Video: GCSE Chemistry Foundation Tier Pack 1 • Paper 2 • Question 6

GCSE Chemistry Foundation Tier Pack 1 • Paper 2 • Question 6

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Video Transcript

Information about the physical properties of a selection of alkenes is given in Table 1. Which of the alkenes in Table 1 is a liquid at room temperature?

Before we have a look at Table 1, we need to first recall what room temperature is. While the real room temperature may vary day to day and time to time and location to location, scientifically, room temperature is taken to be 20 to 25 degrees Celsius. So what we’re looking for is an alkene that’s a liquid at the range of 20 to 25 degrees Celsius.

In order to be a liquid, the temperature has to be high enough to melt the solid and therefore has to be above the melting point and yet be low enough to not evaporate the liquid and turn it into a gas, and therefore the temperature must be below the boiling point. As mentioned before, our temperature is somewhere in the range of 20 to 25 degrees Celsius. Therefore, for our mystery liquid, we expect the melting point to be below this value and the boiling point to be above this value.

Now we can go through the candidates in Table 1 one by one and see which one applies. Let’s start by looking at ethene. We’re going to check whether the melting point of ethene is below the value of room temperature, 20 to 25 degrees Celsius. Minus 169 degrees Celsius is definitely lower than room temperature. The next thing we’re going to check is whether the boiling point of ethene is above room temperature. Minus 104 is not above room temperature. Therefore, ethene is a gas and therefore not a liquid at room temperature.

Now on to propene. Propene’s melting point is below room temperature, but its boiling point is also below room temperature. Therefore, propene is also a gas and therefore not a liquid at room temperature. The same holds true for butene. Butene’s melting point and boiling point are too low.

Now what about pentene? Pentene’s melting point is below room temperature at minus 165 degrees Celsius, and its boiling point is above room temperature at 30 degrees. Therefore, pentene is a liquid at room temperature, while all the others are gases.

We now have the answer we need. Of the alkenes in Table 1, only pentene is a liquid at room temperature.

Which of the following statements is correct? Tick one box. Methene is the smallest alkene. Butene has three carbon atoms. Propene has six hydrogen atoms. Or butene is the most volatile alkene.

This question asks which of the following statements is correct. Therefore, we expect one of them to be true and three of them to be false. Let’s go through the statements one by one.

“Methene is the smallest alkene.” An alkene is a type of organic compound that contains a carbon–carbon double bond. Therefore, an alkene must have at minimum two carbon atoms. However, the prefix “meth” in methene means that there is one carbon atom in the molecule. Therefore, methene is not a proper chemical name. You cannot have a carbon atom with a double bond to itself. This statement is therefore false and an incorrect answer.

Now what about “Butene has three carbon atoms?” Well, we could solve this one of two ways. The easiest way is to look at butene. Remember that “but” means four carbon atoms. Butene therefore has four carbon atoms, not three, and this statement is false. Therefore, this is not a correct answer. The alternative would be to look back at Table 1 at the formula for butene. There you can see that the formula has four carbon atoms and not three.

What about “Propene has six hydrogen atoms?” Again, from Table 1, the formula of propene is CH2CHCH3. Two plus one plus three equals six hydrogen atoms in total. Therefore, this statement is true and the correct answer.

But just to be safe, let’s have a look at the fourth statement, “Butene is the most volatile alkene.” Again, there are a couple of ways of tackling this statement. There are four carbon atoms in butene. Now the relationship between boiling point and volatility is that the lower the boiling point, the more volatile the material. The second relationship we need is that the longer the chain of a hydrocarbon, the higher its boiling point, therefore the lower its volatility is.

So the question we need to answer is- is, is butene the shortest alkene? No, ethene is. Ethene has two carbon atoms and butene has four. Therefore, we would expect ethene to have a lower boiling point and therefore a higher volatility than butene. Therefore, “Butene is the most volatile alkene” is a false statement and an incorrect answer.

The alternative approach to this statement would be to look back at Table 1 at the boiling points of the alkenes. Butene’s boiling point is not the lowest of these alkenes and therefore it cannot be the most volatile alkene. So of the four statements, the only correct statement was that propene has six hydrogen atoms.

Draw the missing bonds in Figure 1 to complete the displayed formula for propene.

For this question, we’re going to need to combine a few pieces of information, starting with the fact that carbon forms four bonds. the second piece of information is that propene has a carbon–carbon double bond. We know this because propene is an alkene. We’ll also need to remember that hydrogen forms only one bond. And if you need it, the formula from Table 1 can be applied here.

Let’s start with the carbon on the far right. This carbon has four neighbouring atoms. Therefore, it will form four single bonds. Now we know that propene has a carbon–carbon double bond. We’ve only got three carbons. Therefore, we’ve only got two bonds to make. Therefore, one of them must be a double bond, and we’ve already drawn one of them, so the carbon–carbon double bond must be here.

Attaching the last three hydrogens, we end up with a complete displayed formula for propene. As you can see, it matches the formula of CH2CHCH3.

Figure 2 shows a flow diagram of the production of ethene by catalytic cracking of long hydrocarbons. Complete the word equation for the production of ethene from hexane. Hexane reacts to form blank plus ethene.

Let’s start off by having a look at Figure 2. Long hydrocarbons enter the reactor where they’re mixed with some form of catalyst, and then products are formed and separated by a distillation column. Ethene is one of the products and there may be others. The slurry from the distillation column is returned to the reactor.

In order to complete the word equation, we need to understand what kind of reaction catalytic cracking is. Catalytic cracking will take a long hydrocarbon, like a long alkane, and break it down into a shorter alkane plus an alkene. In this case, our alkene is ethene. Hexane is an alkane. Therefore, our mystery product must also be an alkane.

Now hexane contains six carbon atoms. We know this because the prefix “hex”mean six. Ethene, on the other hand, has two carbon atoms. We know this because the prefix “eth” means two. So how many carbon atoms must be present in the mystery product?

Well, 𝑥 is equal to six, the number of carbon atoms in hexane, minus two, the number of carbon atoms in ethene. Therefore, 𝑥 equals four, and there must be four carbon atoms in our mystery alkane. “But” is the prefix we use when we want to indicate that a group of an organic compound contains four carbon atoms. Our substance is an alkane. Therefore, the name of our mystery compound is butane. So the complete word equation for the production of ethene from hexane is hexane reacts to produce butane plus ethene.

Why is the slurry returned to the reactor?

As we can see from Figure 2, one of the outputs from the distillation column is the slurry, which is returned to the reactor. Now the slurry is going to be the high melting and boiling point material that’s left at the bottom of the distillation column. As with crude oil distillation, we end up with a mixture of fractions and the one that contains the longest hydrocarbons will be at the bottom.

So ask yourself this: why would you want to return a slurry containing long hydrocarbons to a reactor that breaks down long hydrocarbons? You do it because you want to continue to make highly valuable ethene and shorter hydrocarbons. So why is the slurry returned to the reactor? It is recycled as it still contains crackable hydrocarbons.

Ethene can be polymerised in the presence of a catalyst. Explain which bonds are broken and formed in this reaction and why the product of the reaction is saturated.

Let’s start with the first point: which bonds are broken and formed in this reaction. This is the structure of ethene. When 𝑛 number of molecules of ethene react together and are polymerized, they form polyethene, with this structure. If we look at a few more units of this polymer, we can see new carbon–carbon single bonds. These are produced by the opening out of the double bond in ethene.

So when addressing the first point, our answer will focus on the fact that carbon–carbon double bonds are broken and carbon–carbon single bonds are formed. What about the second statement? For a hydrocarbon to be saturated, there must be no multiple bonds, no carbon–carbon double bonds and no carbon–carbon triple bonds, only carbon–carbon single bonds.

If we look once again at the structure of polyethene, we find no carbon–carbon double bonds, no carbon–carbon triple bonds, only carbon–carbon single bonds. We now have all the information to move on to our answer. Addressing the first point, “which bonds are broken and formed in this reaction,” the carbon–carbon double bond, C double bond C, of ethene breaks during the polymerisation, forming a carbon–carbon single bond, C single bond C, between the monomers. Addressing the second point, “why the product of the reaction is saturated,” the product is saturated because it contains only single bonds.

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