the presence of an acid catalyst. Ethene plus steam gives ethanol. Steam is water in its gaseous state, so it supplies hydrogen and oxygen atoms.

6:41The product contains an oxygen to hydrogen group attached to the carbon chain. For drawing the product, think of water supplying hydrogen to one carbon and an oxygen to hydrogen group to the other. This is a way to track the atoms, not a detailed reaction mechanism. The double bond becomes single, and the water atoms become part of the same product. The resulting ethanol molecule is no longer a hydrocarbon because it contains oxygen.

7:04Hydrogenation and addition of steam sound similar, but they produce different substances. Adding hydrogen produces ethane, while adding steam produces ethanol. Their catalyst requirements differ too: nickel for hydrogen and an acid catalyst for steam. Start with the identity of the added substance, rather than relying on a similar-sounding reaction name.

Worked example: propene hydrogenation

7:24A less familiar question asks you to draw the product when propene reacts with hydrogen using a nickel catalyst. Propene has three carbon atoms, with a double bond between the middle carbon and an end carbon. First locate that double bond, because this is where addition happens. Keep the three-carbon chain intact and change the double bond into a single bond. Next, attach one additional hydrogen atom to each of the carbons that shared the double bond.

7:48The first carbon was not involved, so its attached hydrogens stay unchanged. Finally, check that every carbon has four bonds. The product is propane. Its end carbons each have three attached hydrogen atoms, while the middle carbon has two. There are no double bonds remaining, so the product is saturated. The same reasoning works for other alkenes because hydrogen adds across the double bond without removing any carbon atoms. An exam can combine manufacture, testing and addition in one question.

8:16Notice the command words before answering. State asks for the required information directly, while describe asks you to give the relevant features or sequence. If asked to draw a structure, a product name alone does not fulfil that instruction. For the first two parts, a concise answer would give these separate points. Cracking uses a high temperature and a catalyst. Add aqueous bromine to the hydrocarbon and shake. An alkene changes it from orange to colourless, whereas an alkane leaves it orange.

8:44For the final part, draw ethanol with the structure shown earlier. The carbon atoms are joined by a single bond, and an oxygen to hydrogen group attaches to an end carbon. An acid catalyst is required. Check every bond before finishing, especially the oxygen, which forms two single bonds in this molecule. The main mistakes all come from losing track of the atoms or bonds. Cracking breaks larger molecules into smaller ones, whereas addition combines reactants into only one product.

9:11During addition, the carbon to carbon connection remains, but becomes single. Correct structures conserve atoms and give each carbon four bonds. Alkenes are unsaturated hydrocarbons containing a carbon to carbon double covalent bond. Cracking manufactures them from larger alkanes using a high temperature and a catalyst. It also helps supply short-chain fuels and provides alkenes for making polymers. Aqueous bromine distinguishes an alkene by changing from orange to colourless. The double bond also explains the three addition reactions.

9:42Bromine gives a product with a bromine atom attached to each of the original double-bonded carbons. Hydrogen with nickel produces an alkane, while steam with an acid catalyst produces an alcohol. Once you track the added atoms and check the bonds, these products become understandable rather than isolated facts.