What is a chemical reaction?

What is a chemical reaction?

Chemical Reactions

a system of chemical changes that involves the breaking and reforming of chemical bonds to create new substances.

Chemical Reactions

A chemical reaction is the process through which chemical changes occur. During a chemical change, one substance turns into a different substance that usually have different properties.

EXAMPLE:

If you leave a tarnished copper penny in acid for a few minutes, the penny becomes shiny again.

​The copper oxide (Cu₂O) that tarnished the penny was removed by a chemical reaction with the acid.

Multiple Choice

After a chemical reaction occurs, the substances usually have different properties.

• True

• False

How do we show a reaction?

In cooking, we start with ingredients that you combine to make different foods. In chemical reactions, think of the ingredients as the reactants that are combined to make products.

The image shows the reaction between vinegar and baking soda. When mixed, they react to form 3 completely new substances. The bubbling is a clue that one of the products is a gas.

Multiple Choice

What are the products in a reaction?

• What you put into a chemical reaction

• What you end up with after a chemical reaction occurs

Multiple Choice

What are the reactants in a reaction?

• What you put into a chemical reaction

• What you end up with after a chemical reaction occurs

4 Signs of a reaction

There are 4 observations that a chemical reaction may have occured.

¹ Bubbling

Bubbling is one sign that a chemical reaction has occurred. The bubbling shows that a gas has been created. That means that a new substance has been created, which is what happens during a chemical reaction.

Not always though...

Bubbling doesn't always indicate that a chemical reaction has occurred, sometimes it just shows increased heat, like when your grandma boils water to make mac and cheese.

² Cloudiness

When two reactants react to form a cloudy substance, it could mean that they are forming a new solid substance.

³ Color Change

When two reactants react to show a color change, it could be an indication that a new substance is forming.

⁴ Temperature Change

When new products are formed, sometimes it gives off energy to form these new bonds between atoms. If more energy is required to form new bonds, than to break old bonds, the reaction is called EXOTHERMIC.

Exothermic Reaction

a chemical reaction that releases energy, usually in the form of heat or light.

EXAMPLE: Any kind of combustion reaction (burning something to break bonds).

⁴ Temperature Change

Endothermic Reaction

a reaction that absorbs energy.

EXAMPLE: melting ice, it requires you to put in heat to energize the H₂O particles.

⁴ Temperature Change

Note:

if one of these 4 is observed, it doesn't mean for sure that a chemical reaction has occurred, these are just signs that one could have occurred.

Open Ended

What are the 4 signs that could be made that indicate that a chemical reaction has occurred?

Multiple Choice

This reaction requires heat.

• Endothermic

• Exothermic

Multiple Choice

This reaction releases heat.

• Endothermic

• Exothermic

But what is actually occurring during a chemical reaction?

Recall that earlier we mentioned that a chemical reaction is the rearrangement of atoms in molecules. This rearrangement leads to new substances.

The Copper Oxide compound is show above as a Copper atom bonded to a Oxygen atom. This is the tarnished penny.

The acid (HCl) is shown above as two HCl compounds, each compound containing 1 atom of Hydrogen and 1 atom of Chlorine.

These two compounds are the reactants for this chemical reaction. Notice they are on the left side of the equation.

On the right side you have your products. NEW SUBSTANCES!!!

This product is Copper Chloride, a shiny new penny.

This product is water (H₂O).

Remember, a chemical reaction occurs when atoms are rearranged to create new substances.

Notice how the Copper atom from copper oxide can be found on the right side as copper chloride.

Notice how the Oxygen atom from copper oxide can be found on the right side in water.

Notice the two Hydrogen atoms on the left in the hydrochloric acid are now on the right in water.

Notice the two Chlorine atoms on the left in the hydrochloric acid are now on the right in the Copper chloride.

Multiple Choice

How many hydrogen atoms are on the reactant side?

• 1

• 2

• 3

• 4

Multiple Choice

How many hydrogen atoms are on the product side?

• 1

• 2

• 3

• 4

Conservation of Mass

Notice how there are the same amount of atoms for each element on the left side of the reaction as there are on the right side. This is because even when a chemical reaction occurs, matter is not destroyed or created, therefore the reactants will have the same mass as the products.

Why does this happen?

This rearrangement of atoms in a chemical reaction is because of the attraction of certain atoms to other atoms based on their atomic structure.

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Some elements have more electrons than others, these electrons are what determines how they will bond and what they will bond with.

Here's another example of how compounds rearrange themselves in a chemical reaction. It's kind of like they found a better mate. A mate that has a better match of subatomic particles that will keep them more stable.

Items that should be included in your notes.

1. The 4 Signs of a reaction

2. Exothermic vs. Endothermic reactions

3. Products & Reactants

4. Conservation of Mass