Activation energy chemical compounds

He studied chemistry at university and put together a page dissertation with 56 different theses in to obtain his doctorate. Although the professors in Sweden who reviewed his dissertation did not think highly of his work, almost all 56 theses have been accepted by modern physical chemistry with small changes or no changes at all.

His continued studies of the subjects put forth in these theses would eventually lead to his discovery of activation energy. In , Svante Arrhenius would finally make the discovery of activation energy. By studying chemical reactions and looking at the work of his colleagues, he observed that most reactions needed heat to be successful.

This made him curious as to why heat was so necessary to form chemical reactions. Arrhenius eventually determined that there was an energy barrier that had to be overcome before a chemical reaction could occur. The heat helped the molecules to go over this barrier. This led him to the discovery that the energy required to overcome the barrier was activation energy. As he continued to study this subject, he would eventually come up with an equation to explain this phenomena.

As time went on, more information was gathered about chemical reactions and the elements that were necessary to create these reactions. It was discovered that one of the key factors in the reaction was reaching the transition state.

To reach the transition state, enough activation energy has to be created to disrupt the bonds between elements in a stable state. It is known that molecules with low energy states have stable bonds.

The lower the energy state of a molecule is, the more stable that molecule will be. To create a chemical reaction, it is necessary to use activation energy to weaken the bonds inside a molecule until the molecule is pushed into instability and eventually a transition state.

The graph above shows the process of molecules reacting together in a way that allows them to transform into a new product. The starting point of the graph is a reflection of the energy that was already present inside the molecules. The rise in energy shows the introduction of an activation energy that allowed the molecules to reach their transition state.

Once the molecules have reached their transition state, they are free to break their bonds and rearrange themselves into different bonds. These bonds often lead to new molecules and products being made that are not found freely in nature ex: iron that receives activation energy and is smelted, then refined with oxygen produces steel.

The peak of the graph the transition state , then declines and begins to lower in energy. This is the transition of the molecules going from an unstable transition state back to a stable form.

When the line in the graph reaches a level that is parallel to when the activation energy was introduced, it symbolizes that the chemical reaction is taking place and the bonds inside the molecule s are starting to stabilize.

Looking at the graph, the number of new molecules that are created can be determined by looking at the portion of the line that is to the right of the transition point and parallel to when the activation energy was introduced to the reactants. Everything from this point forward symbolizes the new products that have been obtained through the chemical reaction. Although it is possible for chemical reactions to occur naturally, it is a rare event.

This is because the majority of molecules in nature have low energy levels, resulting in stable bonds. These low energy bonds make it difficult for the molecule to be pushed into a transition state unless they are given enough activation energy. Collisions between molecules that are not given enough activation energy are not able to reach the transition state. This means that while they are receiving some activation energy, they are not able to disrupt the stable bonds that are found with the reactants.

Because there is not enough energy applied to the reaction, the molecules are not able to re-arrange and form the desired end-product.

Failure to create a chemical reaction could also be due to improper orientation. Even if enough activation energy is applied to a chemical reaction, an end-product might not be created if the molecules are not able to collide head-on.

Off-center collisions are not direct enough to disrupt the bonds between molecules, which in turn prevents the forming of new bonds. When enough activation energy can be applied to a chemical reaction, molecules can rearrange their bonds to create new products. At o C the rate constant was found to be 2. Calculate the a activation energy and b high temperature limiting rate constant for this reaction.

All reactions are activated processes. Rate constant is exponentially dependent on the Temperature. We know the rate constant for the reaction at two different temperatures and thus we can calculate the activation energy from the above relation.

First, and always, convert all temperatures to Kelvin, an absolute temperature scale. Then simply solve for E a in units of R. Now that we know E a , the pre-exponential factor, A , which is the largest rate constant that the reaction can possibly have can be evaluated from any measure of the absolute rate constant of the reaction.

The infinite temperature rate constant is 4. Determine graphically the activation energy for the reaction. The value of the slope is -8e so:. This reaction therefore provides the basis for understanding the effect of a catalyst on the rate of a chemical reaction. Four criteria must be satisfied in order for something to be classified as catalyst. A small quantity of catalyst should be able to affect the rate of reaction for a large amount of reactant.

The first criterion provides the basis for defining a catalyst as something that increases the rate of a reaction. The second reflects the fact that anything consumed in the reaction is a reactant, not a catalyst. The third criterion is a consequence of the second; because catalysts are not consumed in the reaction, they can catalyze the reaction over and over again.

The fourth criterion results from the fact that catalysts speed up the rates of the forward and reverse reactions equally, so the equilibrium constant for the reaction remains the same. Catalysts increase the rates of reactions by providing a new mechanism that has a smaller activation energy, as shown in the figure below. A larger proportion of the collisions that occur between reactants now have enough energy to overcome the activation energy for the reaction. As a result, the rate of reaction increases.

To illustrate how a catalyst can decrease the activation energy for a reaction by providing another pathway for the reaction, let's look at the mechanism for the decomposition of hydrogen peroxide catalyzed by the I - ion. In the presence of this ion, the decomposition of H 2 O 2 doesn't have to occur in a single step. It can occur in two steps, both of which are easier and therefore faster. Because there is no net change in the concentration of the I - ion as a result of these reactions, the I - ion satisfies the criteria for a catalyst.

Because H 2 O 2 and I - are both involved in the first step in this reaction, and the first step in this reaction is the rate-limiting step, the overall rate of reaction is first-order in both reagents. Determining the Activation Energy of a Reaction. The rate of a reaction depends on the temperature at which it is run.