Передбачення реакцій за рядом активності

My house is almost 50 years old. In the 1970s they began using copper pipes for plumbing. Copper is a transition metal that has low reactivity with water. Pipes made with copper metal can last for more than 50 years. Unfortunately, they do eventually corrode. The corrosion is caused by the reaction with hypochlorous acid. So, today most plumbing is polyvinyl chloride (PVC). PVC is resistant to water, acids, and bases. In theory, it will never corrode.

How do we know that copper or PVC is a good choice for pipes? As I said, they have low reactivity. But, how do we know what elements have low reactivities? We use an activity series!

What is an activity series? (Easy Answer)

Activity series list elements by their reactivity. More reactive elements are on top. The least reactive are lower.

An activity series is a helpful tool. We can answer complicated questions. For example, alkali metals are more reactive than alkaline earth metals. But is Radium or lithium more reactive? Radium is the most reactive alkaline earth. But, lithium is the least reactive alkali. With only a periodic table, we cannot predict their reactivity. An activity series gives us the answer!

If you are just learning about reactions, you only need the easy answer. Skip the next section. For thermodynamics and electrochemistry, read the next section.

What is an activity series? (Complex Answer)

Unfortunately, activity series have problems. They tell us what elements are more reactive. But, what is reactivity? It has two distinct components: thermodynamics and kinetics.

Thermodynamics tells us if a reaction happens. Gibb's free energy gives us spontaneity. But, it only uses the changes in enthalpy and changes in entropy. So, it's a state function. But, reaction rates depend on the intermediates. It's a path function. Reactivity needs to balance both. For example, what if a reaction is spontaneous but slow?

Let's think of an example. Diamond and graphite are allotropes. The Gibbs free energy for the conversion of diamond to graphite is negative. So, it's a spontaneous reaction. But, it takes over a billion years to convert just one cubic centimeter. So, we could say that diamond is not very reactive even though the reaction is thermodynamically favorable.

So, how do we reconcile different series? We look at the conditions. For example, electrochemistry has standard conditions. Standard electrode potentials (basically an activity series) use aqueous reactants. So, we use 1.0 molar concentrations. They are also at atmospheric pressure and 298.15 kelvin. Changing any factor may alter the reactivity. The standard potentials use only thermodynamics.

Another option uses a metal's reactivity with water. Certain metals react with different phases. For example, sodium reacts with cold liquid water. But, aluminum needs water vapor. So, sodium is more reactive. The reactions with water account for thermodynamics and kinetics.

How do I use an activity series?

We use activity series with single displacement reactions. We want to know if one metal will displace another. A metal higher up replaces any lower metal. Chemical reactions don't occur between lower and higher elements.

We can also apply these rules to halogens. The upper halogens are more reactive. So, no reactions happen between lower and higher halogens.

The Metal Activity Series

In this activity series, the metals react with water. Use this for single displacement reactions.

$Cs>Rb>K>Na>Li>Ba>Sr>Ca>Mg>Be>Al>Ti>Mn>Zn> \\Cr>Fe>Cd>Co>Ni>Sn>Pb>Sb>Bi>Cu>W>Hg>Ag>Au>Pt$

Cesium ( $Cs$ ) through calcium ( $Ca$ ) react with cold water.

Magnesium ( $Mg$ ) reacts slowly in cold water. But, it is very reactive in boiling water. To react magnesium with water we need to add heat.

Beryllium ( $Be$ ) and aluminum ( $Al$ ) react with steam.

The elements from titanium (Ti) through lead (Pb) react with acids. With steam, the reactions are slow.

Antiony ( $Sb$ ) through platinum ( $Pt$ ) only react with strong acids at high concentrations.

This reactivity series comes from the standard electrodes potentials. Use this for electrochemistry.

$Li>Cs>Rb>K>Ba>Sr>Ca>Na>Mg>Be>Al>Zn>Cr>Fe> \\Cd>Co>Ni>Sn>Pb>Sb>Cu>Ag>Hg>Pt>Au$

With this list, some metals have moved. For example, now Calcium (Ca) is more reactive than sodium (Na). For the reaction with water, calcium has to lose two electrons. So, the reaction kinetics are slower. But, for the electrode potentials, calcium gives up two electrons compared to sodium's one. So, under electrochemistry's standard conditions, calcium is more reactive than sodium.

The Halogen Activity Series

F>Cl>Br>I

Example Problems

Use the metal activity series to determine if these reactions occur.

$2Na_{(s)} + 2H_{2}O_{(l)} \rightarrow 2NaOH_{(aq)} + 1H_{2(g)}$

Yes. Sodium reacts with liquid water.

$2Al_{(s)} + 2H_{2}O_{l} \rightarrow 2AlOH_{(aq)} + 1H_{2(g)}$

No. Aluminum cannot react with liquid water. If the water vaporized, it would react.

$2NaI_{(aq)} + Cl_{2(g)} \rightarrow 2NaCl_{(aq)} + I_{2(g)}$

Yes. Chlorine is more reactive than iodine.

$Mg(OH)_{2(aq)} + Be_{(s)} \rightarrow Be(OH)_{2(aq)} + Mg_{(s)}$

No. Magnesium is more reactive than beryllium. So, beryllium cannot displace magnesium.

$Pt_{(s)} + CH_{3}COOH_{(aq)} \rightarrow Pt(CH_{3}COO) + H_{2(g)}$

No. Platinum is one of the least reactive metals. It only reacts with strong acids at high concentrations. Acetic acid is a weak acid.

$Na_{(s)} + CH_{3}COOH_{(aq)} \rightarrow Na(CH_{3}COO) + H_{2(g)}$

Yes!!! Sodium is incredibly reactive. It reacts with liquid water. Furthermore, less reactive elements need acid. So, the more reactive sodium readily reacts with acids. We would expect a violent reaction.