Reaction Rate and Equilibrium

Which part of the energy diagram of a reaction represents the activated complex?

• A

• B

• C

• D

Answer explanation

B represents the activated complex. 

Tags

NGSS.HS-PS1-4

Which of the following would be most likely to result in a reduction in the rate of a chemical reaction?

• lowering the activation energy for a reaction

• warming an exothermic reaction

• stirring a solid and liquid reactant

• crushing a solid reactant

Answer explanation

Heat is a product of an exothermic reaction. Warming an exothermic reaction would slow the reaction rate while cooling it would have the opposite effect. Adding a catalyst, stirring, and lowering the activation energy are all things which tend to increase reaction rates.

Tags

NGSS.HS-PS1-5

Which description below best describes what is happening when a reversible reaction has reached equilibrium?

• There are no more reactants left and only products are present.

• No reactants are combining anymore and no products are being produced.

• Reactants are still combining to form products but the products are forming reactants at the same rate.

• No reactants are combining anymore and products are now breaking apart to make more reactants.

Answer explanation

At equilibrium the forward and reverse reactions are both occurring at the same rate.

A substance which speeds up the rate of a chemical reaction but which is not consumed in the reaction is known as what type of material?

• an equilibrium constant

• an activated complex

• a reactant

• a catalyst

Answer explanation

A catalyst is a substance which increases the rate of a chemical reaction but is not used up during the reaction.

What is meant by the “entropy” of a system?

• Entropy refers to the quickness with which a system will reach equilibrium.

• Entropy refers to the degree of disorder in the system.

• Entropy refers to the average kinetic energy of the particles in a system.

• Entropy refers to the total amount of energy in the system.

Answer explanation

Entropy is a measure of disorder in a system.

Tags

NGSS.HS-PS3-4

Which factors play a role in determining whether a chemical reaction will proceed spontaneously or not?

• entropy, temperature, and solubility

• entropy, enthalpy, and temperature

• entropy, solubility, and enthalpy

• temperature, solubility, and density

Answer explanation

The spontaneity of a reaction is governed by the change in Gibb’s free energy. In order to determine the change in Gibb’s free energy one must know the entropy, enthalpy, and temperature of the products and reactants.

What is the equation for the equilibrium constant for the reaction H_{2 (g)} + Cl_{2 (g)} ↔ 2 HCl?

 

• (See the answer in the explanation.)

• (See the answer in the explanation.)

Answer explanation

A large value for the specific rate constant (k) means the reaction is

• occurs over several steps

• relatively fast

• relatively slow

• incomplete

Answer explanation

When the specific rate constant (k) is large, the products form quickly.

Is the reaction in the diagram endothermic or exothermic?

• endothermic

• exothermic

Answer explanation

The energy of the products (D) is less than the energy of the reactants (A).  Therefore, energy is released and the reaction is exothermic. 

Tags

NGSS.HS-PS1-4

The K_sp value for aluminum hydroxide is very small at 3.0 x 10^-34 at 25 ˚C.  From this we can conclude which statement below is most likely to be true of aluminum hydroxide.

• Aluminum hydroxide will be only very slightly soluble in water.

• Aluminum hydroxide will easily decompose into aluminum, hydrogen and oxygen.

• Aluminum hydroxide will be a gas at room temperature.

• Aluminum hydroxide will be very chemically unstable.

Answer explanation

The solubility product constant (K_sp) for a material indicates the solubility of the material.  The smaller the K_sp, the less soluble the material is in water.