AP Chemistry

HCl(aq)+NH₃(aq)→NH₄⁺(aq)+Cl⁻(aq)

The chemical reaction between and NH₃(aq) is represented above. A student combines equimolar amounts of HCl(aq) and NH₃(aq), both solutions initially at 24°C, in a coffee-cup calorimeter. The student observes that the mixture reaches a temperature of 28°C. Based on the experimental results, which of the following can be concluded about the reaction?

• It is an endothermic process, because energy is released by the reaction and is gained by the reaction mixture.

• It is an endothermic process, because energy is absorbed by the reaction and is lost from the reaction mixture.

• It is an exothermic process, because energy is released by the reaction and is gained by the reaction mixture.

• It is an exothermic process, because energy is absorbed by the reaction and is lost from the reaction mixture.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS1-5

NGSS.HS-PS3-4

When pellets of NaOH(s) are added to a flask of water, it is observed that the temperature of the water increases as the pellets dissolve. Which of the following claims about the observed dissolution of NaOH(s) in water is most accurate?

• It is an endothermic process because heat energy is absorbed by the water as the NaOH(s) dissolves in it.

• It is an endothermic process because heat energy is released by the water as the NaOH(s) dissolves in it.

• It is an exothermic process because heat energy is absorbed by the water as the NaOH(s) dissolves in it.

• It is an exothermic process because heat energy is released by the water as the NaOH(s)) dissolves in it.

Tags

NGSS.HS-PS3-4

An advertisement for a commercial hand warmer claims that the hand warmer works because a chemical reaction in the hand warmer draws out the body’s own natural heat, causing a warming effect. Which of the following states the accuracy of the claim in the advertisement and best provides a correct scientific justification of the claim?

• The advertisement’s claim is inaccurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was endothermic, which would cause the hands to feel colder.

• The advertisement’s claim is inaccurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was exothermic, which would cause the hands to feel colder.

• The advertisement’s claim is accurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was endothermic, which would cause the hands to feel warmer.

• The advertisement’s claim is accurate because heat flowing from the hands to the warmer would only happen if the chemical reaction was exothermic, which would cause the hands to feel warmer.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-4

A student carefully drops a 9.0g solid Zn pellet initially at 50.0°C into an insulated cup containing 30.0g of water at 27.8°C. The student predicts that the temperature of the water will increase after the pellet is added. Which of the following statements is the best justification for the student’s prediction?

• The metallic bonds between Zn atoms will break when the Zn is exposed to the water molecules, releasing energy that will be absorbed by the water molecules.

• Collisions between the water molecules and the surface of the Zn pellet will result in the transfer of energy, increasing the average kinetic energy of the water molecules.

• The strength of the hydrogen bonds between the water molecules will increase when the Zn pellet is added, decreasing the average kinetic energy of the water molecules.

• Collisions between Zn atoms in the solid will increase in frequency when the Zn is exposed to the water molecules, resulting in the transfer of energy to the surroundings.

Tags

NGSS.HS-PS3-2

NGSS.HS-PS3-4

Two aqueous NaCl solutions of equal volume and concentration were kept in flasks and held at different temperatures. The two solutions were combined in a larger flask. Based on this information, which of the following predictions is correct?

• The average kinetic energy of the particles in the cooler solution will decrease as they collide with the particles from the warmer solution.

• The average kinetic energy of the particles in the cooler solution will increase as they collide with the particles from the warmer solution.

• The particles from the cooler solution and the particles from the warmer solution will reach thermal equilibrium through collisions, at which time the average kinetic energy of the particles in the mixture will be lower than the average kinetic energy that the particles had in the cooler solution.

• The particles from the cooler solution and the particles from the warmer solution will reach thermal equilibrium through collisions, at which time the average kinetic energy of the particles in the mixture will be higher than the average kinetic energy that the particles had in the warmer solution.

Tags

NGSS.HS-PS3-4

The table provides data for two CH₃OH samples. Based on this information, which of the following statements describes what happens when these samples are initially mixed, and why?

• Thermal energy is transferred between the CH₃OH molecules in sample 1 and the CH₃OH molecules in sample 2 because both samples contain the same substance.

• The CH₃OH molecules from sample 2 transfer thermal energy to the CH₃OH molecules from sample 1 through collisions because there are more moles of molecules in sample 2.

• The CH₃OH molecules from sample 1 transfer thermal energy to the CH₃OH molecules from sample 2 through collisions because sample 1 has a higher density.

• The CH₃OH molecules from sample 1 transfer thermal energy to the CH₃OH molecules from sample 2 through collisions because the average kinetic energy of the molecules in sample 1 is greater.

Tags

NGSS.HS-PS1-5

NGSS.HS-PS3-4

Define 'Enthalpy'

• a) Energy stored in the movement of molecules in a substance
• b) The opposite of temperature
• c) The temperature of a molecule
• d) Energy stored in the chemical bonds in a substance

Tags

NGSS.HS-PS1-4

Which statement is true regarding endothermic reactions?

• a) Temperature change is positive and enthalpy change is positive
• b) Temperature change is positive and enthalpy change is negative
• c) Temperature change is negative and enthalpy change is positive
• d) Temperature change is negative and enthalpy change is negative

Tags

NGSS.HS-PS1-4

Temperature is...

• A measure of the potential energy stored in a substance

• The same as heat

• A measure of the random motions of the components of a substance

Tags

NGSS.HS-PS3-2

In a laboratory experiment a student was using Styrofoam cups and foil to reduce the energy lost during an exothermic reaction.  The reaction constantly had a temperature probe in it.  Towards the end of the reaction the student noticed the temperature in the cups was going down (opposite what they thought is should).  The student concluded that the energy was destroyed within the system.  This student's reasoning violates what law?

• Law of Conservation of Mass
• Law of Enthalpy
• First Law of Thermodynamics
• Law of Heat Transfer

Tags

NGSS.HS-PS3-4