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

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

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Video Transcript

A student investigated the reactivity of three different halogens. This is the method used. 1) Place 10 centimetres cubed of a sodium halide solution into a test tube. 2) Bubble 10 centimetres cubed of the halogen gas through the solution. 3) Wait one minute and observe. 4) Repeat using the other halogens and sodium halides. The student placed a tick in table one when there was a reaction and a cross when there was no reaction. Which of the following is an independent variable in this investigation? Type of halogen, whether a reaction was observed, volume of sodium halide solution, or time for a reaction to be observed.

Let’s review what an independent variable is. Firstly, it is independent. It does not depend on anything else. Another way of looking at this is that it is the thing that the experimenter chooses to change. And all the variable means is that it is variable and it can or does change. So let’s look at the answers one by one and see which one is the independent variable.

Type of halogen looks like a good candidate. It is being selected in the experiment, and it has been changed between bromine, iodine, and chlorine by the experimenter. It is definitely an independent variable. However, let’s look at the other statements just to be safe. Whether a reaction was observed or not is dependent on the identity of the halogen and the sodium halide. Therefore, it is not independent. Therefore, it is not the correct answer.

Meanwhile, the volume of sodium halide solution is fixed at 10 centimetres cubed. It is not a variable, and therefore it cannot be an independent variable. Therefore, it, too, is not a correct answer. The time for a reaction to be observed is fixed at one minute. It is therefore also a constant, therefore not a variable, and therefore is not the correct answer. The type of halogen is both independent and a variable and therefore is the correct answer.

Give one observation that the student could make during the reaction of chlorine gas with sodium iodide solution.

Here we have our test tube filled with 10 mL of sodium iodide. Sodium iodide does not have a colour. Therefore, the solution is colourless. When chlorine gas is bubbled through it, what they might observe would be the colour of the chlorine, which is a pale yellow green. This is the reaction that occurs. The chlorine is consumed, producing sodium chloride and iodine.

Now one possibility is that as chlorine is being used up, this pale yellow green colour will diminish. However, optically, it’s going to be very difficult to tell the difference between different intensities of this yellow green. What will be noticeable is the production of iodine, which is an intense purple black. Now iodine is not particularly soluble in water, so it would precipitate out as a solid.

Now we need to write this as a full sentence. So, one observation that the student could make during the reaction of chlorine gas with sodium iodide solution is a purple black precipitate of iodine is formed.

The student used different measuring instruments to accurately measure the volumes of the sodium halide solutions and halogen gases. Draw one line from each volume to the measuring instrument used to accurately measure the volume. The volumes are volume of halogen gas and volume of sodium halide solution. And the measurement instruments are measuring cylinder, beaker, balance, gas syringe, test tube, and burette.

The first thing to note here is that the question asks us to find the measurement apparatus that accurately measures volume. Therefore, we can eliminate any measurement apparatus that measures volume inaccurately.

Beakers give rough estimates of volume and are not considered accurate. Test tubes are even worse than beakers because they don’t have graduations. And a balance does not measure volume and therefore is an inappropriate measurement instrument for measuring a volume.

The first volume for which we need to find an appropriate measurement instrument is a gas. The only measurement instrument suitable for measuring out a gas volume is a gas syringe. This leaves us with two choices for the volume of sodium halide solution: using the measuring cylinder or using the burette. Both the measuring cylinder and burette will give accurate volumes. However, the burette is less practical than using a measuring cylinder. Therefore, the volume of sodium halide solution should be measured with the measuring cylinder.

Use the results in table one to place bromine, iodine, and chlorine in order of reactivity.

In order to answer this question, you need to recall that the more reactive element will always displace a less reactive one. We can see from the table that chlorine displaces bromide and iodide and therefore is more reactive than bromine and iodine. We can also see from the table that bromine is more reactive than iodine because bromine displaces iodide.

We now have enough information to rank chlorine, bromine, and iodine in reactivity. Chlorine is the most reactive, bromine is the medium reactive, and iodine is the least reactive.

Suggest one reason why the student should not use fluorine gas in this investigation.

If you look at your periodic table, you can find fluorine above chlorine, in group seven. Going up the group, reactivity increases. Chlorine is more reactive than bromine is more reactive than iodine. Therefore, fluorine is even more reactive than chlorine. Fluorine is so reactive that it will burn almost any material. It is extremely difficult to contain and very very hazardous. So, one reason why the student should not use fluorine gas is that fluorine is a highly reactive gas and is therefore difficult to handle safely.

Metal halide salts can be prepared by reacting a solid metal with an acid. Which metal would not react with an acid to produce a metal halide salt? Tick one box. Copper, Magnesium, Zinc, or Iron.

When a metal reacts with an acid, it forms a salt. In order to form a halide, it needs to react with a halide acid like hydrochloric acid. Whether there is a reaction or not is dictated by the position of the metal relative to hydrogen in the reactivity series. So in order to answer this question, we need to assess each metal and ask the question, “Is it above or below hydrogen in the reactivity series?”

Copper is below, and magnesium, zinc, and iron are all above. So magnesium, zinc, and iron would all react to produce metal halides, while copper would not. Therefore, the answer is copper.

Aluminium reacts with hydrochloric acid to produce aluminium chloride. Aluminium is oxidised in this reaction. Balance the equation for the reaction. Blank Al plus blank HCl react to form blank AlCl3 plus blank H2.

If we assume we start with one equivalent of each, we have one aluminium on the left and one aluminium on the right. However, we are sure of two chlorines on the left-hand side. We can fix that by having three equivalents of hydrochloric acid on the left.

Now to look at the hydrogens, we have three on the left and only two on the right. We can fix that by having three over two hydrogen equivalents on the right. And this produces a perfectly valid answer. However, to eliminate the fraction, I’m going to multiply through by two and make sure all the coefficients are round numbers. And we can see that that still balances. So the balanced equation for the reaction of aluminium with hydrochloric acid is 2Al plus 6HCl react to form 2AlCl3 plus 3H2.

Name the element produced by the reaction of aluminium with hydrochloric acid.

An element is a substance composed of only one type of atom. In the reaction of aluminium with hydrochloric acid, there are two products: aluminium chloride and hydrogen. You should be able to see that there is only one element produced by this reaction, and that’s hydrogen. Aluminium chloride is a compound of aluminium and chlorine, whereas hydrogen is a pure element. So, the name of the element produced by the reaction of aluminium with hydrochloric acid is hydrogen.

What is meant by oxidation in the reaction of aluminium with hydrochloric acid? Tick one box. Loss of oxygen, loss of electrons, gain of oxygen, or gain of electrons.

You should recall OILRIG — oxidation is loss, reduction is gain — in order to answer this question. If we look back at the reaction of aluminium and hydrochloric acid, we can see that aluminium is neutral to start with. In the product, aluminium chloride, the aluminium has a charge of three plus to balance these three minus one charges of the chloride ions. The aluminium has therefore lost electrons in order to become oxidised. Therefore, the answer is loss of electrons.

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