Chemical Reactions

In this unit, students build on their knowledge of energy by exploring the relationship between energy and matter. In this lesson students conduct an investigation into the science phenomenon of how energy is transferred in an endothermic reaction. This page provides an overview of this lesson.

Science Background for Teachers:

Science background provides teachers with more in-depth information on the phenomena students explore in this unit. Below is an excerpt from this section on chemical reactions. 

Molecules and compounds are formed as a result of a chemical reaction between two or more atoms. In a chemical reaction, the atoms that make up the original substances are rearranged into new substances that have different properties from the original substances. For example, at normal room temperature, both oxygen and hydrogen elements are gasses. When hydrogen and oxygen bond in a molecule of water, they change into a liquid instead of a gas at room temperature. These changes are chemical changes because they rearrange the chemical structure of the substances through a chemical reaction.

In any chemical reaction, the atoms and molecules that interact together are called reactants. The atoms and molecules produced by the reaction are called products. When the reactants come together, energy breaks the bonds holding the reactants together and rearranges the atoms and molecules to form new products with different properties.

The total number of atoms does not change in a chemical reaction. Because of this, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction. This is called conservation of mass, which is the theory that states that matter is never created or destroyed. This means that the more reactants you add to the chemical reaction, the more products will form.

It is always true that mass is conserved in a chemical reaction. However, this can be difficult to measure in the real world because matter can interact with the environment. For example, if a gas is produced, it will fill whatever space it is in, which is impossible to measure. For this reason, scientists sometimes conduct closed-system experiments, in which matter cannot be exchanged with the environment, when they want to isolate the reaction from the environment.

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Science Lesson: Discovering Chemical Reactions

In this lesson, students conduct an experiment that explores how the amount of energy transferred into an endothermic reaction changes when the mass of a reactant changes. Because matter cannot change without enough energy, all chemical reactions involve the transfer of energy between the reaction and the environment. Depending on the energy required in the chemical reaction, energy is either transferred from the chemical reaction to the environment or from the environment to the chemical reaction. 

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Science Big Ideas

  • Energy transfers into or out of chemical reactions in a similar way to how energy transfers in energy systems. 
  • Every chemical reaction needs energy to get started. This initial input of energy is called activation energy. Once the reaction begins, some reactions absorb more energy from the environment than they release, while others release more energy into the environment than they absorb.
  • When a reaction is exothermic, energy is released into the environment. It is transferred from the reaction to the environment.
  • When a reaction is endothermic, energy is absorbed from the environment. Energy is transferred from the environment into the reaction.

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Science Essential Questions

  • How can energy transfer within an energy system when two objects collide?
  • How is energy transferred in a chemical reaction?
  • How would you compare endothermic reactions with those that are exothermic?
  • What evidence can be used to show when a reaction is exothermic or endothermic?
  • How is the mass of the reactants related to the mass of the products in both endothermic and exothermic reactions?

Common Science Misconceptions

Misconception: Cold/frozen substances do not have any thermal energy.
Fact: All matter has thermal energy. The atoms or molecules that make up a substance are always moving because of thermal energy.
Misconception: In a chemical reaction, the atoms of the reactants are transformed into other kinds of atoms.
Fact: The number and kinds of atoms in the reactants do not change in a chemical reaction.

Science Vocabulary

Endothermic : a process that absorbs energy from the environment

Exothermic : a process that releases energy into the environment

Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

Chemical Reactions

A Professional Glassblower

Kiva Ford spends his days making complex glass instruments that scientists use in the laboratory. When he comes home from work, he focuses on artistic glass structures. He designs goblets, flower vases, and intricate animal sculptures. For professionals like Kiva, glassblowing combines art and science. To create scientific instruments and elegant glassware, glassblowers need to have a deep understanding of what glass is, how it is formed, and its molecular structure.

Kiva’s interest was sparked in part by one property in particular: glass’s state of matter. “My experience with glass was that it’s solid,” Kiva said in an interview with the e-commerce website Etsy. “And then when I was introduced to glassblowing, all of a sudden it’s not a solid anymore. It’s a liquid. It almost feels like honey in between your hands.”

 

Energy Transfer: Exothermic Processes

Glassblowers all work with torches. A torch is a tool that produces controlled fire at the end of it. The torch produces the extreme temperatures needed to shape and form the particular shapes needed in glassblowing. Torches work because of a chemical reaction.

In any chemical reaction, the reactants and products form a system. The environment is everything else, including the air or any substance mixed with the reactants. As the reactants combine and rearrange, energy is exchanged between the system and the environment.

Every chemical reaction needs energy to get started. This initial input of energy is called activation energy. For example, when someone strikes a match to light a candle, they provide the activation energy needed to start a fire, which is a chemical reaction.

Every chemical reaction needs energy to get started. This initial input of energy is called activation energy. For example, when someone strikes a match to light a candle, they provide the activation energy needed to start a fire, which is a chemical reaction.

Once the reaction begins, some reactions absorb more energy from the environment than they release. Others release more energy into the environment than they absorb. Any process in which the system loses heat to the environment is called exothermic. “Exo-” means to give off. Because the energy is released as heat, the environment’s temperature will increase. The environment’s temperature increases because it means that the reaction has released thermal energy into the environment.

Chemical Reactions
 
Chemical Reactions

Endothermic Processes

Whenever a process occurs in which the system absorbs heat, it is called endothermic. “Endo-” means to draw in. In an endothermic reaction, the environment’s temperature decreases. This is because the reaction has absorbed energy from the environment.

A chemical ice pack uses an endothermic reaction to get cold. Many ice packs have ammonium nitrate and water kept in separate sections with a thin barrier between them. When you break the barrier, the water and ammonium nitrate combine. They absorb energy from the environment. This is why the ice packs get cold. When the reactants have fully reacted, the chemical reaction stops.

The strength of a chemical reaction can be measured by the amount of energy absorbed or released by the reaction. When more reactants are combined in a chemical reaction, the amount of energy that is absorbed or released increases.

 

Hands-on Science Activity

In this lesson, students use the scientific process to analyze the phenomena of endothermic chemical reactions, specifically how increasing one of the reactants affects the magnitude of the temperature change of the reaction. Students record the temperature of their chemical reaction experiments over a set amount of time and calculate the temperature change for each chemical reaction. Students then graph their data to help them identify patterns in how thermal energy is transferred in an endothermic chemical reaction.

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Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.

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