CAST Chemistry-Segment 1-Combustion

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-PS1-5

How does the strength of the electrostatic interactions between particles in a substance correlate with the substance's physical properties, such as boiling point?

• A stronger electrostatic interaction between particles typically results in a lower boiling point, as the substance requires less energy to overcome its interactions.

• Stronger electrostatic interactions between particles typically result in a higher boiling point, as more energy is required to overcome these forces.

• The strength of electrostatic interactions has no significant impact on the boiling point, which is mainly governed by molecular size.

• A weaker electrostatic interaction generally results in a lower boiling point, but only in ionic compounds.

Answer explanation

The boiling point is a measure of the energy needed to overcome the intermolecular forces between particles. Substances with stronger electrostatic forces (such as ionic or network covalent structures) require more energy to break these bonds and transition to the gas phase, resulting in a higher boiling point.

Tags

NGSS.HS-PS2-4

How is energy released from fuels such as carbohydrates or hydrocarbons during metabolism or combustion?

• By the release of electrical charges when chemical bonds are broken

• By the rearrangement of atoms into new molecules, releasing chemical energy

• By the release of light and heat through nuclear reactions

• By the compression of particles, causing an increase in energy

Answer explanation

During combustion or metabolism, chemical bonds in fuels (such as carbohydrates or hydrocarbons) are broken and rearranged, releasing energy in the form of heat and light. This release of energy comes from the chemical potential energy stored in the bonds between atoms.

Tags

NGSS.HS-PS1-4

How does the structure of a substance at the bulk scale help infer the strength of electrical forces between its particles?

• Substances with tightly packed structures tend to have weaker electrical forces.

• Substances with rigid and orderly structures (like network solids) tend to have weaker electrical forces.

• Substances with tightly packed and orderly structures tend to have stronger electrical forces.

• Substances with irregular, loose structures tend to have stronger electrical forces.

Answer explanation

 In substances like network solids (e.g., graphite or diamond) or ionic compounds, the particles are closely packed and organized in a way that maximizes the strength of the electrical forces between them. This results in high melting and boiling points. In contrast, substances with irregular or less tightly packed structures typically have weaker electrical forces between particles

Tags

NGSS.HS-PS1-3

In a chemical reaction, the release or absorption of energy depends on the difference in energy between which two factors?

• The temperature of the reactants and products

• The total bond energy of the reactants and products

• The surface area of the reactants and products

• The volume of the reactants and products

Answer explanation

The release or absorption of energy in a chemical reaction depends on the difference in the total bond energies between the reactants and products. If the bonds formed in the products are stronger than those in the reactants, energy is released (exothermic reaction). If the bonds in the products are weaker, energy is absorbed (endothermic reaction).

Tags

NGSS.HS-PS1-4

In an exothermic reaction, what happens to the total bond energy as the reaction progresses?

• The bond energy of the reactants decreases, and the bond energy of the products increases, releasing energy.

• The bond energy of the reactants is greater than that of the products, releasing energy.

• The bond energy of the reactants and products remains the same, so no energy is released or absorbed.

• The bond energy of the reactants is weaker than that of the products, absorbing energy.

Answer explanation

In an exothermic reaction, the total bond energy of the reactants is greater than that of the products. When the bonds in the reactants break and new bonds are formed in the products, the energy released is greater than the energy needed to break the bonds in the reactants. This excess energy is released as heat.

Tags

NGSS.HS-PS1-4

Which of the following best describes the relationship between bond energy and the absorption or release of energy in a chemical reaction?

• If the total bond energy of the products is greater than that of the reactants, the reaction absorbs energy.

• If the total bond energy of the products is greater than that of the reactants, the reaction releases energy.

• If the total bond energy of the products is equal to the reactants, no energy is absorbed or released.

• If the total bond energy of the reactants is equal to that of the products, the reaction will always absorb energy.

Answer explanation

If the total bond energy of the products is greater than that of the reactants, the reaction requires more energy to form the bonds in the products, which results in the absorption of energy. This is characteristic of endothermic reactions.

Tags

NGSS.HS-PS1-4

Which mathematical approach can be used to demonstrate the conservation of atoms during a chemical reaction?

• By calculating the difference between the total volume of reactants and products.

• By converting the mass of reactants to moles, using the molar mass, and comparing the moles of reactants and products.

• By measuring the temperature change during the reaction and comparing it to the initial temperature.

• By measuring the pH of the reaction and comparing it to the pH of the products.

Answer explanation

To demonstrate conservation of atoms, we use the mole concept. By converting the mass of reactants into moles (using the molar mass), we can compare the moles of atoms in the reactants to the moles in the products. This shows that the number of atoms is conserved, even though they may be rearranged in different products.

Tags

NGSS.HS-PS1-7

In a balanced chemical equation, how can you demonstrate the conservation of mass in terms of atoms?

• By showing that the number of atoms of each element on both sides of the equation is equal.

• By comparing the energy released in the reaction to the mass of the products.

• By calculating the difference in the number of molecules of reactants and products.

• By adjusting the volume of the reactants and products to match each other.

Answer explanation

A balanced chemical equation shows that the number of atoms of each element in the reactants is equal to the number of atoms of that element in the products. This reflects the conservation of atoms, which in turn ensures the conservation of mass during the reaction.

Tags

NGSS.HS-PS1-7

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