Key Idea: Chemical energy is associated with the arrangement of atoms that make up the molecules of the reactants and products of a chemical reaction. Because the arrangement of atoms making up the molecules is different before and after the chemical reaction takes place, the amount of chemical energy in the system is also different.
Students should know that:
- The amount of chemical energy in a chemical reaction system before and after the reaction occurs depends on the types and number of atoms in the system and how they are arranged in molecules. Different systems of reactants and products typically have different amounts of chemical energy because they have different configurations of atoms.
- All chemical reactions involve both bond breaking and bond forming. Energy is always required to separate the atoms that make up molecules (bond breaking), and energy is always released when atoms connect to form molecules (bond forming).
- The relative amounts of energy required to break bonds and released when bonds are formed determines whether energy is given off to or taken in from the
surroundings during a chemical reaction.
- For exothermic chemical reactions (reactions that release more energy than they take in), the amount of energy released as the product molecules form is greater than the energy required to separate the atoms of the reactant molecules. The energy transferred to the surroundings can be manifest as an increase in thermal energy or motion energy or the production of light or sound.
- For endothermic chemical reactions (reactions that take in more energy than they release), the amount of energy released as the product molecules form is less than the energy required to separate the atoms of the reactant molecules. The energy transferred from the surroundings can be detected by a decrease in temperature of the surroundings or the absorption of light or sound.
- If energy is released during a chemical reaction, then the reverse reaction takes in energy [but the reverse reaction is not always possible]. If energy is taken in during a chemical reaction, then the reverse reaction releases energy (e.g., photosynthesis and cellular respiration).
Boundaries:
- For this idea, dissolving is considered a chemical change (because the configuration of and interactions between atoms change). Therefore, item contexts may include the dissolution of ionic solids (e.g. cold packs).
- Students are not assessed on the definitions of the words “endothermic” and “exothermic.”
- Students are not expected to know that the energy that can be released from or absorbed by the reactants in a chemical reaction depends on other conditions such as temperature or type of solvent.
- Students are not expected to quantify how much chemical energy is stored in any particular chemical reaction system. Students are not expected to know or use formulas associated with chemical potential energy such as DHº = S(nDHfº)(products) - S(nDHfº)(reactants). The sub-ideas above describe semi-quantitative relationships.
Item ID Number |
Knowledge Being Assessed | Grades 4–5 |
Grades 6–8 |
Grades 9–12 |
Select This Item for My Item Bank |
---|---|---|---|---|---|
N/A | 44% |
48% | |||
N/A | 31% |
32% | |||
Energy is always required to separate atoms of reactant molecules. | N/A | 29% |
34% | ||
N/A | 26% |
31% | |||
N/A | 28% |
24% | |||
N/A | 26% |
24% | |||
N/A | 16% |
20% | |||
N/A | 15% |
16% |
Misconception |
Student Misconception |
Grades 4–5 |
Grades |
Grades |
---|---|---|---|---|
Bond making requires energy (Tabor, 1998b; Ayyildiz & Tarhan, 2012). | N/A | 49% |
48% | |
Energy is released when chemical bonds break (Ross, 1993; Ayyildiz & Tarhan, 2012). | N/A | 46% |
43% | |
Chemical energy does not depend on the arrangement of molecules. | N/A | 39% |
34% | |
Chemical energy doesn't depend on the number or types of atoms. | N/A | 30% |
28% |
Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.
Code |
Statement |
---|---|
Some chemical reactions release energy, others store energy. |
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