Science Assessment

Natural gas is a fossil fuel that is found deep beneath the surface of the earth. It is made mostly of methane. When natural gas is burned, methane (CH₄) reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O) as represented below.

CH₄ + 2 O₂ → CO₂ + 2 H₂O

As methane reacts with oxygen, a flame is observed, and the temperature of the surroundings increases. The video below shows what happens when a soap bubble filled with methane gas reacts with oxygen in the air. The flame from a candle is used to start the reaction.

A student notices that the flame from the methane-oxygen reaction is much bigger than the flame from the candle. The student wonders how such a small flame from the candle could produce such a large flame from the bubble filled with methane. The student knows that a flame is an indicator that energy is being released during the chemical reaction and that all chemical reactions involve breaking and forming bonds between atoms.

1. To answer the question about why such a large flame was produced, the student starts to think about the energy changes that occur during the breaking and forming of bonds. Which of the following describes the energy changes associated with breaking and forming bonds during a chemical reaction?

A. Energy is given off as bonds are broken, and energy is given off when bonds are formed.

B. Energy is given off as bonds are broken, but energy is required to form bonds.

C. Energy is required to break bonds, but energy is given off as bonds are formed.

D. Energy is required to break bonds, and energy is required to form bonds.

Next, he decides to draw models to help him think about the energy associated with the atoms and molecules in the chemical reaction system. He knows that to form the products, bonds between the atoms of the reactants need to break and new bonds need to form. So, he decides to model a hypothetical transition state where all the bonds are broken and the atoms are separated.

2. Which bar graph best represents the energy associated with the methane and oxygen molecules and with the separated carbon, hydrogen, and oxygen atoms?

A.			B.
C.

3. Which bar graph best represents the energy associated with the separated carbon, hydrogen, and oxygen atoms and with the water and carbon dioxide molecules?

A.			B.
C.

The student knows that there are attractive forces between the atoms that make up the molecules in the chemical reaction system.

4. Using the idea that there are attractive forces between atoms, explain why the graphs you selected in Questions 2 and 3 best represent the energy associated with the different configurations of atoms. Be sure to link your explanation to the graphs you selected.

5. The student wants to use the graphs to make a simplified model that represents the energy changes that occur during the reaction. Which model represents the potential energy in the chemical reaction system for each configuration?

A.			B.
C.			D.
E.			F.

6. Why was a little bit of energy from the candle needed to start the methane-oxygen reaction even though a lot of energy was released during the reaction?

A. An input of energy was needed to start the reaction because bonds between the atoms of the product molecules need to be formed, and an initial input of energy is needed to form bonds.

B. An input of energy was needed to start the reaction because bonds between the atoms of the reactant molecules must be broken for a reaction to occur, and an initial input of energy is required to break bonds.

C. An input of energy was needed to start the reaction because energy is stored in the bonds between the atoms of the reactant molecules, and a small amount of energy is needed to trigger the release of that stored energy.

D. An input of energy was needed to start the reaction because chemical reactions can only occur at high temperatures, and the flame from the candle heated the methane up to a temperature high enough for the reaction to occur.