In your study of chemistry to date, you may have assumed that all reactions are spontaneous. This is not the case. Mixing two reactants together does not necessarily mean that a reaction occurs. Consider single replacement reactions in this context.

First, realize the following:

1. Metals differ in their ability to reduce metallic ions.
   
   
2. Metallic ions differ in their ability to oxidize metals

Metals that are more likely to react with the ions of other metals are considered more reactive or stronger reducing agents.

Metallic ions that are more likely to react with other metals are considered stronger oxidizing agents.

If two reactants are mixed in a reaction vessel, is it possible to determine whether a redox reaction is spontaneous? A spontaneous chemical reaction occurs without a continuous input of energy.

Spontaneous redox reactions in solution generally provide empirical (observable) evidence of a reaction within a few minutes. Evidence for a spontaneous chemical reaction can take many forms, including the following:

• colour change
• pH change
• formation of gas bubbles
• formation of a precipitate
• temperature change

To interpret colour change, use the Table "Colours of Common Aqueous Ions" on page 11 of the Chemistry 30 Data Booklet. To test for pH change or to verify the presence of certain substances, refer to Table 1 on page 805.

If no reaction evidence is observed, the reaction is said to be non-spontaneous. In other words, the combination of reactants does not lead to a reaction.

Being able to list spontaneous empirical evidence for redox reactions is an important skill. Study the following examples:

Example 1

In the Lesson 1.1 video, you observed the following reaction:

\( \mathrm { Cu(s) + 2 AgNO_3(aq) \rightarrow Cu(NO_3)_2(aq) + 2 Ag(s) } \)
\( \mathrm { Cu(s) \rightarrow Cu^{2+}(aq) + 2 e^- } \)	oxidation half-reaction
\( \mathrm { Ag^+(aq) + 1e^- \rightarrow Ag(s) } \)	reduction half-reaction

We can conclude that this reaction is spontaneous because of the following empirical data:

• Silver is deposited on the copper wire.
  
  
• Solution becomes blue as the concentration of copper ions increases. (Refer to the Table "Colours of Common Aqueous Ions" on page 11 of the Chemistry 30 Data Booklet.)

Example 2

\( \mathrm { Ni(s) + 2 H^+(aq) \rightarrow Ni^{2+}(aq) + H_2(g) } \)
\( \mathrm { Ni(s) \rightarrow Ni^{2+}(aq) + 2e^- } \)	oxidation half-reaction
\( \mathrm { 2 H^+(aq) + 2e^- \rightarrow H_2(g) } \)	reduction half-reaction

We can conclude that this reaction is spontaneous because of the following empirical data:

• Solid nickel decreases in mass as it is oxidized.
  
  
• As nickel(II) ions form, a blue-green colour appears (see Learning Tip).
  
  
• pH increases as hydrogen ions are reduced.
  
  
• Gas bubbles are observed.