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

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

07:12

Video Transcript

The Haber process is an equilibrium between hydrogen and nitrogen, the reactants, and ammonia, the product. Write the word equation for the Haber process, using the correct reaction arrow.

To start with, a word equation only uses the names of the chemicals, not their formulas. A word equation essentially has this structure: reactants on the left, then a reaction arrow, and then the products on the right. The question, if you read it closely, has been very helpful, telling us that the reactants are hydrogen and nitrogen. It’s also told us that ammonia is the only product. So our reactants are hydrogen and nitrogen, our product is ammonia, and we’ve been asked to use the correct reaction arrow. This is a hint that the standard reaction arrow might not be the right one.

This is what the standard reaction arrow looks like. It’s a rightward-pointing long arrow. It can be read as “react to form”, as in the reactants react to form the products. If we look back at the question, remember we’ve been told that the Haber process is an equilibrium. Now, an equilibrium has its own special type of arrow. This is what it looks like. It’s two fishhook arrows, one pointing right, one pointing left. And what it means is that there are two reactions happening, the forward and the reverse, simultaneously. When you say it aloud, you can read it as “are in equilibrium with”, as in the reactants are in equilibrium with the products.

Now, we have all the information to write our answer. We’re going to write the reactants, then the reaction arrow, then the products. The reactants are added together, hydrogen plus nitrogen. The reactants, hydrogen plus nitrogen, are in equilibrium with the product ammonia. So altogether, hydrogen plus nitrogen are in equilibrium with ammonia. So there we have our complete word equation for the Haber process, using the correct reaction arrow.

If a student wished to test for the presence of hydrogen, what test would they perform?

The most important thing to recognize here is that this question is not asking for what’s done in industrial settings, only in the lab. Hydrogen has many properties. To start with, it’s the least dense gas possible. To get anything less dense, you would need to break it up into individual hydrogen atoms at ridiculously high temperatures. The second major property of hydrogen is that it reacts with oxygen; it’s flammable. Now, which of these do you think a student might be able to make a test for?

Well, the student could take a balloon of hydrogen and weigh the gas. Then to work out the density, they would have to measure the volume. This method would work, but there’s a simpler way. If you take a full test tube of hydrogen and expose it to a lit splint or any flame source, there’ll be a loud pop as the hydrogen detonates. Hydrogen reacts with oxygen in the air producing water. The loud pop has a high pitch, much higher than the pitch of most other explosions.

If you did the same test to a tube of methane gas, what you might get out of a tap, you’ll get a bang, something with a lower pitch. This is because the speed of sound in hydrogen is almost three times that of the speed of sound in methane. Both produce an explosion, but hydrogen is the only one that will make the pop. So how do we write this as a full sentence? The test that the student would perform to test for the presence of hydrogen would be testing with a lit splint. If hydrogen is present, there will be a pop.

At the industrial scale, the Haber process is used to make many millions of tons of ammonia per year. Table one shows the annual volumes of ammonia produced in the US for 2006 to 2008. What is the mean production of ammonia in the US over this three-year period?

This question is asking for the mean average of the values over 2006 to 2008. And mean is defined as the sum of all the values divided by the number of those values. So for this question, we need to take 8.2, 8.5, and 7.9, sum them together, and divide them by three. So 8.2 plus 8.5 plus 7.9 divided by three equals 8.2 million tons. So the mean annual production of ammonia across the years 2006 to 2008 in the US was 8.2 million tons.

The reaction of hydrogen and nitrogen is an example of an exothermic reaction. The reverse reaction, the decomposition of ammonia into hydrogen and nitrogen, can also occur. What is the name for this type of reaction?

So the first reaction, the combination of hydrogen and nitrogen forming ammonia, is exothermic. This means that it gives out heat energy. So what’s the name for this type of reaction? Well, since it’s the reverse process, it must do the opposite of an exothermic reaction. So it must take in heat. The name for this type of reaction is endothermic. In exothermic, exo means giving out. In endothermic, endo means taking in. So the name for the type of reaction that decomposition of ammonia is, is an endothermic reaction.

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