Spontaneous Redox Reactions: Study with Video Lessons, Practice Problems & Examples

Redox reactions occur when one element displaces another in a compound, indicating reduction. The activity series chart ranks elements by their ability to displace others; higher elements can displace lower ones. For example, sodium can displace zinc due to its higher position. The strongest reducing agents, like lithium, have a greater tendency to lose electrons, while the weakest oxidizing agents, like gold, are less likely to undergo reduction. Remember, an oxidizing agent is reduced, and a reducing agent is oxidized in these reactions.

Spontaneous Redox Reactions Concept 1

Redox reactions are spontaneous when an element can successfully displace another element within a compound. And when we say the term "displace," that means to remove an element from its compound, thereby reducing it. So to displace an element means I'm actually reducing that element. And we're going to say here we utilize what's called an activity series chart to determine if an element can displace another element. Now what we're going to say here is we're going to say an element that is higher in the activity series will displace an element lower in the activity series chart. And what we need to also remember is that recall an oxidizing agent is reduced, and a reducing agent will be oxidized. So if we take a look here at this activity series chart, we have lithium, potassium, calcium, all the way down to gold down here. And what we can say here is up near lithium, that's where we have the strongest reducing agent, which means that we have the greatest propensity for oxidation. So we want to be oxidized a lot. And up here is where we have the weakest, by default, the weakest oxidizing agent, which means reduction does not want to happen up here. Okay. So we're going to say at this end of the activity series chart, this is the greatest tendency to lose electrons. If we do the opposite end, down here we have the weakest reducing agent, which means that we have the least likely to be oxidized. So oxidation is very weak here. And down here we have the strongest oxidizing agent, which means reduction really wants to happen down here. So down here on the activity series chart is where we have the greatest tendency to gain electrons. So just remember, when looking at this activity series chart, an element that's higher up will displace another element that's below it. So if we had sodium, it could displace zinc from another compound. Why? Because sodium is higher up than zinc is.

Spontaneous Redox Reactions Example 1

Based on the activity series chart, determine if the following reaction represents a spontaneous redox reaction. Basically, we're looking to see if calcium can displace this silver. And we know that we're trying to compare those two because they're the elements on the activity series chart mentioned earlier. Now, take a look. Is calcium higher up than silver? If it is, then we can successfully displace the silver, essentially kicking it out. So, if you look, calcium is indeed higher up. Right? So, step 1, we locate the monoatomic element on the activity series chart. We found out where both are located. And remember, step 2, if it is higher on the activity series chart, it will displace the element within the nearby compound. So, silver is going to get kicked out, metals by themselves exist as solids at room temperature except for mercury. Plus, now calcium and chlorine are going to combine together to give us our new ionic compound. Remember, calcium is in group 2A, so it has a 2+ charge. Chlorine is in group 7A, so it has a 1− charge. We crisscross the numbers to give us our ionic compound, which would come out to be CaCl₂, and here we can make it a solid if we want. The states are not that important. What's important is that calcium can successfully displace silver within this redox reaction, and as a result, it is spontaneous. Now, if we wanted to balance this, we'd say we have 2 chlorines here and only 1 here. So, I put a 2 here, and then as a consequence, I'd also have to put a 2 here for this silver. So these would be our coefficients for our balanced redox reaction: 1:2:2:1 as the coefficients.

Which element is the best reducing agent?

• A. Manganese
• B. Aluminum
• C. Lithium
• D. Nickel
• E. Chromium

Determine whether which of the following redox reactions will occur spontaneously in the forward direction?

a) Ni(s) + Zn²⁺(aq) → Ni²⁺(aq) + Zn(s)
b) Fe(s) + Pb⁴⁺(aq) →   Fe²⁺(aq) + Pb(s)
c) Al(s) + Ag⁺(aq) →  Al³⁺(aq) + Ag(s)
d) Pb(s) + Mn²⁺ (aq) → Pb²⁺(aq) + Mn(s)

• A. Only b)
• B. Both b) and c)
• C. a), b) and c)
• D. Both c) and d)

Suppose you wanted to cause Ni²⁺ ions to come out of solution as solid Ni. Which metal could you use to accomplish this?

• A. Au
• B. Sn
• C. Mn
• D. Ag
• E. Cu

Here’s what students ask on this topic:

What is a spontaneous redox reaction?

A spontaneous redox reaction is a chemical reaction where one element displaces another element in a compound, leading to the reduction of the displaced element. This occurs without the need for external energy input. The spontaneity of the reaction is determined by the activity series chart, which ranks elements based on their ability to displace others. Elements higher in the activity series can displace those lower in the series. For example, sodium (Na) can displace zinc (Zn) from a compound because sodium is higher in the activity series. In these reactions, the oxidizing agent is reduced, and the reducing agent is oxidized.

How does the activity series chart determine the spontaneity of redox reactions?

The activity series chart ranks elements based on their ability to displace other elements in compounds. Elements higher in the chart can displace those lower in the chart, indicating a spontaneous redox reaction. For instance, lithium (Li) is at the top of the chart and is a strong reducing agent, meaning it has a high tendency to lose electrons and undergo oxidation. Conversely, elements like gold (Au) at the bottom are weak reducing agents and strong oxidizing agents, meaning they are more likely to gain electrons and undergo reduction. Thus, the position of elements in the activity series chart helps predict the spontaneity of redox reactions.

What is the role of reducing and oxidizing agents in redox reactions?

In redox reactions, a reducing agent donates electrons and is itself oxidized, while an oxidizing agent accepts electrons and is itself reduced. The reducing agent undergoes oxidation, losing electrons, and the oxidizing agent undergoes reduction, gaining electrons. For example, in the reaction between sodium (Na) and zinc (Zn), sodium acts as the reducing agent and loses electrons, while zinc acts as the oxidizing agent and gains electrons. The activity series chart helps identify which elements are strong reducing or oxidizing agents, thereby predicting the direction and spontaneity of redox reactions.

Why is lithium considered a strong reducing agent?

Lithium (Li) is considered a strong reducing agent because it is at the top of the activity series chart, indicating a high tendency to lose electrons and undergo oxidation. This means lithium readily donates electrons to other elements, reducing them in the process. The high reactivity of lithium is due to its low ionization energy, which makes it easier for lithium atoms to lose their outermost electron. As a result, lithium is highly effective in displacing other elements from their compounds, making it a strong reducing agent in redox reactions.

How can you predict if a redox reaction will occur using the activity series chart?

To predict if a redox reaction will occur using the activity series chart, compare the positions of the elements involved. An element higher in the activity series can displace an element lower in the series from its compound, indicating a spontaneous redox reaction. For example, if you have a compound containing zinc (Zn) and you introduce sodium (Na), the reaction will occur because sodium is higher in the activity series than zinc. This means sodium will displace zinc, leading to the reduction of zinc and oxidation of sodium. The activity series chart thus helps determine the feasibility of redox reactions.