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

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


Video Transcript

A student is investigating the reaction of magnesium hydroxide with dilute hydrochloric acid. Figure 1 shows the apparatus used in this investigation. Complete the reaction equation by inserting state symbols for the reactants and products. Mg(OH)₂ blank plus 2HCl aqueous react to form MgCl₂ blank plus H₂O liquid.

So here we can see we have two missing state symbols: one for the magnesium hydroxide in the reactants and one for the magnesium chloride, which is in the products. We can work out which state each of these is in by using Figure 1.

We can see that magnesium hydroxide, the reactant, is labelled and shown as solid pellets in the figure. The state symbol for a solid is simply the letter “s.” So we can fill in the first part of the question.

The second part asks us about the magnesium chloride product. And if we look at Figure 1, it’s not clearly shown. The reason that the magnesium chloride product is not shown in Figure 1 is that it’s dissolved in the hydrochloric acid. The technical term for this is aqueous, and the state symbol for aqueous is “aq.” So to complete the reaction equation, we have magnesium hydroxide solid plus 2HCL aqueous reacting to form magnesium chloride aqueous plus H₂O liquid.

The student tests the reaction mixture with a pH meter during the reaction. Why does the pH meter reading increase during the reaction? Tick one box. An acid is consumed by the reaction. Water is produced by the reaction. A salt is produced by the reaction. A base is produced by the reaction.

So the key part of this question is to work out why the pH meter increases during the reaction. Now let’s review what we mean by pH. Remember that the pH reading tells us how acidic or basic a solution is. Very low numbers mean that the solution is acidic. Very high numbers mean that the solution is basic.

In this example, our reaction mixture has a pH meter reading which increases throughout the reaction. This means that the solution becomes less acidic or more basic. The next thing we need to do is look back at our reaction equation. Our reaction involves magnesium hydroxide, which is a base, reacting with two lots of hydrochloric acid to form magnesium chloride, a salt, plus water.

We can now use all of this information to answer our question. Let’s look at each option in turn. The first option is that an acid is consumed by the reaction. Now let’s check if this is what’s going on in our case. We can see that, on the reactant side of the equation, we do have an acid and that acid is not present in the products. So it is true that an acid is consumed by this reaction.

The second option says that water is produced by the reaction. Again, if we look at the products, we can see that indeed there is water there, so this is also true. The third option is that a salt is produced by the reaction, and magnesium chloride is considered a salt, so this is true. And the final option says that a base is produced by the reaction. In our reaction equation, the only base present is in the reactants, so there is no base produced, so we can rule out this answer.

We’ll now consider the true statements to see which one could be responsible for increasing the pH of our solution. The first option, “An acid is consumed,” would mean that the solution becomes less acidic because there is less acid around. Less acidic is the same as saying more basic, which means that an acid being consumed by the reaction would indeed increase the pH meter reading. So this is possibly the correct answer, but let’s check the others just to be sure.

Water being produced by the reaction: would this have an effect on the pH reading? Technically, producing water would dilute the solution that we have. If we dilute an acid, it does make the solution less acidic, but you need a lot of water simply to increase the pH by one. So whilst this is probably true, you would need a lot of water, so this is unlikely to be the right answer.

The third option, “A salt is produced by the reaction,” would have no effect on the pH of the solution, so this cannot be the right answer. So for this reaction, the reason that the pH increases during a reaction is that the acid is consumed.

Describe a safe method for preparing pure magnesium chloride crystals from magnesium hydroxide and dilute hydrochloric acid. Use the information in Figure 1 for assistance. Name all apparatus used in the preparation of the crystals.

Let’s remind ourselves of Figure 1. We can also draw some important information from the question itself. We can see that the method we suggest needs to be safe and to prepare pure magnesium chloride. It suggests which reactants we need to use, magnesium hydroxide and dilute hydrochloric acid, and it also asks us to name all the apparatus we use.

We can consider this process in two parts. Step one is the reaction itself to create the magnesium chloride, and step two is purification so that we get pure magnesium chloride crystals.

Let’s look at step one first. A lot of the information we need for the reaction is provided in Figure 1. We can see that both the magnesium hydroxide and dilute hydrochloric acid are placed into a beaker. So that’s the first piece of apparatus that we need. We’re going to add the magnesium hydroxide, which is a solid, using a spatula, and we’re gonna keep adding it until no more magnesium hydroxide dissolves. We know that once the magnesium hydroxide stops dissolving, the reaction has gone to completion. This means we have created our magnesium chloride.

So let’s put that first bit into sentences. Dilute hydrochloric acid is placed into a beaker. Solid magnesium hydroxide is added using a spatula until no further solid reacts and dissolves.

So now that we’ve covered the reaction step, let’s look at purification, how we get those pure magnesium chloride crystals out of our mixture. Our next step needs to be removing the excess solid magnesium hydroxide. Now because this is a solid, the easiest way to do this is by filtering. To filter our solution, we’re going to need a funnel, a conical flask, and some filter paper. You could use a funnel with a beaker, but it’s much easier to use a conical flask because the neck is narrower and it sits in really nicely.

We’re gonna pour our mixture through the filter paper where the magnesium hydroxide solid will get stuck, and the rest of our solution will go through the filter paper and into the conical flask. The magnesium chloride that we’re after is now in the solution in the bottom of the conical flask. And what we have to do now is to get the crystals out of solution.

So in this next step, we’re gonna pour our solution into an evaporating dish. We’re then going to apply heat underneath using a Bunsen burner to evaporate off a little bit of the water. Once we’ve made this solution more concentrated, we’re going to cool it down. As we cool this solution down, the magnesium chloride crystals will come out of solution. Once this happens, we can decant off the rest of the liquid and dry off the crystals on a piece of filter paper. This completes our method as we’ve produced pure magnesium chloride crystals.

So now we need to put this into sentences. Excess solid magnesium hydroxide is removed by filtering the product using filter paper in a glass funnel inserted into a conical flask. The filtrate is poured into an evaporating dish and heated using a Bunsen burner to remove some of the water. Note that we’re not removing all of the water, just some of the water. This is because any impurities will then stay in solution and when we decant the water off, they leave, so we’re left with only pure magnesium chloride. The concentrated filtrate is cooled on ice until magnesium chloride crystallizes. The remaining solution is decanted off, and the crystals are dried on filter paper.

The equation for the reaction of magnesium hydroxide and hydrochloric acid is as shown. Mg(OH)₂ plus 2HCl react to form MgCl₂ plus 2H₂O. Relative formula masses in grams per mole for magnesium hydroxide equals 58.3, for HCl it equals 36.5, for magnesium chloride 95.2, and for water 18.0. The atom economy of a reaction is calculated using the equation atom economy equals relative formula mass of desired product divided by the sum of the formula masses of all reactants times 100 percent. Calculate the atom economy for the production of magnesium chloride from magnesium hydroxide.

We have all the numbers and equations that we need given to us in the question. it’s our job to put it all together. So let’s start with the relative formula mass of the desired product. The product that we desire in this reaction is magnesium chloride, and magnesium chloride has a relative formula mass of 95.2. Next, we need the sum of the formula masses for all reactants. Our reactants are magnesium hydroxide and 2HCl. The relative formula mass of magnesium hydroxide is 58.3. The relative formula mass of HCl is 36.5. But remember we have 2HCl molecules, so we need to times this by two. So the sum of the formula masses of all of our reactants is 131.3 grams per mole.

So if we now put the numbers into our atom economy equation, we get 95.2, which is the formula mass of our product, divided by 131.3 times 100 percent. This works out to be 72.5057 percent.

Now because the numbers we were given in the question are correct to three significant figures, we should put our answer in three significant figures. So the answer is that the atom economy equals 72.5 percent.

Give one disadvantage of using a reaction with a low atom economy when preparing a material.

First, let’s consider what we mean by atom economy. The atom economy is a measure of the efficiency of a reaction. A reaction with a high atom economy is very efficient, producing little waste. A reaction with a low atom economy is very inefficient, producing lots of by-product and waste. Any reaction which produces a lot of by-product and waste can’t be good for the environment. And any waste produced may be expensive to dispose of.

Now let’s put this into sentences. So one possible disadvantage of using a reaction with a low atom economy is that there will be a large proportion of by-product, which may be pollutants or be expensive to dispose of.

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