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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:

  1. 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.  
  2. 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). 
  3. 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.
    1. 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.
    2. 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.
  4. 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:

  1. 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).
  2. Students are not assessed on the definitions of the words “endothermic” and “exothermic.”
  3. 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.
  4. 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.
Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID
Number
Knowledge Being Assessed Grades
4–5
Grades
6–8
Grades
9–12
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RG036003

The amount of chemical energy depends on the types and number of atoms inside the battery and how they are arranged in molecules.

N/A

44%

48%

RG212001

If the chemical energy of the products of a chemical reaction is greater than the chemcial energy of the reactants, energy was taken in from the surroundings because the amount of energy required to break bonds of the reactant molecules is greater than the amount of energy released when bonds of the product molecules are formed.

N/A

31%

32%

RG034004

Energy is always required to separate atoms of reactant molecules.

N/A

29%

34%

RG071003

During photosynthesis, energy is required to separate atoms of the reactants. This amount of energy is greater than the amount of energy released when atoms connect to form the products.

N/A

26%

31%

RG213001

If the chemical energy of the products of a chemical reaction is less than the chemcial energy of the reactants, energy was released to the surroundings because the amount of energy required to break bonds of the reactant molecules is less than the amount of energy released when bonds of the product molecules are formed.

N/A

28%

24%

RG035003

The amount of energy required to separate atoms of the reactants in a glow stick is less than the amount of energy released when the atoms connect to form the product.

N/A

26%

24%

RG085004

During the reaction between KI and Pb(NO3)2, energy is required to break the bonds of the reactant molecules, and energy is released when the bonds of the product molecules are formed.

N/A

16%

20%

RG085003

During the reaction between potassium iodide and lead nitrate, energy is required to break the bonds of the reactant molecules, and energy is released when the bonds of the product molecules are formed.

N/A

15%

16%

Frequency of selecting a misconception

Misconception
ID Number

Student Misconception

Grades
4–5

Grades
6–8

Grades
9–12

RGM025

Bond making requires energy (Tabor, 1998b; Ayyildiz & Tarhan, 2012).

N/A

49%

48%

RGM024

Energy is released when chemical bonds break (Ross, 1993; Ayyildiz & Tarhan, 2012).

N/A

46%

43%

RGM093

Chemical energy does not depend on the arrangement of molecules.

N/A

39%

34%

RGM094

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.