Worksheet: Exothermic and Endothermic Reactions

In this worksheet, we will practice describing exothermic and endothermic reactions and examining the energy transfers involved.

Q1:

A chemical reaction has a reaction enthalpy of +430 kJ/mol. Is it endothermic or exothermic?

  • A Endothermic
  • B Exothermic

Q2:

Which of the following is the best definition of an exothermic reaction?

  • A A reaction that is accelerated by heating
  • B A reaction that absorbs heat
  • C A reaction that is accelerated by cooling
  • D A reaction that is spontaneous
  • E A reaction that gives out heat

Q3:

A chemical reaction has a reaction enthalpy of 5 1 2 kJ/mol. Is it endothermic or exothermic?

  • A Exothermic
  • B Endothermic

Q4:

Which of the following is the best definition of an endothermic reaction?

  • A A reaction that is spontaneous
  • B A reaction that absorbs heat
  • C A reaction that gives out heat
  • D A reaction that is accelerated by heating
  • E A reaction that is accelerated by cooling

Q5:

The complete combustion of methanol proceeds according to the equation:

The reaction enthalpy is 7 1 5 kJ/mol. Is the combustion of methanol endothermic or exothermic?

  • A Exothermic
  • B Endothermic

Q6:

The thermal decomposition of calcium carbonate proceeds according to the equation: The reaction enthalpy is +110 kJ/mol. Is the thermal decomposition of calcium carbonate endothermic or exothermic?

  • A Exothermic
  • B Endothermic

Q7:

Consider the chemical reaction equation shown.

Is this reaction endothermic or exothermic?

  • AExothermic
  • B Endothermic

Q8:

When 10 mL of gasoline freezes at 5 7 C , the total amount of heat transferred is approximately 1 kJ. However, 300 kJ of heat is transferred when the same volume of gasoline is burned at room temperature. Why does combustion result in a greater transfer of heat than freezing?

  • A Freezing is exothermic, while combustion is endothermic.
  • B Bonds formed during combustion are stronger than those formed during freezing.
  • C Bonds broken during combustion are weaker than those broken during freezing.
  • D Combustion occurs at a higher temperature than freezing.
  • E Freezing is endothermic, while combustion is exothermic.

Q9:

When reporting an experiment, a student writes that “combustion is an exothermic reaction because energy is produced in the form of heat.”

Why is this description not correct?

  • A Energy cannot be produced.
  • B Combustion is an endothermic reaction.
  • C There are no energy changes during an exothermic reaction.
  • D Exothermic reactions absorb heat.
  • E Heat is not a form of energy.

Which of the following is a more accurate description of the combustion reaction?

  • A Combustion is an endothermic reaction because thermal energy is released.
  • B Combustion is an exothermic reaction because energy is not produced.
  • C Combustion is an exothermic reaction because heat is converted to chemical energy.
  • D Combustion is an endothermic reaction because energy is produced in the form of heat.
  • E Combustion is an exothermic reaction because chemical energy is converted to heat.

Q10:

Exothermic and endothermic reactions both involve changes in bond strength and chemical potential energy.

Which of the following is the best definition of chemical potential energy?

  • A The energy that must be absorbed to form chemical bonds
  • B The energy stored in the vibration of chemical bonds
  • C The energy stored in the movement of molecules
  • D The energy that can be released through the formation of chemical bonds
  • E The energy that can be released through the breaking of chemical bonds

How do bond strength and chemical potential energy change during an exothermic reaction?

  • A Bond strength decreases and chemical potential energy increases.
  • B Bond strength and chemical potential energy both decrease.
  • C Bond strength remains constant and chemical potential energy increases.
  • D Bond strength and chemical potential energy both increase.
  • E Bond strength increases and chemical potential energy decreases.

Q11:

Which of the following is the best definition of an endothermic reaction?

  • AHeat is absorbed from the surroundings and there is a decrease in the total bond energy.
  • BHeat is absorbed from the surroundings and there is no change in the total bond energy.
  • CHeat is released to the surroundings and there is an increase in the total bond energy.
  • DHeat is absorbed from the surroundings and there is an increase in the total bond energy.
  • EHeat is released to the surroundings and there is a decrease in the total bond energy.

Q12:

Hand warmers are sealed packets of liquid that freeze when a metal disc is pressed. These devices are commonly used during cold weather.

Why does freezing of the liquid in a hand warmer cause the temperature to change?

  • AFreezing is exothermic because solids contain stronger bonds than liquids.
  • BFreezing is exothermic because solids contain weaker bonds than liquids.
  • CFreezing is endothermic because solids contain stronger bonds than liquids.
  • DFreezing is endothermic because solids contain weaker bonds than liquids.

How is it possible to reuse a hand warmer after freezing takes place?

  • AHeating reverses the process by breaking bonds between molecules.
  • BCooling reverses the process by breaking bonds between molecules.
  • CCooling reverses the process by forming bonds between molecules.
  • DHeating reverses the process by forming bonds between molecules.

Q13:

Paraffin wax is a soft solid used in candles and lubricants. Listed in the table are three processes that are observed when paraffin is heated and the temperatures at which these changes take place.

Process Melting Combustion Boiling
Temperature ( C ) 60 245 320

Identify the exothermic processes listed in the table.

  • ABoiling
  • BMelting, boiling, and combustion
  • C Combustion
  • D Melting and boiling
  • E Boiling and combustion

Q14:

The air temperature in a sealed, insulated box is 2 0 C . An ice cube at 0 C is placed in the box, where it slowly melts.

How and why does the melting of the ice affect the air temperature in the box?

  • A Melting is exothermic, so the air temperature decreases.
  • B Melting is endothermic, so the air temperature decreases.
  • C Melting is exothermic, so the air temperature increases.
  • D Melting is endothermic, so the air temperature increases.

How and why does the temperature of the remaining ice vary while melting takes place?

  • A The absorbed energy is used only to form bonds, so the ice temperature remains constant.
  • B Energy is released as bonds are formed, so the ice temperature increases.
  • C Energy is absorbed as bonds are broken, so the ice temperature decreases.
  • D Energy is released as bonds are broken, so the ice temperature increases.
  • E The absorbed energy is used only to break bonds, so the ice temperature remains constant.

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