Science Assessment ~ Items ~ EB007003

Methanol (CH₃OH) reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O). Molecular models of the molecules involved in this reaction are shown below. In the models, gray circles represent carbon atoms, red circles represent oxygen atoms, and white circles represent hydrogen atoms. Lines represent bonds between the atoms that make up molecules.

During this reaction, a flame is observed and the temperature of the surroundings increases. Scientists have determined the amount of energy associated with the breaking and forming bonds of the reactant and product molecules. The table below lists the sum of the bond energies for all the bonds in the reactant molecules and for all the bonds in the product molecules when two moles of methanol react with three moles of oxygen. [A kilocalorie (kcal) is a unit of energy and a mole is a very large quantity of molecules.]

Sum of the bond energies in kilocalories (kcal)
Reactants	2 CH₃OH + 3 O₂	1,345 kcal
Products	2 CO₂ + 4 H₂O	1,651 kcal

Does this chemical reaction give off energy or take in energy? Support your answer using the models and data given above. In your response, be sure to describe the changes in both matter and energy.

Details
Performance
NGSS

Topic

Energy Changes

Scoring Rubric

Students receive three scores for this item. One is based on whether or not they made the correct claim (0-1 point), one is based on whether they included substance-level evidence and reasoning (0-2 points), and the third is based on whether they included atomic-level evidence and reasoning (0-2 points).

Claim: Energy is given off by the chemical reaction system to the surroundings.
Substance-level evidence and reasoning (one point each for a maximum of 2 points):
1. Evidence from observations: A flame/light was observed and/or an increase in temperature of the surroundings was observed.
2. Reasoning with science ideas: Because we see light and/or an increase in temperature, we can conclude that energy was given off.
Atomic-level evidence and reasoning (one point each for a maximum of 2 points):
1. Evidence from the table: Data show that less energy is required to break bonds of the reactants (1345 kcal) than is given off when bonds form between products (1651 kcal) or that the total bond energy of the products is greater than the total bond energy of the reactants.
2. Reasoning with science ideas: Because less energy was required to break bonds of reactants than was given off when bonds of products formed, we can conclude that there was a net release of energy to the surroundings.

Pre-Test

Pre-Test

Post-Test (Treatment)

Post-Test

Points Earned

Group	Earned	Possible	Percent
Overall	109	2675	4%
Grades
6–8	N/A	N/A	N/A
9–12	94	2360	4%
Gender
Male	71	1390	5%
Female	28	990	3%
Primary Language
English	89	2235	4%
Other	7	150	5%

View data table

Points Earned

Group	Earned	Possible	Percent
Overall	221	2675	8%
Grades
6–8	N/A	N/A	N/A
9–12	208	2445	9%
Gender
Male	118	1405	8%
Female	91	1000	9%
Primary Language
English	199	2260	9%
Other	9	155	6%

View data table

Disciplinary Core Ideas: PS1.A A stable molecule has less energy than the same set of atoms separated; one must provide at least this energy in order to take the molecule apart.
PS1.B Chemical processes, their rates, and whether or not energy is stored or released can be understood in terms of the collisions of molecules and the rearrangements of atoms into new molecules, with consequent changes in the sum of all bond energies in the set of molecules that are matched by changes in kinetic energy.

Science Assessment

Item EB007003: The reaction between methanol and oxygen releases energy because the amount of energy released when bonds form is greater than the amount of energy required to break bonds.