Intro to Chem Reactions

Match

Do Now:

Match the following

• Molecule

• Homogeneous mixture

• Compound

• Heterogeneous mixture

• Element

Water H20

Saltwater

Carbon Dioxide CO2

Trail mix

Oxygen O2

• Water H20

• Saltwater

• Carbon Dioxide CO2

• Trail mix

• Oxygen O2

I. Introduction to Chemical Reactions

• Definition: A chemical reaction occurs when substances (reactants) change into new substances (products) with different properties.

• Importance: Chemical reactions are responsible for many processes, including cooking, rusting, and digestion.

Multiple Choice

Which of the following best describes a chemical reaction?

• A technique for separating mixtures.
• A method for measuring temperature changes.
• A process that only involves physical changes.
• Substances (reactants) change into new substances (products) with different properties.

​II. Chemical vs. Physical Change

• Physical Change: No new substance is formed. The change might involve a change in size, shape, or state (e.g., melting, freezing).

  • Example: Ice melting into water.

• Chemical Change: A new substance is formed with different chemical properties.

  • Example: Wood burning to produce ash and smoke.

Multiple Choice

Which of the following is an example of a physical change?

• Wood burning to produce ash and smoke

• Iron rusting when exposed to air

• Ice melting into water

• Baking a cake

III. Evidence of a Chemical Reaction

• Clues that a chemical reaction has occurred:

1.     Bubbles: Indicates gas is being produced (e.g., vinegar reacting with baking soda).

2.     Precipitate: A solid that forms when two liquids react (e.g., milk curdling when lemon juice is added).

3.     Light: Some reactions give off light (e.g., fireworks).

4.     Temperature: Heat is absorbed or released (e.g., an ice pack becoming cold when activated).

5.     Color Change: The substance changes color (e.g., iron rusting).

6.     Smell: A new odor is produced (e.g., spoiled food).

Multiple Choice

Which of the following is not a clue that a chemical reaction has occurred?

• Bubbles are produced

• A solid forms from two liquids

• A substance changes color

• A substance changes shape

​Memory Trick:
Bad Puppies Like to Chew Shoes

• Bubbles

• Precipitate

• Light

• Temperature

• Color

• Smell

IV. Endothermic and Exothermic Reactions

• Endothermic Reactions:

  • Definition: Reactions that absorb heat, causing the temperature of the surroundings to decrease.

  • Example: Ice packs used for injuries feel cold because they absorb heat from the body.

  • Key Point: More energy is required to break the bonds of the reactants than is released when forming the products.

• Exothermic Reactions:

  • Definition: Reactions that release heat, causing the temperature of the surroundings to increase.

  • Example: Fireworks explode and release heat and light.

  • Key Point: More energy is released when forming products than is required to break the bonds of the reactants.

Multiple Choice

Which of the following statements correctly describes an exothermic reaction?

• The reaction absorbs heat, causing the surroundings to feel colder.

• The reaction releases heat, causing the temperature of the surroundings to increase

• More energy is absorbed than is released during the reaction

• The reaction causes the temperature of the surroundings to decrease.

​V. Law of Conservation of Mass

• Definition: Matter cannot be created or destroyed in a chemical reaction.

  • The mass of the reactants (starting materials) must equal the mass of the products (new substances formed).

• Example: When wood burns, the mass of the ash, gases, and smoke equals the mass of the original wood and oxygen used in the reaction.

Multiple Choice

Which of the following statements best describes the Law of Conservation of Mass?

• Matter is destroyed during a chemical reaction.

• The mass of the products is always greater than the mass of the reactants.

• Matter cannot be created or destroyed, and the mass of the reactants equals the mass of the products.

• The mass of the reactants is always greater than the mass of the products.

​VI. Reactants and Products

• Reactants: The substances you start with in a chemical reaction (on the left side of a chemical equation).

• Products: The new substances formed in a chemical reaction (on the right side of the chemical equation).

Example:
CH₄ + O₂ → CO₂ + H₂O

• Reactants: Methane (CH₄) and Oxygen (O₂)

• Products: Carbon Dioxide (CO₂) and Water (H₂O)

Multiple Choice

In the chemical reaction CH₄ + O₂ → CO₂ + H₂O, which of the following correctly identifies the reactants and products?

• The reactants are CO₂ and H₂O, and the products are CH₄ and O₂.

• The reactants are CH₄ and O₂, and the products are CO₂ and H₂O.

• The reactants are CO₂ and CH₄, and the products are O₂ and H₂O.

• The reactants are H₂O and CH₄, and the products are CO₂ and O₂.

​VII. Common Chemical Reactions

• Combustion: A reaction with oxygen that produces heat and light (e.g., burning fuel).

• Rusting: Iron reacts with oxygen to form rust (iron oxide).

Open Ended

Exit Ticket:

A student observes that when wood burns, ash, smoke, and gases are made. Using the Law of Conservation of Mass, explain how the mass of the wood before burning compares to the mass of the products (ash, smoke, gases) after burning. Then, compare this to a physical change like ice melting. How would the mass and properties be different in these two processes?