C. Chemical Reactions · Chemical ReactionsSingle Displacement: Halogens, and Planning an Activity-Series Inquiry
C2 C3 A1.1 A1.2 A1.4 A1.5 A1.6 A1.8 A1.10About 75 minutes
Learning goals- I can use the halogen series to predict whether a halogen will displace a halide ion, and write the equation.
- I can explain the halogen series using atomic size and the attraction for electrons.
- I can plan a safe, controlled investigation to rank metals by reactivity, including variables, safety (WHMIS) and a data table.
- I can analyse results, identify sources of error and draw a conclusion.
Words to know
halogen
An element in group 17: fluorine, chlorine, bromine, iodine (and astatine). Halogens exist as diatomic molecules.
halide ion
A halogen atom that has gained one electron: F⁻, Cl⁻, Br⁻, I⁻.
halogen series
The order of halogen reactivity: F₂ > Cl₂ > Br₂ > I₂.
independent / dependent variable
The variable you change on purpose / the variable you observe or measure in response.
controlled variable
A variable kept the same so it cannot affect the result.
WHMIS
Workplace Hazardous Materials Information System: Canada's system of hazard pictograms, labels and safety data sheets (SDSs) for chemicals at work and in school labs.
Non-metals can cut in too
Metals are not the only elements that displace each other. The halogens (group 17) do it as well, but in reverse: they react by gaining electrons to become negative halide ions. A more reactive halogen pulls electrons away from the ions of a less reactive halogen.
This matters in real life. Bromine is produced commercially by bubbling chlorine through salty brine that contains bromide ions, and chlorine is used to treat drinking water and swimming pools because it reacts so readily.
The halogen series
F₂ > Cl₂ > Br₂ > I₂ (most reactive to least). This is simply the order going down group 17.
Why? Going down the group, the atoms get larger. In a larger atom, the outer energy level is farther from the nucleus and more shielded by inner electrons, so the nucleus attracts an extra electron less strongly. Fluorine, the smallest, attracts electrons most strongly of all elements (it has the highest electronegativity). You studied these trends in the Matter, Chemical Trends, and Chemical Bonding unit.
The rule: a halogen displaces any halide ion below it in the series.
- Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) ✔ chlorine is above bromine
- Br₂(aq) + 2NaI(aq) → 2NaBr(aq) + I₂(aq) ✔ bromine is above iodine
- I₂(aq) + NaCl(aq) → NR ✘ iodine is below chlorine
Notice: the halogens are diatomic, so the halide compound usually needs a coefficient of 2.
What you would see in the lab
The products are coloured, which makes these reactions easy to detect. In water, chlorine is very pale yellow-green, bromine is yellow-orange to brown, and iodine is yellow-brown. Shaking the mixture with a small amount of a non-polar solvent makes the colours clearer, because the halogen molecules move into the solvent layer: bromine looks orange and iodine looks violet. A teacher does these tests in a fume hood, because halogen vapours are toxic. Fluorine is far too reactive and dangerous for school labs.
Worked examplePredicting halogen displacement
Predict the products or write NR: (a) chlorine + aqueous sodium iodide; (b) bromine + aqueous potassium chloride; (c) fluorine gas + aqueous sodium bromide (a thought experiment only).
- (a) Chlorine is above iodine, so it displaces iodide. Chlorine becomes chloride; iodide becomes I₂. Two NaI are needed for one Cl₂.
- (b) Bromine is below chlorine: NR.
- (c) Fluorine is above bromine, so it would displace bromide. (In reality fluorine also reacts violently with the water itself, which is one reason nobody does this.)
(a) Cl₂(g) + 2NaI(aq) → 2NaCl(aq) + I₂(aq) (b) NR (c) F₂(g) + 2NaBr(aq) → 2NaF(aq) + Br₂(aq)
Planning an investigation: ranking metals
Your curriculum asks you to investigate reaction types, not just read about them. Here is how a chemist would plan a lab to rank four metals: magnesium, zinc, iron and copper.
Question: In what order are Mg, Zn, Fe and Cu in reactivity?
Hypothesis: If each metal is placed in solutions of the other metals' compounds, then the most reactive metal will react in the most solutions, because a metal displaces only the metals less reactive than itself. (Prediction from the activity series: Mg > Zn > Fe > Cu.)
Variables:
- Independent: which metal is placed in which solution.
- Dependent: whether a reaction is observed (a deposit forms, the metal changes colour, bubbles appear, or the solution's colour changes).
- Controlled: solution concentration (for example 0.1 mol/L) and volume (the same number of drops), size of metal pieces, how clean the metal is (sand off oxide), temperature, observation time (for example 5 minutes).
Equipment: a spot plate (well plate), dropper bottles of MgSO₄, ZnSO₄, FeSO₄ and CuSO₄ solutions, small sanded pieces of each metal, forceps, a white sheet of paper underneath, and a hand lens.
Safety (WHMIS): read the safety data sheet for each chemical. Wear splash goggles and a lab apron. Copper(II) sulfate is harmful if swallowed and irritates the eyes and skin, and it is toxic to aquatic life, so it goes in the waste container, never down the drain. Wash hands after the lab.
Procedure: place 5 drops of each solution in a row of wells, add one metal piece per well (each metal in each other metal's solution), and observe for 5 minutes. Record observations in a grid.
Analysing the results
Conclusion: the more solutions a metal reacts in, the more reactive it is. Typical results support the hypothesis: Mg > Zn > Fe > Cu.
Sources of error (things that could make results differ from the prediction):
- An oxide coating on the metal (especially magnesium and aluminium) can hide or slow a reaction. Sanding helps.
- Some reactions are slow, so a short observation time can miss them (a false "no reaction").
- Iron(II) sulfate solution slowly reacts with oxygen from the air, so it should be freshly made.
- Small amounts of a deposit can be hard to see; a hand lens and white background help.
A good report suggests improvements: a longer observation time, cleaner metals, a magnifier, and repeating each test (replicates) to check results are reliable.
Hands-on activityDesign and run a safe mini-inquiry at home
You need: the copper-containing vinegar-salt solution from the last lesson's activity (or make a new batch: 60 mL white vinegar, 5 mL salt, 15 dull pennies soaked for 10 minutes, then pennies removed), four small clear cups, a plain steel nail (iron), a galvanized nail (zinc-coated), a strip of aluminium foil (5 cm × 1 cm, folded twice), a copper wire or a penny, masking tape and a marker, and a grown-up.
Safety: the solution is not food. Do not taste it, wash your hands after, and pour it down the sink with plenty of water. Handle nails carefully.
- Write a testable question and a hypothesis using the activity series (which metals will become coated with copper?).
- Name your independent, dependent and controlled variables (for example: same volume of solution in each cup, same time, same temperature).
- Label the cups, pour the same amount of solution into each, and add one metal to each. Observe after 15 minutes and after 1 hour. Record in a table.
- Write a conclusion and at least two sources of error.
What to expect: the steel nail and the galvanized nail get a reddish-brown copper coating. The copper shows no change. Aluminium is above copper and should react, but its tough oxide coating can make it slow to start, a good example of a source of error to discuss.
Just for funFor decades, chemists believed the noble gases could not form compounds at all. Then in 1962, Neil Bartlett at the University of British Columbia in Vancouver made the first one, a compound of xenon, using a substance containing fluorine. Even the most "noble" atoms could not resist the most reactive halogen.
Your turn
1Predict or NR: Cl₂(g) + KI(aq) →
Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(aq). Chlorine is above iodine.
2Predict or NR: Br₂(l) + NaCl(aq) →
NR. Bromine is below chlorine.
3Predict or NR: Br₂(l) + CaI₂(aq) →
Br₂(l) + CaI₂(aq) → CaBr₂(aq) + I₂(aq). Bromine is above iodine; Ca²⁺ keeps two halide ions.
4Why does a halogen's reactivity decrease down the group?
The atoms get larger, so the outer energy level is farther from the nucleus and more shielded. The nucleus attracts an extra electron less strongly.
5In the metal-ranking lab, give the independent, dependent and two controlled variables.
Independent: which metal is in which solution. Dependent: whether a reaction is seen. Controlled (any two): concentration, volume, metal size, cleanliness, temperature, time.
6Why should copper(II) sulfate never go down the drain?
It is toxic to aquatic life; it must go into a labelled waste container for proper disposal.
More practice: warm-up
1List the halogens in order of reactivity, most reactive first.
2What charge does a halide ion have?
3Is bromine a solid, liquid or gas at room temperature?
A liquid (it is one of only two elements that are liquids at room temperature, the other being mercury).
4What does WHMIS stand for?
Workplace Hazardous Materials Information System.
5What is the purpose of a controlled variable?
To make sure only the independent variable could have caused the change in the dependent variable (a fair test).
More practice: on your own
1Predict or NR: fluorine + aqueous potassium chloride (thought experiment).
F₂(g) + 2KCl(aq) → 2KF(aq) + Cl₂(g). Fluorine is the most reactive halogen. (In practice fluorine also reacts with water.)
2Predict or NR: iodine + aqueous magnesium bromide.
NR. Iodine is below bromine.
3A student mixes chlorine water with potassium bromide solution and adds a few drops of a non-polar solvent. What colour should the solvent layer turn, and why?
Orange. Chlorine displaces bromide, forming Br₂, which dissolves in the non-polar layer and looks orange.
4In the metal lab, a student's magnesium did not react with zinc sulfate after 1 minute. Suggest two reasons and how to fix them.
The magnesium may have an oxide coating (sand it first), or the reaction is slow and 1 minute is too short (observe for longer, for example 5 to 10 minutes). Concentration that is too low could also slow it.
5Write a hypothesis, in "If..., then..., because..." form, for testing whether chlorine, bromine and iodine solutions displace each other's halide ions.
If each halogen solution is added to solutions of the other halide ions, then chlorine will displace bromide and iodide, bromine will displace only iodide, and iodine will displace neither, because halogen reactivity decreases down group 17 (the larger atoms attract electrons less strongly).
Challenge
1Bromine is extracted from brines (very salty water containing bromide ions) by bubbling chlorine through them. Write the net ionic equation and explain why the reverse would not work.
Cl₂(g) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq). Chlorine is above bromine in the halogen series, so it takes electrons from bromide ions. The reverse (bromine taking electrons from chloride ions) does not happen, because bromine attracts electrons less strongly than chlorine.
2A student tested metals P, Q, R and S in each other's nitrate solutions. P reacted in 0 solutions, Q in 3, R in 1, and S in 2. Rank them and explain. Which metal ion solution would be best for showing that all the others react?
Q > S > R > P (more reactions means more reactive). A solution of P's ions would be best, because every other metal is more reactive than P and should displace it.
3Explain why a halogen's place in the series is the reverse of what you might expect from a metal's position in its group (metals get more reactive down a group).
Metals react by losing electrons. Larger atoms down a group hold their outer electrons less tightly, so they lose them more easily and are more reactive. Halogens react by gaining electrons. Larger atoms attract an extra electron less strongly, so they gain electrons less easily and are less reactive. The same size trend causes opposite reactivity trends.
Quiz
1. Which halogen is the most reactive?
2. Which reaction will occur?
3. The products of Br₂ + 2KI are:
4. In the metal-ranking lab, which is the dependent variable?
5. Why are metal pieces sanded before the lab?
6. Halogen reactivity decreases down group 17 because the atoms:
This lesson has two parts: a short one on halogens and a longer one on planning an investigation, a skill used in every science course.
How to help: ask your teen to explain the difference between independent, dependent and controlled variables using the home activity. A good plan should let someone else repeat the experiment exactly.
Common difficulties:
- Forgetting the coefficient 2 on the halide compound (because halogens are diatomic).
- Confusing the halogen series direction with the metal activity series (both are "top displaces bottom", but the reasons are opposite: metals lose electrons, halogens gain them).
- Listing "human error" as a source of error. Encourage specific sources (oxide coatings, short observation time).
Safety: halogens are never used at home. The home activity uses vinegar, salt and copper from pennies only; do not taste the solution, wash hands, and handle nails carefully.