CAST Chemistry Review

The table below shows the boiling points of several substances. Which substance is most likely to have the strongest intermolecular forces?

• Substance A

• Substance B

• Substance C

• Substance D

Answer explanation

The boiling point of a substance is directly related to the strength of its intermolecular forces. Higher boiling points generally indicate stronger forces because more energy is required to overcome them. Therefore, Substance D, with the highest boiling point, is likely to have the strongest intermolecular forces.

Tags

NGSS.HS-PS1-3

The combustion of methane (CH₄) is represented by the following balanced chemical equation:

CH₄ + 2O₂ → CO₂ + 2H₂O.

If 4 moles of water (H₂O) form, how many moles of methane (CH₄) react?

• 1 mole

• 2 moles

• 4 moles

• 8 moles

Answer explanation

The mole ratio of water to methane is 2:1 so if 4 moles of water are produced, half as much methane reacts. Therefore the answer is 2 moles.

Tags

NGSS.HS-PS1-7

In a chemical reaction, a system is modeled to show the bond energy changes between reactants and products. The diagram shows that the reactants start at a higher energy level, and the products are at a lower energy level. What is the most likely outcome of this reaction?

• The reaction is endothermic and absorbs energy from the surroundings.

• The reaction is exothermic and releases energy to the surroundings.

• The reaction requires an external source of energy to proceed.

• The reaction does not involve any energy change.

Answer explanation

When the reactants start at a higher energy level and the products are at a lower energy level, this indicates that the reaction releases energy. The energy difference between the reactants and products is released into the surroundings, making the reaction exothermic.

Tags

NGSS.HS-PS1-4

The combustion of methane (CH₄) is represented by the following balanced chemical equation:

CH₄ + 2O₂ → CO₂ + 2H₂O.

If 16 g of methane (CH₄) react with excess oxygen (O₂), how many grams of carbon dioxide (CO₂) will be produced? (Molar masses: CH₄ = 16 g/mol, CO₂ = 44 g/mol)

• 22 g

• 44 g

• 88 g

• 16 g

Answer explanation

First, convert 16.0 g of CH₄ to moles:

16.0 g CH4 / 16.0 g/mol=1 mol CH4

From the balanced equation, 1 mole of CH₄ produces 1 mole of CO₂. Therefore, 1 mole of CH₄ produces 44.0 g of CO₂. 

Tags

NGSS.HS-PS1-7

What is the most direct method of measuring energy flow in a system during a chemical reaction?

• By measuring the change in pressure

• By measuring the temperature change in the system

• By observing the color change of the substance

• By measuring the volume of gas produced

Answer explanation

Energy flow in a system can be measured by observing the temperature change. In exothermic reactions, the system releases energy in the form of heat, which is detected as an increase in temperature. Similarly, in endothermic reactions, the system absorbs heat, leading to a decrease in temperature.

Tags

NGSS.HS-PS3-4

During a chemical reaction, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water. What can be concluded from this reaction about the conservation of mass and atoms?

• The total mass of the reactants is different from the total mass of the products.

• The number of atoms of hydrogen and oxygen is conserved in the products.

• The total number of hydrogen atoms decreases during the reaction.

• The number of moles of hydrogen atoms is less than the number of moles of oxygen atoms in the products.

Answer explanation

The reaction demonstrates that the number of hydrogen and oxygen atoms in the reactants is conserved in the products. The 2 moles of hydrogen atoms and 1 mole of oxygen atoms rearrange to form 2 moles of water, maintaining the atom balance as required by the law of conservation of mass.

Tags

NGSS.HS-PS1-7

In the context of climate feedback loops, a rise in global temperatures has been shown to lead to the melting of Arctic ice. If the melting continues at an accelerated rate, how might this change the Earth's surface energy budget, and what feedback effect could it produce?

• The Earth’s albedo decreases as ice melts, causing more solar radiation to be absorbed by the surface, leading to further warming and more ice melt. This is a reinforcing feedback.

• The Earth’s albedo increases as ice melts, causing less solar radiation to be absorbed, which cools the surface. This is a counterbalancing feedback.

• The melting ice causes a decrease in atmospheric CO₂, leading to cooling. This is a counterbalancing feedback.

• The loss of ice reduces ocean circulation, lowering global temperatures and further ice melt. This is a reinforcing feedback.

Answer explanation

Explanation: Ice has a high albedo, meaning it reflects most of the incoming sunlight. As ice melts, darker ocean or land surfaces are exposed, which absorb more sunlight and heat, leading to more warming and further ice melt, forming a reinforcing feedback loop.

Tags

NGSS.HS-ESS2-2

NGSS.HS-ESS2-4

The reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) is at equilibrium. Which of the following would result in an increase in ammonia (NH₃) production?

• Increasing the temperature.

• Decreasing the pressure.

• Adding a catalyst.

• Increasing the concentration of nitrogen gas (N₂).

Answer explanation

Explanation: Adding more nitrogen gas will shift the equilibrium toward the right, producing more ammonia (NH₃), as the system will try to counteract the change by consuming more N₂ and H₂ to form NH₃.

Tags

NGSS.HS-PS1-5

NGSS.HS-PS1-6

Ocean acidification occurs when carbon dioxide (CO₂) in the atmosphere dissolves in seawater and reacts with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).

If the concentration of CO₂ in the atmosphere increases, what effect will this have on the rate of ocean acidification?

• The rate of acidification will decrease because the system will reach equilibrium more quickly.

• The rate of acidification will remain unchanged because the reaction does not depend on the concentration of CO₂.

• The rate of acidification will increase because there are more CO₂ molecules available to react with water.

• The rate of acidification will decrease because higher concentrations of CO₂ will limit the number of water molecules available for reaction.

Answer explanation

Explanation: According to the collision theory, increasing the concentration of CO₂ increases the frequency of collisions between CO₂ molecules and water molecules, which results in a faster rate of reaction, leading to greater production of H⁺ ions and increased ocean acidification. A system at equilibrium will be pushed forward if the concentration of reactants increases.

Tags

NGSS.HS-ESS2-6

NGSS.HS-PS1-5

When a hot metal block is placed into a container of cooler water, which of the following best describes the energy transfer that occurs?

• Thermal energy flows from the water to the metal block until both reach the same temperature.

• Thermal energy flows from the metal block to the water until both reach the same temperature.

• No thermal energy transfer occurs because the system is closed.

• The metal block loses all of its thermal energy, and the water gains none.

Answer explanation

Explanation: Thermal energy always moves from a region of higher temperature to a region of lower temperature until thermal equilibrium is reached, according to the second law of thermodynamics.

Tags

NGSS.HS-PS3-4