Energy Diagrams: Videos & Practice Problems

An energy diagram illustrates the energies of reactants, products, and the transition state during a chemical reaction. The activation energy (E_a) is the minimum energy required for the reaction, influencing the reaction speed; higher E_a results in slower reactions. The overall energy change (ΔE) between reactants and products determines reaction favorability, with negative ΔE indicating exothermic and favorable reactions. Enthalpy (ΔH) and Gibbs free energy (ΔG) are specific forms of overall energy, where negative values signify stability and product formation.

Energy Diagrams Concept 1

An energy diagram is a graphical representation that illustrates the energies of reactants, products, and the transition state during a chemical reaction. In this diagram, the reactants, abbreviated as r, are positioned on the left side, while the products, represented by p, are found on the right side at the end of the curve. The transition state, denoted as ts, corresponds to the maximum energy point along the reaction coordinate, which is the path from reactants to products. This transition state is also known as the activated complex.

The reaction coordinate, depicted along the x-axis, indicates the progress of the reaction, while the y-axis shows the change in energy, ranging from 80 kilojoules to 200 kilojoules. The activation energy, abbreviated as E_a, represents the minimum energy required for the reaction to proceed. It is defined as the height difference between the transition state and the energy level of the reactants. Understanding these key components—reactants, products, transition state, and activation energy—is essential for interpreting energy diagrams and grasping the energy changes that occur during chemical reactions.

Energy Diagrams Example 1

In an energy diagram, the energy value of the products can be determined by locating the endpoint of the reaction curve, which represents the final state of the products. In this case, the product line is positioned at 140 kilojoules on the y-axis of the diagram. This indicates that the energy associated with the products of the reaction is 140 kilojoules. Understanding this value is crucial for analyzing the energy changes that occur during a chemical reaction, as it helps in assessing the stability and reactivity of the products compared to the reactants.

Energy Diagrams Concept 2

The speed of a chemical reaction is significantly influenced by the activation energy, denoted as \( E_a \). Activation energy is defined as the difference in energy between the transition state and the reactants. In an energy diagram, the transition state represents the peak of the curve, while the starting point of the curve indicates the energy level of the reactants. The relationship between activation energy and reaction speed is crucial: a higher activation energy means that fewer molecules possess the necessary energy to convert into products, resulting in a slower reaction rate. Conversely, a lower activation energy allows more molecules to overcome the energy barrier, leading to a faster reaction.

To illustrate this concept, consider two reactions represented by energy diagrams. For the first reaction, if the transition state is at 90 kilojoules and the reactant line is at 20 kilojoules, the activation energy can be calculated as:

\[ E_a = 90 \, \text{kJ} - 20 \, \text{kJ} = 70 \, \text{kJ} \]

This indicates a relatively high activation energy, suggesting a slower reaction. In contrast, for the second reaction, with a transition state at 60 kilojoules and the same reactant line at 20 kilojoules, the activation energy is:

\[ E_a = 60 \, \text{kJ} - 20 \, \text{kJ} = 40 \, \text{kJ} \]

This lower activation energy corresponds to a faster reaction. Thus, the activation energy, or activation barrier, is a critical factor in determining the rate of a chemical reaction: the higher the \( E_a \), the slower the reaction, while a smaller \( E_a \) results in a faster reaction.

Energy Diagrams Example 2

In chemical reactions, the activation energy (Ea) is a crucial factor that determines the rate at which a reaction occurs. The activation energy is the minimum energy required for reactants to transform into products. Generally, a lower activation energy correlates with a faster reaction rate. In the comparison of three reactions, we have:- Reaction A with an activation energy of 143 kilojoules- Reaction B with an activation energy of 80 kilojoules- Reaction C with an activation energy of 215 kilojoulesAmong these, Reaction B has the lowest activation energy of 80 kilojoules. This indicates that it requires less energy to initiate the reaction compared to the others. Consequently, Reaction B will occur in the shortest amount of time, making it the fastest reaction among the three options. In summary, when evaluating reaction rates based on activation energy, the reaction with the smallest activation energy will proceed the quickest, confirming that Reaction B is the correct choice for the fastest reaction.

Energy Diagrams Concept 3

The overall energy change in a chemical reaction, represented by the symbol \(\Delta E\), is crucial in determining the favorability of that reaction. This change is calculated as the difference between the energy of the products and the energy of the reactants, expressed as:

\[\Delta E = E_{\text{products}} - E_{\text{reactants}}\]

Within this context, two important concepts arise: enthalpy and Gibbs free energy. Enthalpy, denoted as \(\Delta H\), specifically refers to thermal energy, which is the energy associated with heat. In contrast, Gibbs free energy, represented as \(\Delta G\), relates to the favorability of product formation in a reaction. A reaction is considered favorable if it can produce products from reactants, indicating a successful transformation.

To illustrate these concepts, consider energy diagrams where the product and reactant energy levels are plotted. For example, if the product line is at 15 kilojoules and the reactant line is at 40 kilojoules, the overall energy change would be:

\[\Delta E = 15 \, \text{kJ} - 40 \, \text{kJ} = -25 \, \text{kJ}\]

This negative value indicates that the reaction is exothermic, meaning it releases energy. Similarly, if \(\Delta H\) is negative, it confirms that the reaction is exothermic. When \(\Delta G\) is also negative, the reaction is termed exergonic, which is favorable as it results in products with lower energy, thus greater stability.

Conversely, if the product line is at 50 kilojoules and the reactant line is at 30 kilojoules, the calculation would yield:

\[\Delta E = 50 \, \text{kJ} - 30 \, \text{kJ} = +20 \, \text{kJ}\]

This positive value indicates an endothermic reaction, where energy is absorbed. In this case, both \(\Delta H\) and \(\Delta G\) would be positive, leading to the classification of the reaction as endergonic, which is unfavorable since it results in products that possess more energy than the reactants, indicating lower stability.

In summary, understanding the distinctions between overall energy, enthalpy, and Gibbs free energy is essential for evaluating the favorability and stability of chemical reactions. A negative \(\Delta E\), \(\Delta H\), or \(\Delta G\) signifies a favorable reaction, while positive values indicate unfavorable conditions.

Which of the following statements is true regarding the energy diagram provided?

i. The reaction is endothermic.
ii. The activation energy is +10 kJ.
iii. The reaction releases energy.
iv. The enthalpy of the reaction is –25 kJ.

• A. I only
• B. II and IV
• C. I and III
• D. II, III and IV

Answer: (D)

Which of the following reactions proceeds the slowest?

Reaction 1 Reaction 2 Reaction 3

• A. Reaction 1
• B. Reaction 2
• C. Reaction 3

Answer: (B)

Here’s what students ask on this topic:

What is an energy diagram in chemistry?

An energy diagram in chemistry is a graphical representation that illustrates the energy changes during a chemical reaction. It shows the energies of reactants, products, and the transition state. The y-axis represents energy, while the x-axis represents the reaction coordinate, which is the progress of the reaction. Key components include the reactants (R) at the beginning, the products (P) at the end, and the transition state (TS) at the peak of the curve. The activation energy (Ea) is the energy difference between the reactants and the transition state, indicating the minimum energy required for the reaction to occur.

How does activation energy affect the speed of a chemical reaction?

Activation energy (Ea) significantly affects the speed of a chemical reaction. It is the minimum energy required for reactants to transform into products. A higher activation energy means that fewer reactant molecules have sufficient energy to overcome the energy barrier, resulting in a slower reaction. Conversely, a lower activation energy allows more reactant molecules to participate in the reaction, leading to a faster reaction. Mathematically, the activation energy can be expressed as:

$E_a=E_{\mathrm{TS}}-E_R$

What is the difference between exothermic and endothermic reactions in energy diagrams?

In energy diagrams, exothermic and endothermic reactions are distinguished by the overall energy change (ΔE) between reactants and products. For exothermic reactions, ΔE is negative, indicating that energy is released, and the products have lower energy than the reactants. This is represented by a downward slope from reactants to products. For endothermic reactions, ΔE is positive, meaning that energy is absorbed, and the products have higher energy than the reactants, shown by an upward slope. Mathematically, ΔE can be expressed as:

$\Delta E\; =\; E$_products - E_reactants

What is the role of the transition state in an energy diagram?

The transition state in an energy diagram represents the highest energy point along the reaction coordinate. It is the point where the reactants are transformed into products, and it is often referred to as the activated complex. The energy difference between the reactants and the transition state is the activation energy (Ea), which is the minimum energy required for the reaction to proceed. The transition state is crucial because it determines the activation energy and, consequently, the reaction rate. A lower transition state energy results in a lower activation energy and a faster reaction.

How do enthalpy (ΔH) and Gibbs free energy (ΔG) relate to energy diagrams?

Enthalpy (ΔH) and Gibbs free energy (ΔG) are specific forms of overall energy change (ΔE) in energy diagrams. Enthalpy (ΔH) refers to the thermal energy change and indicates whether a reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0). Gibbs free energy (ΔG) relates to the favorability of a reaction, with negative ΔG indicating a spontaneous and favorable reaction (exergonic) and positive ΔG indicating a non-spontaneous reaction (endergonic). Both ΔH and ΔG can be derived from the overall energy change:

$\Delta E\; =\; E$_products - E_reactants