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?

  • AExothermic
  • BEndothermic

Q2:

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

  • AA reaction that is accelerated by cooling
  • BA reaction that gives out heat
  • CA reaction that is accelerated by heating
  • DA reaction that is spontaneous
  • EA reaction that absorbs heat

Q3:

A chemical reaction has a reaction enthalpy of 512 kJ/mol. Is it endothermic or exothermic?

  • AEndothermic
  • BExothermic

Q4:

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

  • AA reaction that gives out heat
  • BA reaction that is accelerated by heating
  • CA reaction that is accelerated by cooling
  • DA reaction that is spontaneous
  • EA reaction that absorbs heat

Q5:

The complete combustion of methanol proceeds according to the equation: 2CHOH+3O2CO+4HO3222.

The reaction enthalpy is 715 kJ/mol. Is the combustion of methanol endothermic or exothermic?

  • AExothermic
  • BEndothermic

Q6:

The thermal decomposition of calcium carbonate proceeds according to the equation: CaCOCaO+CO32. The reaction enthalpy is +110 kJ/mol. Is the thermal decomposition of calcium carbonate endothermic or exothermic?

  • AEndothermic
  • BExothermic

Q7:

Consider the chemical reaction equation shown.

Is this reaction endothermic or exothermic?

  • AExothermic
  • BEndothermic

Q8:

When 10 mL of gasoline freezes at 57C, 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?

  • AFreezing is exothermic, while combustion is endothermic.
  • BBonds broken during combustion are weaker than those broken during freezing.
  • CBonds formed during combustion are stronger than those formed during freezing.
  • DCombustion occurs at a higher temperature than freezing.
  • EFreezing 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?

  • AThere are no energy changes during an exothermic reaction.
  • BCombustion is an endothermic reaction.
  • CHeat is not a form of energy.
  • DEnergy cannot be produced.
  • EExothermic reactions absorb heat.

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

  • ACombustion is an endothermic reaction because energy is produced in the form of heat.
  • BCombustion is an exothermic reaction because energy is not produced.
  • CCombustion is an exothermic reaction because chemical energy is converted to heat.
  • DCombustion is an exothermic reaction because heat is converted to chemical energy.
  • ECombustion is an endothermic reaction because thermal energy is released.

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?

  • AThe energy that can be released through the formation of chemical bonds
  • BThe energy stored in the vibration of chemical bonds
  • CThe energy that can be released through the breaking of chemical bonds
  • DThe energy stored in the movement of molecules
  • EThe energy that must be absorbed to form chemical bonds

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

  • ABond strength decreases and chemical potential energy increases.
  • BBond strength remains constant and chemical potential energy increases.
  • CBond strength and chemical potential energy both increase.
  • DBond strength and chemical potential energy both decrease.
  • EBond 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 an increase in the total bond energy.
  • BHeat is released to the surroundings and there is a decrease in the total bond energy.
  • CHeat is absorbed from the surroundings and there is no change in the total bond energy.
  • DHeat is absorbed from the surroundings and there is a decrease in the total bond energy.
  • EHeat is released to the surroundings and there is an increase 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 weaker bonds than liquids.
  • DFreezing is endothermic because solids contain stronger bonds than liquids.

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

  • ACooling reverses the process by breaking bonds between molecules.
  • BHeating reverses the process by forming bonds between molecules.
  • CCooling reverses the process by forming bonds between molecules.
  • DHeating reverses the process by breaking 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.

  • ACombustion
  • BMelting and boiling
  • CBoiling
  • DMelting, boiling, and combustion
  • EBoiling and combustion

Q14:

The air temperature in a sealed, insulated box is 20C. An ice cube at 0C 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?

  • AMelting is exothermic, so the air temperature decreases.
  • BMelting is exothermic, so the air temperature increases.
  • CMelting is endothermic, so the air temperature decreases.
  • DMelting is endothermic, so the air temperature increases.

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

  • AThe absorbed energy is used only to break bonds, so the ice temperature remains constant.
  • BThe absorbed energy is used only to form bonds, so the ice temperature remains constant.
  • CEnergy is released as bonds are broken, so the ice temperature increases.
  • DEnergy is absorbed as bonds are broken, so the ice temperature decreases.
  • EEnergy is released as bonds are formed, so the ice temperature increases.

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