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Chemguide: Core Chemistry 14 - 16 The reactions between metals and acids This page shows how the position of a metal in the reactivity series affects its reactions with common dilute acids such as hydrochloric acid or sulfuric acid. Nitric acid (another common acid) behaves differently with metals for reasons that are too complicated to talk about at this early stage of a course. Its reactions with metals are rarely covered at 14-16 year-old level. It would be helpful if you have read the previous page in this sequence about the reactions of metals with water or steam before you go on. The position of hydrogen in the reactivity series
The overall pattern of reactivity
As with the reactions with water, there are odd cases where the reactivity isn't quite what you would expect. We will talk about those as we go along. Reactions of the individual metals Potassium, sodium and lithium These are too dangerous to react with dilute acids. You will know how reactive they are with cold water - their reactions with acids would be far more violent than that. Calcium It is safe to carry out the reaction between calcium and hydrochloric acid as long as the acid is very dilute. The next piece of video talks about the acid being "0.5 molar". Molarity is a measure of concentration. Most dilute hydrochloric acid found in school labs will be either 1 molar or 2 molar. So the acid being used here is more dilute than usual.
You get a colourless solution of calcium chloride formed together with hydrogen gas. The reaction with dilute sulfuric acid is more complicated, because calcium sulfate which is formed is only very slightly soluble in water.
The effect of that is that you get a coating of insoluble calcium sulfate formed around the calcium which quickly stops the reaction. This is why I have included state symbols in the last two equations. The state of the salt formed (calcium chloride or calcium sulfate) matters. The other metals There are two useful bits of video which follow on from each other - I'm not really sure why they didn't simply edit the second one on to the end of the first one. The first one shows reactions of dilute hydrochloric acid with Mg, Al, Zn, Fe, Pb and Cu. In each case the metal is present as a foil rather than a powder. That stopped abruptly, and the next video completes it, starting a few minutes later. And finally, here is what happens if you treat lead with moderately concentrated hydrochloric acid. "6M HCl" is an abbreviation for 6 molar hydrochloric acid. Concentrated hydrochloric acid is 10 molar. Magnesium is definitely the most reactive, and the tube will get quite hot. It is an exothermic reaction - one in which heat is given off. The extra heat also makes it go faster, and so the reaction accelerates. Hydrogen and a colourless solution of magnesium chloride are produced.
The reaction with dilute sulfuric acid doesn't look any different. In this case, magnesium sulfate and hydrogen are produced.
As you have seen on the videos, aluminium is very slow to start because ot its coating of aluminium oxide. Once the acid has broken through that, the reaction is very fast, especially if you use powdered aluminium. Hydrogen is produced together with a colourless solution of aluminium chloride. Sulfuric acid behaves similarly. Again, hydrogen is produced together with a colourless solution of aluminium sulfate. Zinc reacts steadily with both acids to give colourless solutions of zinc chloride or zinc sulfate together with hydrogen. Traditionally, the reaction between zinc and dilute sulfuric acid has been used as a way of making hydrogen gas in the lab. Iron has a slower reaction with both acids to give iron(II) chloride or iron(II) sulfate and hydrogen. If the reaction went to completion, you would get very pale green solutions formed. (But few people normally have the patience for that to happen!) | |||||||||||||||||||||||||||||||||||||||||||
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Note: I haven't given any equations for the reactions of aluminium, zinc or iron with dilute hydrochloric or dilute sulfuric acid. That's because I want you to work them out yourself. Write them down and check your answers by following this link. Include the state symbols. Don't short-cut this - you have to be able to write equations for simple reactions. | |||||||||||||||||||||||||||||||||||||||||||
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Lead has no noticeable reaction with either of the dilute acids because lead(II) chloride and lead(II) sulfate are insoluble in water. Any reaction which occurs will coat the lead with insoluble lead(II) chloride or lead(II) sulfate and stop any further acid getting at it. The reason it reacts with concentrated hydrochloric acid is because of a further reaction between the lead(II) chloride and excess chloride ions in the acid to give a soluble complex ion, [PbCl4]2-, and so the surface of the lead is kept clean. You don't need to know about this at this level. It does, however, show that lead is above hydrogen in the reactivity series, otherwise there would have been no reaction at all. There is no reaction between dilute hydrochloric acid or sulfuric acid and the metals below hydrogen in the reactivity series.
© Jim Clark 2020 |
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CaCl2(aq) + H2(g)