2.5 Reactivity Series
We briefly discussed about the reactivity series in the "Principles of Chemistry section' when we talked about electrochemistry. We will learn about how this series affects reactions involving metals and we will look at a few examples of such reactions.
A lot of the metals in the periodic table are very reactive. You could describe them as competitive species. The reactive series is just a representation of which metals are strong and capable of winning a reaction when competing with other metals in the series.
As you can see potassium is the most competitive metal in the series. It will do anything to form a compound with the element or molecules it reacts with. In fact, even if the element or molecule has formed a compound with another metal, potassium will force away the metal from the compound so that it can take its place. The new compound formed is more stable; therefore, more difficult to separate.
Before moving on we are going to have to make a quick change to the reactivity series above. Carbon and hydrogen, even though they are not metals, should be included because they are important when dealing with reactions involving metals. Since hydrogen is already included we just have to include carbon. Where do you think carbon will fit in?
Carbon comes in between aluminium and zinc. Carbon and hydrogen are included in the reactivity series because they can be used to extract metals from their metallic compounds or oxides. For example if you react zinc oxide with carbon, carbon will take the place of zinc and release it from its compound because carbon is more reactive than zinc. Similarly, if you react a copper compound with hydrogen or carbon, copper will be displaced and released from its compound.
These types of reactions are known as displacement reactions.
A displacement reaction is a reaction where a reactive element will take the place of a less reactive element that has already formed a compound. For example, assume Mg, greyish in color, was reacted with CuO [Copper (II) Oxide], black in color, by heating them together. The greyish black mixture will turn white. Since Mg is more reactive than Cu, Mg will take the place of CuO ( blackish powder) and form the new compound MgO (white powder).
Such reactions happen because a highly reactive element hates to exist by itself. It always wants to react with another element to form a stable compound that will not break down easily. This is why you cannot find reactive elements in their pure forms in nature. They are always found bonded to other elements and compounds. Therefore, extracting these elements are difficult and expensive.
What happens in the above example is that Mg pulls away the Cu atom from the CuO compound. It is able to do this because it is much stronger than Cu in terms of reactivity. It then bonds with oxygen to form MgO. Cu has no choice but accept defeat and exist in its elemental form for that particular system. Furthermore, if you were to introduce Ca into this system, then Mg will concede and allow Ca to bond with oxygen to form CaO (calcium oxide). But if you were to add gold instead, nothing will happen since it's the weakest in terms of reactivity.
The chemical equation for the example above is shown below:
You might have noticed by now that we could use this reaction to extract metals from their oxides. Metals are usually found in the form of oxides in nature. For example iron is naturally found in iron ore . Iron ores mostly contain iron (III) oxide. If you could look at the reactivity series it is evident that we could heat up iron ore along with zinc to isolate and extract iron from the iron oxide. But is using large amounts of zinc, another useful metal, for this purpose feasible?
Well, it's not because zinc is also more useful in its pure form since it can be used to reduce rusting of iron. We will talk about this later on in this section. We can conclude than using zinc for extraction of iron is expensive and not feasible. This is where carbon, a non-metal, comes into play. If you look at the reactivity series, you can see that carbon is more reactive than iron. Therefore, heating the iron ore along with carbon is a cheaper method and more efficient method of extracting iron.
Lets look at a couple more examples of such reactions. To understand these reactions it is important that you understand what redox reactions are. If you cant remember or dont understand redox ractions then I suggest you read 'Redox Reactions' chapter. The link is on the left hand side.
Zinc and Copper
Zinc is more reactive than copper; therefore, it will always displace copper. This can be proven by observing the reaction between zinc and copper (II) sulfate solution. The chemical equation is shown below:
Copper (II) sulfate is a blue solution. It is blue because copper forms complex ions that absorb red light and reflect blue light. Complex ions was discussed in the 'Transition Metals' chapter.
When we add zinc to a blue copper (II) sulfate solution the blue color disappears to form a colorless solution. This means that copper (II) sulfate does not exist anymore. Due to zinc's electron configuration, zinc does not form colored compounds unless the molecules attaching onto zinc are already colored. Therefore, we can conclude that the colorless compound is definitely zinc sulfate as shown in the equation above.
In the previous example it was clear which reactant was being oxidized and which was being reduced because there was a clear transfer of oxygen between compounds (Mg gained oxygen and Cu lost oxygen). But in this example we cannot clearly see such an exchange. Therefore, we will have to rewrite the equation in the ionic form so that we can determine how the electrons are gained and lost between the reactants. The ionic equation is shown below:
We can cancel out the sulfate ions since they are on both sides of the equation and are spectator ions. After cancelling them out, we get the final equation in terms of only zinc and copper which makes our job to identify the oxidant very easy.
As you can see zinc loses electrons to become a cation and copper gains electrons to lose its positive charge. We discussed earlier that atoms become ions because the ion form is more stable. In this case since zinc is the more reactive element, it has the power to choose its stable ionic form whereas copper will have to obey zinc and exist in its uncharged state.
Therefore, zinc is oxidized and copper is reduced. For zinc to be oxidized, it has to reduce copper which means that zinc is the reducing agent. For copper to be reduced, it has to oxidize zinc which means copper is the oxidizing agent.
Copper and Silver Nitrate
Copper is more reactive than silver; therefore, you can expect copper to switch its role from oxidizing agent to reducing agent compared to the above example. The chemical equation is shown below:
Silver nitrate is a colorless solution. But when copper is added to this solution the solution starts turning blue and you can start seeing greyish deposits in the solution. This happens because copper displaces silver to form copper (II) nitrate solution which is blue. We can study the ionic equation for the reaction to determine the oxidizing agent and reducing agent.
Since nitrate is a spectator ion, it can be canceled out. If you look at the final equation, it is clear that copper is loosing electrons and getting oxidized which means that it is the reducing agent. Vice versa for silver.
Metals and Water (Hydrogen)
Water is made up of hydrogen and oxygen. If we look at the reactivity series we can see that hydrogen is in the lower spectrum of reactivity. If the metals that are more reactive than hydrogen react with water or steam, then the hydrogen will be displaced from the water molecules and released as gas.
If the metal reacts with cold water then not all hydrogens will be displaced from the water molecule. Only one of the two hydrogen atoms will be released. Lets look at the chemical equation for calcium reacting with cold water.
As you can see the water molecule has broken down to a hydroxide (OH) anion and hydrogen cation. Therefore, you get a metal hydroxide as a product. However, if the metal reacts with steam, the product is most likely to be a metal oxide rather than a metal hydroxide. This because the heat in steam will cause the hydroxide molecule to break down further and release the remaining hydrogen. The chemical equation for reaction with steam is shown below:
If the metal is less reactive relative to hydrogen then no reaction will happen with water. This is why copper pipes are usually used for transporting steam in heating systems. It does not react or break down when exposed to water at high temperatures.
One could argue that iron is the most important metal for humans. It is used to make all types of steel and other alloys, is an essential material used in construction and is vital in the human biology. However, it has one huge drawback. It rusts very easily. That is, it reacts with oxygen in air to from layers of brown flaky powder. This basically weakens the metal and deteriorates its structural integrity. Therefore, finding methods to reduce and prevent rust is very important.
Rust forms when iron is exposed to water and oxygen. The chemical equation is shown below:
The equation above shows rust to be a iron (III) hydroxide. This is the hydrated form of rust. This hydroxide undergoes dehydration to form the oxides that you see as the orange reddish powder coating iron. When the iron hydroxide undergoes dehydration it forms either FeO, [aka ferrous oxide or iron (II) oxide] or Fe2O3, [ferric oxide or iron (III) oxide].
The straight forward method to prevent rust is to paint or coat the metal in oil or plastic. This will limit the contact of the metal surface with water or air. Even though this method is inexpensive and fairly straight forward, its sucess entirely depends on the interety of the coating. Even if the coating gets damaged in a small area, the enitre metal is compromised and will rust entirely if the damage is not repaired
This problem can be solved if we coat iron with a thin layer of zinc. A zinc coated iron is known as galvanized iron. Since zinc is more reactive than iron, zinc will corrode instead of iron even if the coating is damaged.
Coating iron in zinc or any other more reactive element can be expensive especially if the structure being coated has a large surface area like ship hulls and long pipelines. In these instances rather than coating the surface, blocks of the more reactive metal can be attached to the iron surfaces. This is known as sacrificial protection since the metal blocks will corrode instead of the ship metal.