Why gold stays shiny
Gold jewelry dug up after thousands of years still gleams, while ancient iron tools are crusted with rust. And although aluminum is more reactive than iron, aluminum pans hardly corrode at all. Why do metals behave so differently?
Physical properties of metals
At room temperature most metals are solids (mercury is a liquid). They’re shiny, conduct electricity and heat well, and are malleable and ductile. They also differ: copper is reddish, gold is yellow, and tungsten has the highest melting point of all metals, which is why it’s used for light-bulb filaments.
An alloy合金 is a metal melted together with other metals or nonmetals, such as steel (iron with a little carbon), brass, and stainless steel. Alloys are mixtures, and they’re usually harder and more corrosion-resistant than the pure metals they’re made from.
Chemical properties of metals
1. With oxygen: magnesium and aluminum react with oxygen at room temperature; iron and copper need high temperatures; gold doesn’t react even when heated. Aluminum quickly forms a thin, tough layer of aluminum oxide that protects the metal underneath, so aluminum resists corrosion.
2. With dilute hydrochloric or sulfuric acid: magnesium, zinc, and iron react and release hydrogen gas; copper doesn’t.
3. With some salt solutions: put an iron nail in copper(II) sulfate solution and red copper coats the nail while the blue solution turns pale green. Copper was once extracted this way.
These are all single replacement reactions (lesson 1.6.4).
The activity series
From many experiments like these, chemists ranked common metals from most to least reactive. This ranking is the activity series金属活动性顺序:
How to use it:
- The higher a metal is in the series, the more reactive it is.
- Metals above hydrogen can replace the hydrogen in dilute hydrochloric or sulfuric acid.
- A metal can replace any metal below it from a solution of that metal’s salt. Potassium, calcium, and sodium are exceptions: they’re so reactive that they react with the water first.
Note that iron forms iron(II) compounds in these reactions (, ), and the solution turns pale green.
Pick a metal and a solution, predict whether they react, then check the observations and the equation:
Activity series: most reactive to least reactive
Rusting and how to stop it
Iron rusts only when it’s in contact with both oxygen and water. Rust is crumbly and porous, so it doesn’t protect the iron underneath. To prevent rusting, take away one of the conditions: keep iron clean and dry; coat it with oil, paint, or another metal (galvanized steel is coated with zinc); or change its makeup, as in stainless steel.
Example 1: Ranking iron, copper, and silver
You have iron, copper, and silver wire, plus dilute hydrochloric acid and solutions of iron(II) sulfate, copper(II) sulfate, and silver nitrate. Design a simple test to rank the three metals.
- Strategy. Use the metal in the middle of the three. Copper is in the middle, so work with copper.
- Option 1: two metals, one solution. Put the iron wire and the silver wire into copper(II) sulfate solution. Red copper forms on the iron, so iron is more reactive than copper. Nothing happens to the silver, so silver is less reactive than copper.
- Option 2: one metal, two solutions. Put copper wire into iron(II) sulfate solution and into silver nitrate solution. Copper can’t replace iron but does replace silver.
- Conclusion. Reactivity: iron > copper > silver.
Example 2: Will they react?
Decide whether each pair reacts, and write an equation if it does: (1) zinc and copper(II) sulfate solution; (2) copper and dilute hydrochloric acid; (3) iron and silver nitrate solution.
- (1) Zinc and copper(II) sulfate. Zinc is above copper, so they react: .
- (2) Copper and hydrochloric acid. Copper is below hydrogen, so no reaction.
- (3) Iron and silver nitrate. Iron is above silver, so they react, and iron forms iron(II): .
- Zinc and dilute sulfuric acid
Zinc is above hydrogen.
- Copper and dilute hydrochloric acid
Copper is below hydrogen, so it can't replace the hydrogen in the acid.
- Iron and copper(II) sulfate solution
Iron is above copper: .
- Silver and copper(II) sulfate solution
Silver is below copper.
- Copper and silver nitrate solution
Copper is above silver: .
- Iron and zinc sulfate solution
Iron is below zinc, so it can't replace zinc.
- Magnesium and dilute hydrochloric acid
Magnesium is well above hydrogen and reacts vigorously.
- Copper and solid silver chloride
Silver chloride doesn't dissolve in water, so there's no salt solution for copper to react with.
- Aluminum and copper(II) sulfate solution
Aluminum is above copper: .
Common mistakes
Mistake 1: “Iron and hydrochloric acid make iron(III) chloride.” In single replacement reactions iron forms iron(II), so the product is and the solution is pale green.
Mistake 2: “Aluminum is more reactive than iron, so aluminum objects corrode faster.” Aluminum is covered by a tough layer of aluminum oxide that protects it, so aluminum objects actually resist corrosion well.
Mistake 3: “A more reactive metal can replace a metal from any of its compounds.” The compound has to be dissolved in water. And potassium, calcium, and sodium react with the water instead of replacing the other metal.
Quick check
Which reagent can NOT be used to compare the reactivity of zinc and copper?
Zinc and copper are both below magnesium, so neither reacts with magnesium sulfate and you learn nothing. In A zinc reacts but copper doesn't; in B zinc replaces copper; in C copper can't replace zinc. All three show zinc is more reactive.
An iron nail is placed in copper(II) sulfate solution. What do you see after a while?
: red copper forms, and iron(II) sulfate solution is pale green.
Summary
- Metals are shiny, conduct electricity and heat, and are malleable and ductile. Alloys are usually harder and more corrosion-resistant than pure metals.
- Activity series: K Ca Na Mg Al Zn Fe Sn Pb (H) Cu Hg Ag Pt Au.
- Metals above hydrogen replace hydrogen in dilute acids; a metal replaces any metal below it from a solution of its salt (except K, Ca, Na). Iron forms iron(II).
- Iron rusts only with both oxygen and water; oil, paint, metal coatings, and stainless steel prevent rusting.
On to AP (for later)
Metal replacement reactions are electron transfers: loses electrons to become , and gains electrons to become . AP topic 4.9 analyzes these redox reactions with oxidation numbers, and the table of standard reduction potentials in Unit 9 is a quantitative version of the activity series, used to build galvanic cells and calculate their voltages.