Chemical Reactions: Definition and Examples

Chemical and Physical Changes

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Creating Models of Chemical Reactions

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Vocabulary

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Discussion Starter

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Chemical Change Indicators

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Mentos and Coke Experiment

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Essential Questions for Chemical Reactions

1. What is a chemical reaction?
2. How do we know if a chemical reaction has occurred?
3. What are the difference between chemical and physical changes?

Chemical Reactions

Throughout history, scientists have been fascinated by the way that some substances can transform into other substances. During the early years of science, many scientists were also engaged in the practice of alchemy, which included attempting to turn base metals, like lead, into gold.

The total number of atoms and the total mass of the atoms is conserved during a chemical reaction. It was French chemist Antoine Lavoisier who showed that the mass of the reactants in a reaction is equal to the mass of the products. This means that no atoms are created or destroyed in a chemical reaction. However, bonds between atoms are broken, the atoms rearrange, and then new bonds are made. When using Storyboard That to model chemical reactions, you’ll want to use the “equipment” and “symbol” assets listed under “Science.”

Word equations provide the names of each of the reactants and products. For example methane + oxygen → carbon dioxide + water . In this equation, methane and oxygen are the reactants and carbon dioxide and water are the products. These equations are helpful when looking only to understand exactly what is produced in a reaction.

Another way of describing chemical reactions is by using symbol equations. Symbol equations not only allow us to know the reactants and products involved in the reaction, they also let us know the chemical composition of the substances involved. For example, the word equation for the combustion of hydrogen gas in air is hydrogen + oxygen → water. The symbol equation is H₂ + O₂ → H₂O. There’s only one issue: the equation is not balanced. On the right hand side there are 2 hydrogen atoms and 2 oxygen atoms. On the left hand side, there are two hydrogen atoms and one oxygen. We know that atoms are not created or destroyed in chemical reactions. In order to rectify this, we need to balance the equation using coefficients. 2H₂ + O₂ → 2H₂O is a balanced equation. Now we have four hydrogen atoms and two oxygen atoms on the right hand side. The left hand side remains the same.

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From a balanced symbol equation, students can create stick and ball models on Storyboard That. This helps students understand how bonds are broken and how the atoms are rearranged during a chemical reaction. It also allows them to visualize symbol equations in a more easily digestible manner.

Students can often find it difficult to differentiate between physical and chemical reactions. Physical changes do no create new substances. The old substance is the same as the new substance, and the old substance can easily be recovered. Melting and dissolving are two examples of physical reactions. Those substances can be solidified again or recovered via evaporation. Chemical changes occur during chemical reactions, and this means new substances are produced.

There are several different indicators that a chemical reaction has occurred. Often, the products of a chemical reaction can be very different to the reactants.

Indicators that a chemical reaction has occurred:

• A gas is produced
• A noticeable odor
• Temperature change
• Change of color
• A precipitate forms

This is a great opportunity to show your students a range of different and exciting chemical reactions, some of which the students will be able to complete themselves. Others are better suited to teacher demonstrations. As with any practical work, make sure they are properly risk-assessed and the students are made aware of safe practices.

The Next Generation Science Standards push the importance of getting students to develop and use models to understand phenomena. In the real world, scientists will make models to aid their understanding of a system or part of a system. Models are used in science to make predictions and communicate ideas or data to other people. There are activities here that focus on honing the skill of creating models. Students will easily be able to create their own models of atoms to describe the arrangement of atoms in chemical reactions. This gives you a great opportunity to discuss the limitations of using models, giving students the opportunity to evaluate and refine them.

Other Ideas for Chemical Reaction Activities

1. Students could produce a spider diagram showing different everyday chemical reactions.
2. Have your students make a storyboard showing other students how to balance symbol equations.
3. Get your students to create a T-Chart storyboard comparing different chemical reactions.