AP Chemistry Kinetics

Which of the following correctly justifies why a reaction is found to occur more quickly in a solution of 1.0M HCl?

• The acid increases the temperature of the mixture causing the rate of collision between particles to increase.

• The acid provides an alternative pathway for the reaction which increases the activation energy.

• The acid acts as a catalyst, providing an alternative pathway for the reaction.

• The acid solution increases the volume of the solution, decreasing the concentration of the reactants.

Answer explanation

A catalyst is a substance that increases the rate of a reaction by providing an alternative pathway. This lowers the activation energy of the overall reaction.

Tags

NGSS.HS-PS1-5

A reaction is represented by the equation XA -> X + A. At a certain temperature, the rate constant is 2.5 1/M·s. What is the rate of the reaction if the initial [XA] = 1.1 M?

• A. 2.8 M/s

• B. 2.5 M/s

• C. 3.0 M/s

• D. Not enough information was provided.

Answer explanation

Based on the units for the rate constant, it's determined that the reaction is second order with respect to XA. The rate law is rate=k[XA]^2. Therefore, the rate of the reaction is rate= (2.5)(1.1)^2 = 3.0

Tags

NGSS.HS-PS1-5

Which of the following defines the collision theory?

• A. A reaction will only occur if the particles collide with the right amount of energy and in the right orientation.

• B. Collisions between gas particles and other particles or the wall are elastic.

• C. The energy created due to the collision between atoms in mass spectrometers.

• D. The minimum amount of energy required to initiate a chemical reaction.

Answer explanation

The collision theory states that a reaction will only occur if the reactants collide with the right amount of energy and in the right orientation.

Tags

NGSS.HS-PS1-5

The decomposition of diamond to graphite is thermodynamically favorable. Why does this reaction not occur at an observable rate?

• Because diamonds cannot conduct electricity while graphite can.

• The distance between the carbon atoms in diamonds are closer than the bonds between carbon in graphite.

• A large amount of pressure is necessary for the reaction to occur.

• The activation energy for the reaction is extremely high.

Answer explanation

The main reason a reaction does not occur at an observable rate, even when the reaction is thermodynamically favorable, is that the activation energy is very large.

Tags

NGSS.HS-PS1-5

The following data table shows the concentration of a reaction and the initial rate data. Based on the table, what is the order of the reaction with respect to the reactant?

• A. 2nd

• B. 0th

• C. 1st

• D. 3rd

Answer explanation

The rate of the reaction changes the same as the change of the concentration of the reactant. When the reaction of the concentration doubles, the rate of the reaction doubles; therefore, the order of the reaction with respect to the reactant is 1st order.

Tags

NGSS.HS-PS1-5

What is the rate law for the overall reaction represented by the following mechanism?

• Rate = k[X][XY][Y]^2

• Rate = k[X][Y_2]

• Rate = k[XY][Y]

• Rate = k[X]

Answer explanation

The slow, elementary step determines the rate of the overall reaction. Since the coefficients for X and Y_2 are 1 and 1 respectively, the exponents for X and Y_2 are 1 and 1.

Tags

NGSS.HS-PS1-5

Given the data, what is the rate law for the reaction X + Z -> XZ?

• Rate = k[X]

• Rate = k[Z]

• Rate = k[X][Z]

• Rate = k[X]^2

Answer explanation

When the concentration of X halved, the rate halved. When the concentration of Y halved, the rate did not change. Therefore, the reaction is 1st order with respect to X and 0th order with respect to Y.

Tags

NGSS.HS-PS1-5

Which of the following correctly justifies why most chemical reactions slow down as they proceed?

• As a reaction proceeds, there are less products.

• As a reaction proceeds, there are less reactants.

• As a reaction proceeds the temperature decreases.

• As a reaction proceeds it consumes the catalyst.

Answer explanation

The rate of the reaction depends on the concentration of the reactants. Because a chemical reaction consumes the reactants, the rate of the reaction decreases.

Which of the following elementary reactions is first order with respect to A?

• A.  A+B->AB

• B. AB -> A+B

• C. A+A -> A_2

• D. A_2 -> A+A

Answer explanation

An elementary reaction is one in which the coefficients of the reactants equal the order of the reaction with respect to the reactants. A + B -> AB is the only reaction where the coefficient for A is equal to 1.

Tags

NGSS.HS-PS1-5

Permanganate, a purple solution, reacts with hydrogen peroxide to produce a colorless solution. Which of the following methods is used to study the rate of reaction?

• Photospectrometry

• B. Mass Spectrometry

• C. Gravimetric Analysis

• D. Titration

Answer explanation

Since the solution changes from purple to clear, one can study the rate of disappearance of the color purple in solution using Beer's Law and photo-spectrometry.

Tags

NGSS.HS-PS1-5