الأسبوع 1 · الوحدة الأولى «التفاعلات الكيميائية وآثارها البيئية» (افتتاحية) والدرس الأول «أنواع التفاعلات الكيميائية» (١): الاتحاد المباشر والانحلال وتفاعل الفلزات مع الأكسجين والأحماض والماء ومتسلسلة النشاط الكيميائي · 3 دروس
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Metals and acids: the reactivity series
Science Prep 3 Term 1, Unit 1: pp. 1–8
GoalUse the reactivity series to decide which metals replace hydrogen from a dilute acid.
New words
reactivity series · a list of metals from the most reactive to the least reactivesingle replacement · a reaction in which one element takes the place of another in a compound: A + BC → AC + Bdilute acid · an acid mixed with a lot of water, so it is weak enough to be used safely in a laboratory
Explanation
The reactivity series lists metals from the most to the least reactive: K, Na, Ca, Mg, Al, Zn, Fe, Pb, then hydrogen, then Cu, Ag, Au. Hydrogen is not a metal, but it is placed in the list as a marker. A metal above hydrogen replaces the hydrogen of a dilute acid, so bubbles of hydrogen gas appear: Mg + 2HCl → MgCl₂ + H₂. The higher the metal, the faster the bubbles. In the laboratory zinc gives bubbles faster than iron, and copper gives none, because copper is below hydrogen. Aluminium is above zinc but reacts slowly at first, because a thin layer of its oxide protects it. Hydrogen is flammable, and in a test tube it burns with a small pop, but this is the clue to read about, not a test to repeat. Careful: reactivity is how readily a metal reacts, not how hard or heavy it is. Also, silver and gold do not replace hydrogen from dilute acids, but that does not make them unreactive in every way.
Metal
Place in the series
With dilute acid
Mg
high
fast bubbles
Zn
above H
bubbles
Fe
above H, lower
slow bubbles
Cu
below H
no reaction
Worked examples
Zn→Fe→Pb→H→Cu→Ag
Will lead (Pb) release hydrogen from dilute acid? Pb is above H in the series, so yes, although slowly, because it is close to hydrogen. Will silver (Ag)? It is below H, so no.
Four test tubes hold the same dilute acid. Metal strips of equal size, magnesium, zinc, iron and copper, are put in one at a time. The order of bubble speed is Mg, then Zn, then Fe, and nothing for Cu: the same as the order of the series.
A new aluminium strip shows almost no bubbles for the first minute in acid, then many. Aluminium is high in the series, so why the delay? Its protective oxide layer must first be attacked by the acid; then the metal underneath reacts.
Class activity
Reactivity card game. Write the metals K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au on twelve cards and shuffle. Race to put them in the order of the series, then turn over the cards that come after H and say what they do with dilute acid. For a safe home test, a grown-up cleans a dull copper coin: leave it for a few minutes in a cup with a pinch of salt in vinegar, no tasting, rinse and dry. Describe the colour change. The acid reacts with the dull surface layer, not with the shiny copper below.
Practice
1
Which of Mg, Cu, Zn releases no hydrogen from dilute acid?
Cu, because it is below hydrogen.
2
Write the reaction of magnesium with hydrochloric acid and name the gas.
Mg + 2HCl→MgCl2 + H2
Mg + 2HCl → MgCl₂ + H₂; the gas is hydrogen.
3
Zinc and iron strips of equal size are put in the same acid. Which gives bubbles faster, and why?
Zinc, because it is higher in the series, so more reactive.
4
A friend says gold is useless because it does not react with acid. What is wrong with this?
Not reacting with dilute acid says only that gold is below hydrogen; it does not say whether the metal is useful.
