Question

Draw the energy profile for the following endothermic reaction: $$ \mathrm{H}_{2}(g)+\mathrm{I}_{2}(g) \rightleftarrows 2 \mathrm{HI}(g) $$

Short answer

Draw energy starting low for reactants, peak at activation energy, and end higher for products.

Step-by-step solution

Define the Reaction Type

The given chemical equation represents an endothermic reaction. In an endothermic reaction, energy is absorbed from the surroundings, meaning the products have higher energy than the reactants.

Identify the Reactants and Products

The reactants are \(\mathrm{H}_2(g)\) and \(\mathrm{I}_2(g)\), and the product is \(2\mathrm{HI}(g)\). You'll need these to correctly label the energy profile.

Draw the Energy Axis

Draw a vertical axis to represent energy. Label it as 'Energy' on the vertical side, with arbitrary units.

Plot the Reactants

Start by plotting your reactants \(\mathrm{H}_2(g) + \mathrm{I}_2(g)\) at a lower energy level on the graph, due to being the starting point of the reaction.

Draw the Activation Energy Hump

Draw a hill or hump, which represents the activation energy (\(E_a\)). This is the energy barrier that needs to be overcome for the reaction to proceed.

Plot the Products

At the end of the hump, draw a line above the level of the reactants representing the products \(2\mathrm{HI}(g)\). Since this is an endothermic reaction, the energy level of the products should be higher than that of the reactants.

Label the Energy Changes

Label the energy difference between the reactants and products as \(\Delta H > 0\) because the reaction is endothermic. Also, label the vertical distance from the reactant level to the top of the hill as \(E_a\).

Key concepts

Chemical Reaction Diagram

Understanding a chemical reaction diagram is pivotal when studying reactions like the one presented in this exercise. A chemical reaction diagram visually represents the changes in energy that take place during a reaction. It’s a graphical depiction where the y-axis represents energy levels and the x-axis represents the progress of the reaction.

When you draw the reaction diagram for the process of converting hydrogen and iodine gases into hydrogen iodide, the energy axis is shown as the vertical line. As the reaction progresses, you first mark the energy of the reactants — \(\mathrm{H}_2(g) + \mathrm{I}_2(g)\) — low on the y-axis.

As the reaction proceeds, the graph forms a hump, illustrating that energy is required to initiate the reaction: the activation energy. Following this hump, the energy level stays elevated, ending higher with the products — \(2\mathrm{HI}(g)\). The graph effectively showcases how the energy input required to start the reaction is overcome to form a higher energy product in an endothermic reaction.

Activation Energy

Activation energy is a fundamental concept in understanding how reactions occur and their rates. This is the minimum quantity of energy needed for reactants to successfully collide and react to form products.

In your reaction diagram, the activation energy is the peak of the hump. As the reaction progresses, you need sufficient energy to reach this peak. This hump represents the transition state — a high energy state that must be achieved for the reaction to proceed.

The height of this energy barrier, known as activation energy (denoted as \(E_a\)), is critical. It is the measure of how strong or intense the energy needs to be for the reaction to occur. For the endothermic reaction between \(\mathrm{H}_2(g) + \mathrm{I}_2(g)\), the activation energy is particularly significant because it reflects the energy intake needed to achieve the transformation into \(2\mathrm{HI}(g)\).

If the energy provided is insufficient to reach this level, the reaction cannot proceed, as there won't be enough energy to convert reactants into products.

Energy Profile

The concept of an energy profile is integral to chemistry. It offers a full depiction of the energy landscape a reaction traverses. In this specific endothermic reaction, the energy profile is used to show the energy absorbed and the transformational energy journey from reactants to products.

An energy profile typically follows a curve illustrating how energy levels change from the start to the end of a reaction. \(\Delta H\), representing the enthalpy change or the total energy absorbed by the system, is positive (\