Quiz 13.1 - Heat Changes In Reactions

1.

In this energy profile for a reaction, what is represented by the arrow labeled W?
- A.
  
  Activation energy
- B.
  
  Energy of products
- C.
  
  Energy of activated complex
- D.
  
  Change in enthalpy
Correct Answer
C. Energy of activated complex

Explanation
The arrow labeled W in the energy profile for a reaction represents the energy of the activated complex. The activated complex is an intermediate state that forms during a chemical reaction, where the reactant molecules are in a highly unstable and transition state. The energy of the activated complex is the highest energy point in the reaction pathway, and it represents the energy barrier that must be overcome for the reaction to proceed.

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2.

In this energy profile for a reaction, what is represented by the arrow labeled Z?
- A.
  
  Activation energy
- B.
  
  Energy of products
- C.
  
  Energy of activated complex
- D.
  
  Change in enthalpy
Correct Answer
D. Change in enthalpy

Explanation
The arrow labeled Z in the energy profile represents the change in enthalpy. Enthalpy is a measure of the heat energy absorbed or released during a reaction. The change in enthalpy represents the difference in energy between the reactants and the products. In this context, the arrow labeled Z indicates the overall change in energy that occurs during the reaction.

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3.

In this energy profile for a reaction, what is represented by the arrow labeled R?
- A.
  
  Energy of reactants
- B.
  
  Energy of products
- C.
  
  Energy of activated complex
- D.
  
  Change in enthalpy
Correct Answer
A. Energy of reactants

Explanation
The arrow labeled R represents the energy of reactants in the energy profile for a reaction. The energy of reactants refers to the initial energy level of the molecules or atoms before the reaction takes place. It represents the minimum amount of energy required for the reaction to occur. As the reaction progresses, the energy of the reactants is either increased or decreased depending on the nature of the reaction and the energy changes involved.

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4.

In this energy profile for a reaction, what is being plotted on the y-axis?
- A.
  
  Progress of the reaction
- B.
  
  Time
- C.
  
  Potential energy
- D.
  
  Mass of the reactants
Correct Answer
C. Potential energy

Explanation
The y-axis in the energy profile for a reaction represents the potential energy. This is because the energy profile shows the energy changes that occur during the progress of the reaction. The potential energy of the reactants is plotted on the y-axis, and any changes in energy, such as activation energy or energy released or absorbed during the reaction, can be observed on this axis.

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5.

This reaction is best described as
- A.
  
  Endothermic
- B.
  
  Exothermic
- C.
  
  Catalyzed
- D.
  
  A multi-step reaction mechanism
Correct Answer
B. Exothermic

Explanation
The reaction is described as exothermic because it releases heat energy to the surroundings. In an exothermic reaction, the products have lower energy than the reactants, resulting in a net release of energy in the form of heat.

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6.

A catalyst is best described as
- A.
  
  A substance which speeds up a reaction without being consumed.
- B.
  
  A substance which adds sufficient heat to a chemical reaction to allow it to react.
- C.
  
  A substance which grinds the reactants into smaller pieces so they can react faster.
- D.
  
  A substance which prevents a reaction from happening at all.
Correct Answer
A. A substance which speeds up a reaction without being consumed.

Explanation
A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It works by lowering the activation energy required for the reaction to occur, allowing the reactants to more easily overcome the energy barrier and form products. The catalyst itself remains unchanged at the end of the reaction and can be used again in subsequent reactions. This property makes catalysts highly efficient and cost-effective in industrial processes, as they can be continuously reused.

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7.

In the equation Q=MCDT, the variable Q stands for
- A.
  
  Question (in grams).
- B.
  
  Heat (in joules).
- C.
  
  Specific heat capacity (in Joules/gram-degrees Celsius).
- D.
  
  Change in temperature (in degrees Celsius).
Correct Answer
B. Heat (in joules).

Explanation
In the equation Q = MCDT, Q represents heat in joules. This equation is derived from the formula for heat transfer, where Q is the amount of heat transferred, M is the mass of the substance, C is the specific heat capacity of the substance, and DT is the change in temperature. Therefore, Q in this equation specifically refers to the amount of heat energy in joules.

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8.

A good way to speed up a chemical reaction would be to
- A.
  
  Lower the temperature of the reaction vessel.
- B.
  
  Use larger chunks of reactants.
- C.
  
  Increase the concentration of the reactants.
- D.
  
  Use a larger beaker for the reaction, instead of a smaller one.
Correct Answer
C. Increase the concentration of the reactants.

Explanation
Increasing the concentration of the reactants would lead to more collisions between the reactant molecules, increasing the likelihood of successful collisions and therefore speeding up the reaction. This is because a higher concentration means there are more reactant particles in a given volume, resulting in a higher frequency of collisions. On the other hand, lowering the temperature would decrease the kinetic energy of the molecules, slowing down their movement and reducing the frequency of collisions. Using larger chunks of reactants or a larger beaker would not have a significant effect on the reaction rate.

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9.

Generally, an easy way to double the rate of a chemical reaction would be to
- A.
  
  Use larger chunks of material in the reactions.
- B.
  
  Lower the temperature by 15 degrees Celsius.
- C.
  
  Stir the reaction beaker vigorously.
- D.
  
  Raise the temperature by 10 degrees Celsius.
Correct Answer
D. Raise the temperature by 10 degrees Celsius.

Explanation
Raising the temperature by 10 degrees Celsius increases the kinetic energy of the reactant molecules, which leads to more frequent and energetic collisions. This increases the chances of successful collisions, resulting in a higher reaction rate. Stirring the reaction beaker vigorously may increase the rate of mixing, but it does not directly affect the energy of the reactant molecules. Using larger chunks of material may increase the surface area available for the reaction, but it does not increase the energy of the reactant molecules. Lowering the temperature by 15 degrees Celsius would decrease the kinetic energy of the reactant molecules, slowing down the reaction.

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10.

A high-energy, unstable compound that is formed after the reactant phase but before the product phase is called a(n) ___.
- A.
  
  Catalyst.
- B.
  
  Substrate.
- C.
  
  Joule.
- D.
  
  Activated complex.
Correct Answer
D. Activated complex.

Explanation
An activated complex refers to a high-energy and unstable compound that is formed during a chemical reaction after the reactant phase but before the product phase. It is a transitional state where the reactant molecules are in the process of forming new bonds and breaking old ones. The activated complex is crucial in determining the rate of the reaction and the formation of products. Therefore, the correct answer is activated complex.

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Quiz 13.1 - Heat Changes In Reactions

In this energy profile for a reaction, what is represented by the arrow labeled W?

In this energy profile for a reaction, what is represented by the arrow labeled Z?

In this energy profile for a reaction, what is represented by the arrow labeled R?

In this energy profile for a reaction, what is being plotted on the y-axis?

This reaction is best described as

A catalyst is best described as

In the equation Q=MCDT, the variable Q stands for

A good way to speed up a chemical reaction would be to

Generally, an easy way to double the rate of a chemical reaction would be to

A high-energy, unstable compound that is formed after the reactant phase but before the product phase is called a(n) ___.