Spontaneous vs Nonspontaneous Reactions: Videos & Practice Problems

Spontaneity in chemical reactions refers to the natural tendency of a process to occur without the need for continuous external energy. A spontaneous reaction favors the formation of products at equilibrium, such as the slow conversion of diamond to graphite over millions of years. Conversely, a non-spontaneous reaction, like the formation of diamonds from graphite, requires significant energy input to proceed. Importantly, spontaneity does not equate to the speed of a reaction—some spontaneous reactions can be slow. Photosynthesis, for example, is non-spontaneous as it requires sunlight to produce glucose from ${\mathrm{CO}}_2$ and water, while cellular respiration is spontaneous, converting glucose and oxygen into ${\mathrm{CO}}_2$ and water naturally. Understanding these concepts is crucial for grasping thermodynamics in chemistry, which is distinct from kinetics, the study of reaction rates.

Spontaneity of Processes

Spontaneity in chemical reactions is a crucial concept that determines whether a reaction can occur naturally under specific conditions. A spontaneous reaction is one that happens without the need for a constant external energy source, favoring the formation of products at equilibrium. In contrast, a non-spontaneous reaction requires continuous energy input to proceed and does not favor product formation at equilibrium, often remaining in its reactant state.

It is important to distinguish between spontaneity and the speed of a reaction. Spontaneity is related to thermodynamics, which assesses whether a reaction can occur, while kinetics focuses on the rate at which it happens. For instance, the transformation of diamond (carbon) into graphite is a spontaneous process, albeit extremely slow, taking millions of years. Conversely, converting graphite back into diamond is non-spontaneous, requiring significant energy input, such as heat and pressure, making diamonds rare and valuable.

Another example is photosynthesis, where carbon dioxide (CO₂) and water react to form glucose and oxygen, driven by sunlight as an external energy source. This reaction is non-spontaneous without sunlight. In the reverse process, cellular respiration, glucose and oxygen naturally produce CO₂ and water, demonstrating a spontaneous reaction. Thus, spontaneous processes can occur over varying timescales, from seconds to millions of years, as long as they can happen without external energy input.

In summary, spontaneous reactions are natural processes that can occur without external energy, while non-spontaneous reactions require energy to proceed. Understanding these concepts is essential for grasping the dynamics of chemical reactions and their implications in various biological and geological processes.

Spontaneity of Processes Example

Understanding the distinction between spontaneous and non-spontaneous processes is crucial in thermodynamics. A spontaneous process occurs naturally without the need for external energy, while a non-spontaneous process requires a continuous input of energy to take place.

For instance, consider the dissolution of a sugar cube in room temperature water. This is a spontaneous process because it happens naturally; when the sugar cube is placed in water, it dissolves without any additional energy input. Similarly, when three different gases are mixed in a container, they will naturally diffuse and mix due to their random motion, making this process spontaneous as well.

However, not all processes are spontaneous. For example, heat typically flows from a hotter object to a colder one. If we were to observe heat flowing from a cold object to a warm one, this would be a non-spontaneous process, as it contradicts the natural flow of heat. Another example is the melting of ice at negative 1 degree Celsius. Since water freezes at 0 degrees Celsius, we would not expect ice to melt at a lower temperature, categorizing this as a non-spontaneous process.

On the other hand, a ball rolling down a hill is a spontaneous process. If a gust of wind pushes the ball over the edge, gravity will naturally cause it to roll down, demonstrating a natural progression without external energy. Lastly, plant respiration, which involves the conversion of oxygen and glucose into carbon dioxide, water, and ATP, is a spontaneous process. This is in contrast to photosynthesis, which requires an external energy source, making it non-spontaneous.

In summary, recognizing whether a process is spontaneous or non-spontaneous helps in understanding the natural tendencies of systems and the energy dynamics involved in various reactions.

Here’s what students ask on this topic:

How can one tell the difference between spontaneous and nonspontaneous reactions?

To differentiate between spontaneous and nonspontaneous reactions, you need to consider the thermodynamic properties of the system, specifically the Gibbs free energy change ($\Delta G$). A spontaneous reaction is one that occurs without the need for continuous external energy; it tends to happen naturally once initiated. For a reaction to be spontaneous, the change in Gibbs free energy must be negative ($\Delta G<0$) under constant temperature and pressure.

In contrast, a nonspontaneous reaction requires an input of energy to proceed and will not occur on its own under the given conditions. These reactions have a positive change in Gibbs free energy ($\Delta G>0$).

Temperature can also play a role in spontaneity. Some reactions may be spontaneous at certain temperatures and nonspontaneous at others due to the temperature dependence of $\Delta G$.

In summary, check the sign of the Gibbs free energy change: if $\Delta G$ is negative, the reaction is spontaneous; if $\Delta G$ is positive, the reaction is nonspontaneous. Temperature must also be considered, as it can affect the spontaneity of a reaction.

Which of the following reactions will be spontaneous at low temperatures and nonspontaneous at high temperatures?

A reaction that is spontaneous at low temperatures but nonspontaneous at high temperatures is characterized by having a negative change in enthalpy (ΔH) and a positive change in entropy (ΔS). This is because the spontaneity of a reaction is determined by the change in Gibbs free energy (ΔG), which is given by the equation:

$\mathrm{\Delta G}=\mathrm{\Delta H}-T\mathrm{\Delta S}$

where T is the temperature.

When ΔH is negative (exothermic reaction) and ΔSis positive (increase in disorder), at low temperatures the TΔSterm will be relatively small, so the negative ΔH will dominate, making ΔG negative and the reaction spontaneous. However, as the temperature increases, the TΔSterm becomes larger. If it becomes large enough to outweigh the negative ΔH, then ΔG will become positive, and the reaction will no longer be spontaneous. Thus, a reaction with a negative ΔH and a positive ΔSfits the criteria of being spontaneous at low temperatures and nonspontaneous at high temperatures.

Which of these reactions is/are predicted to be spontaneous, nonspontaneous, or at equilibrium?

To determine if a reaction is spontaneous, nonspontaneous, or at equilibrium, you need to consider the Gibbs free energy change (ΔG) for the reaction. The Gibbs free energy equation is:

$\mathrm{\Delta G}=\mathrm{\Delta H}-T\mathrm{\Delta S}$

where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔSis the change in entropy.

• If ΔG is negative ($-\mathrm{\Delta G}$), the reaction is spontaneous, meaning it can occur without any continuous external influence.
• If ΔG is positive ($+\mathrm{\Delta G}$), the reaction is nonspontaneous, meaning it will not occur without the input of energy.
• If ΔG is zero ($\mathrm{\Delta G}=0$), the system is at equilibrium, indicating that the forward and reverse reactions occur at the same rate, and there is no net change in the concentrations of reactants and products over time.

To apply this to a specific reaction, you would need to know or be able to calculate the values of ΔH, T, and ΔS. Remember that the sign and magnitude of these values, as well as the temperature at which the reaction is occurring, will influence the spontaneity of the reaction.