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

Redox reactions occur spontaneously when a higher activity series element displaces a lower one, effectively reducing it. The activity series chart ranks elements by their tendency to lose electrons, with lithium and potassium as strong reducing agents. An oxidizing agent is reduced, while a reducing agent is oxidized. Understanding this hierarchy is crucial for predicting reaction outcomes, such as sodium displacing zinc due to its higher position in the activity series. This concept is foundational in electrochemistry and helps in analyzing oxidation-reduction reactions.

Spontaneous Redox Reactions Concept 1

Redox reactions are spontaneous when an element can successfully displace another element within a compound. When we say the term 'displace', that means to remove an element from its compound and thereby reducing it. So to displace an element means I'm actually reducing that element. We utilize what's called an activity series chart to determine if an element can displace another element. We say an element that is higher in the activity series will displace an element lower in the activity series chart. It's also important to recall that an oxidizing agent is reduced and a reducing agent will be oxidized. If we take a look here at this activity series chart, we have elements like lithium, potassium, calcium, all the way down to gold. At the top near lithium, we have the strongest reducing agent, indicating the greatest propensity for oxidation; essentially, these elements want to be oxidized a lot. Conversely, this also means that at the top, we have the weakest oxidizing agent, implying that reduction does not want to happen. At this end of the activity series chart, we observe the greatest tendency to lose electrons. On the opposite end, we have the weakest reducing agent, making it least likely to be oxidized, meaning oxidation is very weak here. At the lower end, we have the strongest oxidizing agent, indicating that reduction really wants to happen and here we see the greatest tendency to gain electrons. Remember, when looking at this activity series chart, an element higher up will displace another element that's below it. For example, if we had sodium, it could displace zinc from another compound because sodium is higher up than zinc.

Spontaneous Redox Reactions Example 1

Based on the activity series chart, determine if the following reaction represents a spontaneous redox reaction. We're looking to see if calcium can displace silver. We need to compare the two because they're the elements on the activity series chart above. Look and see if calcium is higher than silver. It is, indicating calcium can successfully displace the silver, ejecting it from the compound. So step 1, we locate the monoatomic element on the activity series chart. We found out where both are located. Remember, step 2, if it is higher on the activity series chart, it will displace the element within the nearby compound. Thus, silver will be kicked out, as metals by themselves exist as solids at room temperature, except for mercury. Now, calcium and chlorine combine to form a new ionic compound. Calcium belongs to group 2a, so it has a 2+ charge, and chlorine belongs to group 7a, so it has a 1- charge. We crisscross these numbers to form the ionic compound, CaCl₂, which we can denote as a solid. The states are not crucial here; what's important is that calcium can successfully displace silver in this redox reaction, making it spontaneous. To balance this, we note there are two chlorines here and only one here, so we put a 2 here, and as a consequence, we'd also put a 2 here for the silver. These are the coefficients for our balanced redox reaction: 1, 2, 2, 1.

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 from a compound, leading to the reduction of the displaced element. This occurs naturally 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 tendency to lose electrons. Elements higher in the activity series, like lithium and potassium, are strong reducing agents and can displace elements lower in the series, such as zinc or gold. This concept is crucial in understanding electrochemical processes and predicting reaction outcomes.

How does the activity series chart help in predicting redox reactions?

The activity series chart helps predict redox reactions by ranking elements based on their tendency to lose electrons. Elements higher in the chart, such as lithium and potassium, are strong reducing agents and have a greater propensity for oxidation. They can displace elements lower in the chart from their compounds. For example, sodium, which is higher in the activity series, can displace zinc from a compound. This hierarchy allows chemists to determine which elements will undergo oxidation or reduction in a given reaction, making it a valuable tool in electrochemistry and reaction prediction.

What is the role of an oxidizing agent in a redox reaction?

An oxidizing agent in a redox reaction is a substance that gains electrons and is reduced in the process. It facilitates the oxidation of another substance by accepting electrons from it. In the activity series chart, elements lower in the series, such as gold, are strong oxidizing agents because they have a high tendency to gain electrons. Understanding the role of oxidizing agents is essential for analyzing redox reactions, as it helps identify which substances will be reduced and which will be oxidized during the reaction.

Why are lithium and potassium considered strong reducing agents?

Lithium and potassium are considered strong reducing agents because they are high in the activity series chart, indicating a strong tendency to lose electrons. This propensity for oxidation means they can easily displace other elements from their compounds, effectively reducing those elements. For instance, lithium can displace zinc from a compound due to its higher position in the activity series. Their strong reducing nature makes them highly reactive and essential in various chemical processes, including batteries and electrochemical cells.

Can you explain the difference between an oxidizing agent and a reducing agent?

An oxidizing agent is a substance that gains electrons and is reduced in a redox reaction, while a reducing agent is a substance that loses electrons and is oxidized. In the activity series chart, elements lower in the series, like gold, are strong oxidizing agents because they readily gain electrons. Conversely, elements higher in the series, like lithium and potassium, are strong reducing agents because they easily lose electrons. Understanding this distinction is crucial for predicting the direction of redox reactions and identifying which substances will undergo oxidation or reduction.