Metals with Water

Reactions of Metals with Water

Potassium with Water

Let's start with an equation for the reaction of potassium with water (which is also in your workbook in example 10a). This equation is not balanced. We could balance it now, but that won't be necessary for our purposes at the moment. The reaction between potassium and water results in the formation of potassium hydroxide and hydrogen gas.

K + H₂O KOH + H₂

Because potassium hydroxide is soluble in water, we can write it in the ionic form. So this equation does a better job of showing what actually happens. The potassium reacts with the water to form potassium ion and hydroxide ion and also hydrogen gas.

K + H₂O K⁺ + OH^-+ H₂

Let us consider what actually happened in this case. The potassium loses electrons. What we were studying last term was how readily potassium lost electrons compared to other metals. But now let's look at where the electron goes. The hydrogen in the water is what takes the electron. So, the metal loses electrons and the hydrogen gains electrons.

This is the essential quality of oxidation-reduction reactions: one chemical element loses electrons and another gains them. Another way of saying this is that electrons are transferred from one element to another. Note that oxidation and reduction must happen both at the same time. When an electron leaves something, it goes to something else. In a redox reaction the electrons are transferred from one chemical to another.

Let's take a look at potassium's half of the reaction. What happened to the potassium? It changes from potassium metal K to potassium ion K⁺.

K K⁺

In doing so, it gives off an electron. This is a better way of showing what happens to the potassium because it shows that charge is conserved. The electron doesn't just disappear. The equation shows that it's still around but has separated from the rest of the potassium atom.

K K⁺ + e^-

Next, what is happening to the hydrogen? Here is its half of the reaction. The water changes from H₂O to OH^-and gives off hydrogen gas H₂.

H₂O OH^-+ H₂

This form of the equation balances the atoms involved in the reaction. But notice that the charge is not the same on both sides even though the total number of atoms is.

2 H₂O 2 OH^-+ H₂

In order for this reaction to take place, two electrons have to combine with the water in order to form the two hydroxide ions and hydrogen gas. This is what's shown here. This is hydrogen's half of the reaction.

2 e^-+ 2 H₂O 2 OH^-+ H₂

The potassium reacts to give off electrons, so that is an oxidation reaction.

K K⁺ + e^-

The hydrogen in the water is gaining electrons so that is a reduction reaction.

2 e^-+ 2 H₂O 2 OH^-+ H₂

The number of electrons gained by the water must equal the number of electrons lost by the potassium. Consequently two potassium atoms must react to provide the two electrons needed by the water.

2 K 2 K⁺ + 2 e^-
2 e^- + 2 H₂O 2 OH^-+ H₂

When these two half-reactions are added together, the electrons cancel out and we get the complete balanced equation for this redox reaction.

2 K + 2 H₂O 2 K⁺ + 2 OH^-+ H₂

The same kinds of equations can be written for any reaction in which a metal reacts with water.

Practice

Try your hand at writing these kinds of equations by considering the reactions that the metals Na and Ca have with water. From their position on the periodic table, you can tell that Na and Ca form the ions Na⁺ and Ca²⁺. Check your answers below before you continue with the other pages.

Answers

Because sodium, like potassium, loses one electron per atom, the equations are nearly identical to the equations above.

2 Na 2 Na⁺ + 2 e^-
2 e^- + 2 H₂O 2 OH^-+ H₂

2 Na + 2 H₂O 2 Na⁺ + 2 OH^-+ H₂

Because calcium atoms lose two electrons, the equations are slightly different. By the way, because calcium hydroxide is an insoluble compound the calcium and hydroxide ions will combine together to form a precipitate.

Ca Ca²⁺ + 2 e^-
2 e^- + 2 H₂O 2 OH^-+ H₂

Ca + 2 H₂O Ca²⁺ + 2 OH^-+ H₂

Ca + 2 H₂O Ca(OH)₂(s) + H₂

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