HC Energy Changes in Chemical Reactions

If a substance evaporates, is that endothermic or exothermic?

• exothermic

• endothermic

Answer explanation

Evaporation is an endothermic process because it requires energy to break the intermolecular forces in a liquid, allowing molecules to escape into the gas phase. Thus, the correct answer is endothermic.

Energy in exothermic reactions is... (reaction is the system)

• released - it would make us feel cold

• absorbed - it would us feel cold

• released - it would make us feel warm

• absorbed - it would make us feel warm

Answer explanation

In exothermic reactions, energy is released into the surroundings, which increases the temperature and makes us feel warm. Therefore, the correct choice is 'released - it would make us feel warm'.

Tags

NGSS.HS-PS1-4

During an endothermic reaction (reaction is system) in a beaker if we are part of the surroundings and touched the beaker, it would feel _______.

• Warm

• Cold

Answer explanation

During an endothermic reaction, the system absorbs heat from the surroundings. When you touch the beaker, it feels cold because heat is being drawn from your hand, making the beaker lose thermal energy.

Tags

NGSS.HS-PS3-4

Which of the following process is exothermic?

• Candle was melting

• A puddle evaporating

• Dry ice (solid carbon dioxide) subliming to form gaseous carbon dioxide

• Water freezing to form ice

Answer explanation

Water freezing to form ice is an exothermic process because it releases heat to the surroundings as the liquid water transitions to solid ice, unlike the other options which either absorb heat or involve phase changes that do not release heat.

Tags

NGSS.HS-PS1-4

If you go to the beach on a hot sunny day, the sand may feel hot. If you then stand in the ocean, the water may still feel cold. (You are the system - are you absorbing or releasing?!) These are examples of....

• an endothermic process followed by an exothermic process

• an exothermic process followed by an endothermic process

• endothermic processes only

• exothermic processes only

Answer explanation

When you stand on hot sand, your body absorbs heat (endothermic). Then, when you enter the cold ocean, your body releases heat to the water (exothermic). Thus, it's an endothermic process followed by an exothermic process.

Tags

NGSS.HS-PS3-4

A student mixed two chemicals in water to allow them to react. The temperature of the water before the reaction was 25 ° C. The temperature of the water after the reaction was 18° C. Which of the following is true?

• It is an exothermic reaction

• It is an endothermic reaction

Answer explanation

The temperature decreased from 25 °C to 18 °C, indicating that heat was absorbed from the surroundings. This is characteristic of an endothermic reaction, where energy is taken in, leading to a drop in temperature.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-4

When bonds are broken, is that an endothermic or exothermic process?

• Endothermic

• Exothermic

Answer explanation

Breaking bonds requires energy input to overcome the attractive forces between atoms, making it an endothermic process. Therefore, the correct answer is 'Endothermic'.

Tags

NGSS.HS-PS1-4

When bonds are formed, is that an endothermic or exothermic process?

• Endothermic

• Exothermic

Answer explanation

When bonds are formed, energy is released as the system becomes more stable. This release of energy characterizes the process as exothermic, meaning it releases heat to the surroundings.

Tags

NGSS.HS-PS1-4

The minimum amount of energy needed for colliding particles to react is called the

• Chemical Energy

• Kinetic Energy

• Activation Energy

• Potential Energy

Answer explanation

The minimum energy required for colliding particles to undergo a reaction is known as Activation Energy. It is essential for overcoming the energy barrier for a chemical reaction to occur.

Which letter corresponds to the energy of the reactants?

• A

• B

• C

• D

Answer explanation

The letter A corresponds to the energy of the reactants, indicating their initial energy level before the reaction occurs. This is crucial for understanding the energy changes during the reaction.

Tags

NGSS.HS-PS1-4