Kinetics and Energy - AP Chemistry

If the activation energy of the forward reaction is greater than that of the reverse reaction, the products must have a higher enthalpy than the reactants. The net enthalpy change is therefore positive, meaning that it is endothermic.

If the activation energy of a forward reaction is greater than that of the reverse reaction, thenthe products must have a higher enthalpy than the reactants (draw a potential energy diagram to visualize).

Activation energy is the energy barrier that needs to be overcome for a reaction to proceed; the higher the activation energy, the slower the reaction. Activation energy can only be altered via a catalyst. Catalysts are chemical substances that lower the activation energy, allowing reactions to proceed faster. Other physical quantities such as temperature and pressure don’t alter the activation energy.

Reaction rate is a direct measure of the speed of a reaction. As mentioned, increasing activation energy will increase the barrier and, therefore, slow down the reaction.

Catalysts are chemical substances that decrease the activation energy, thereby increasing the reaction rate. They are commonly used in chemical reactions to drastically speed up reactions that might otherwise take hours or days to complete. Enzymes are biological catalysts that facilitate most of the biological reactions happening in our bodies.

This question is asking about the relationship between activation energy of a reaction (kinetics) and the amount of products produced (equilibrium). Remember that altering the speed of a reaction (kinetics) does not change the equilibrium of the reaction. Increasing or decreasing activation energy (which alters the speed of reaction) will simply allow for the reaction to proceed slower or faster, respectively. It will not change the amount of products produced at the end.

Activation energy is the difference between the energy of the transition state and the energy of the reactant. Recall that activation energy is the energy barrier that needs to be overcome by a reaction. The transition state is a higher-energy, intermediary molecule that lies in between the reactants and products. It is created when reactants are in the process of becoming products. Therefore, to get to products, it is necessary to go through this high-energy transition state.

Activation energy is the energy needed to climb this energy “hill” (energy needed to go from reactants to transition state); therefore, the activation energy is the energy of the transition state (top of the energy hill/barrier) minus the energy of the reactant.

To answer this question, we have to consider an energy diagram for an exothermic reaction. We know the forward reaction is exothermic because the question states that energy is released. We are given the activation energy of the reaction (the energy difference between the activated complex, or transition state, and the reactants) and the energy released (the energy difference between the products and the reactants). In the reverse reaction we will be going from products to reactants as follows:

This means that the activation energy for reverse reaction will be higher (has to climb a higher hill from “products” to “activated complex”). Using the given information we can deduce that the activation energy () of the reverse reaction is the SUM of the activation energy of the forward reaction AND the energy released from the forward reaction.

The closest answer is . Note that the forward reaction is exothermic, whereas the reverse reaction is endothermic (energy is being consumed). The reverse reaction is endothermic because the reactant () has lower energy than the product ().

Recall that the only thing that can alter the activation energy of a reaction is the addition of a catalyst. Factors such as enthalpy, entropy, temperature, and pressure don’t change the activation energy.

Catalysts decrease the activation energy and increase the rate of reaction. They have no effect on the energy of the reactants. As mentioned, temperature will not alter the activation energy; however, increasing the temperature will speed up the reaction. This occurs because adding energy in the form of heat will increase the energy of individual molecules in the reaction and will allow more molecules to overcome the energy barrier. Note, however, that temperature does not change the energy barrier (activation energy). Recall that changing activation energy has no effect on the equilibrium of the reaction; therefore, equilibrium constant does not depend on the activation energy.

"Increase the temperature" is incorrect because we have an exothermic reaction and by increasing the temperature the equilibrium constant will become smaller. An increase in temperature will shift the equilibrium toward reagents. According to the Le Chatelier's principle, by extracting continuously from the reactor the equilibrium shifts toward the product, . Furthermore, the use of a catalyst increases the reaction rate. The Haber process to synthesize ammonia is an example of this kind of reaction.

The kinetic molecular theory of gases states that the average kinetic energy of gas particles is proportional to temperature, and it is the same for all gases at a given temperature. This is the opposite of what is stated in the answer choice.

Using the equation , n being the number of moles, R being 8.314, and T being the temperature in Kelvin, we can find the kinetic energy of two moles of gas in a container.

The temperature of a system can be manipulated to increase the reaction rate. The temperature affects the rate of the reaction because as temperature increases, so does the average kinetic energy. This means that molecules (reactants) are moving around more quickly, resulting in more frequent molecular collisions, resulting in an increased rate of product formation.

The average kinetic energy is considered to be a sole function of temperature; pressure is not a factor. All of the other statements are assumptions of kinetic molecular theory.

Recall that and that and are both molar concentrations. Then,

Dividing both sides by gives

For a first order reaction,

Integrating both sides gives

Where is an integration constant.

Solving for gives

Initially there are 3 mols of . Using this we find that

At ,

Percent conversion is calculated as follows:

All of the above describe elementary reactions and how they give an overall mechanism.

Based on the slowest step the rate law would be: Rate = k2 \[NOBr2\] \[NO\], but one cannot have a rate law in terms of an intermediate (NOBr2).

Because the first reaction is at equilibrium the rate in the forward direction is equal to that in the reverse, thus:

and:

Substitution yields:

The reaction can never go faster than its slowest step.

Since the products are higher in energy than the reactions, the reaction is endothermic.

R is the intermediate. It is formed in Step 1 and consumed in Step 2.

A zero-order reaction has a rate of formation of product that is independent of changes in concentrations of any of the reactants; however, since the rate constant itself is dependent on temperture, changing the temperature can alter the rate.