Ch 5 (Hess's Law)

Rohan, Harper, and Priya are studying for their chemistry exam. They come across a problem where they need to calculate the enthalpy change for the reaction of graphite with oxygen to produce carbon monoxide. However, they realize that this cannot be measured directly since some carbon dioxide is always formed in the reaction. They remember that they can use Hess’s Law and the enthalpy changes of combustion of graphite and of carbon monoxide to solve this.

They know that when graphite reacts with oxygen to produce carbon dioxide, the enthalpy change is ΔH=-394 kJmol^–1.

And when carbon monoxide reacts with oxygen to produce carbon dioxide, the enthalpy change is ΔH=-283 kJmol^–1.

Can you help them calculate the enthalpy change for the reaction of graphite with oxygen to give carbon monoxide?

• -677 kJmol^–1

• -111 kJmol^–1

• +111 kJmol^–1

• +677 kJmol^–1

Answer explanation

The enthalpy change for the reaction cannot be measured directly, but it can be calculated using Hess’s Law and the enthalpy changes of combustion of graphite and carbon monoxide. The enthalpy change for the reaction of graphite with oxygen to give carbon monoxide is -111 kJmol^–1.

Aria, Elijah, and Scarlett are conducting a science experiment. They notice that in their endothermic reaction, the system is releasing energy. Is this observation correct?

• True

• False

• Sometimes

Answer explanation

In an endothermic reaction, the system is absorbing energy, not releasing it. This means that the reaction requires an input of energy from the surroundings to proceed. The correct choice is 'False' because the statement contradicts the definition of an endothermic reaction. The question asks about the system's behavior, not the surroundings. Therefore, the answer explanation clarifies that endothermic reactions absorb energy and do not release it.

Tags

NGSS.HS-PS1-4

During a chemistry experiment, Mia, Aria, and Olivia observed that the temperature of the solution increased. This indicates that the reaction is ...

• endothermic, heat is taken in

• exothermic, heat is given out

• neutral, nothing happens

• exothermic, it gets colder

Answer explanation

In an exothermic reaction, heat is given out. This means that during the reaction, energy is released in the form of heat. The reaction itself produces more energy than it consumes, resulting in a net release of heat. This is in contrast to an endothermic reaction, where heat is taken in from the surroundings. In the case of an exothermic reaction, the surroundings become warmer as heat is transferred to them. Therefore, the correct answer is 'given out'.

Tags

NGSS.HS-PS3-4

Sophia is conducting a science experiment in her school lab. She notices that one of her chemical reactions requires additional heat to proceed. This type of reaction is known as a(n)

• exothermic reaction 

• endothermic reaction 

• over reaction 

• redox reaction

Answer explanation

An endothermic reaction is a chemical reaction that requires energy. In this type of reaction, energy is absorbed from the surroundings, resulting in a decrease in temperature. The correct choice is endothermic reaction because it describes a reaction that requires energy. The other options, exothermic reaction, over reaction, and redox reaction, do not fit the description of a reaction that requires energy. The question asks for a chemical reaction that requires energy, not a specific option number.

Tags

NGSS.HS-PS1-4

• 86 kJ

• - 86 kJ

• 68 kJ

• -68 kJ

Answer explanation

When the reaction is reversed, the sign of ΔHrxn changes. Since the original reaction has a ΔHrxn of 68 kJ, the reversed reaction will have a ΔHrxn of -68 kJ. This means that the reversed reaction is exothermic, releasing heat to the surroundings. Therefore, the correct answer is -68 kJ.

Tags

NGSS.HS-PS1-4

Sophia, Isla, and Anika are working on a science project. They need to calculate the energy required to turn 1 mole of N₂O₄(g) into 2 moles N & 4 moles O(g) for their experiment. Can you help them figure it out?

• 1933 kJ

• -1875 kJ

• - 1933 kJ

• 1875 kJ

Answer explanation

To determine the energy required to convert 1 mole of N2O4(g) into 2 moles of N and 4 moles of O(g), we need to consider the balanced chemical equation. The given answer, 1933 kJ, is correct. This value represents the energy change associated with the reaction. The positive sign indicates that energy is absorbed during the reaction. Therefore, the correct answer is 1933 kJ.

Tags

NGSS.HS-PS1-4

NGSS.HS-PS3-1