Chemical Reactions All

Chemical Reactions

Learning Objectives

• Atoms regroup to make new products with new properties.

• Atom numbers don't change in reactions (conservation of atoms).

• Read formulas to find atom types and numbers in molecules.

• Use equations to spot reactants, products, and molecule counts.

• A chemical equation uses symbols to show a reaction.

• It shows reactants (what reacts) and products (what forms).

• Word equations use words. Example: Iron + oxygen → iron oxide.

• Chemical equation example: 4Fe + 3O₂ → 2Fe₂O₃ for rusting.

What is a Chemical Equation?

Multiple Choice

What is the main purpose of using a chemical equation in chemistry?

• To list only the elements involved.

• To provide a shorthand description of a chemical reaction.

• To show the temperature at which a reaction occurs.

• To measure the mass of reactants only.

• Chemical formulas: Symbols and numbers showing a molecule's makeup.

• Element symbols: Short letters for elements (e.g., H for Hydrogen).

• Subscripts: Small numbers show atom count; no subscript means one atom.

• Example H₂O: Water has 2 Hydrogen atoms, 1 Oxygen atom.

Understanding Chemical Formulas

Multiple Choice

In the chemical formula for methane, CH₄, how many hydrogen atoms are there in one molecule?

• 1

• 2

• 3

• 4

• Reactants are starting substances, found on the left side.

• Products are new substances formed, found on the right side.

• An arrow (→) means 'yields'; plus (+) separates multiple items.

• Coefficients (numbers in front) show how many molecules are involved.

Key Parts of a Chemical Equation

Multiple Choice

In the reaction N₂ + 3H₂ → 2NH₃, what does the coefficient '3' in front of H₂ signify?

• There are 3 atoms in a hydrogen molecule.

• 3 molecules of hydrogen react.

• The mass of hydrogen is 3 units.

• Hydrogen is the third reactant.

• Matter is like a magic trick: never made, never lost, just rearranged!

• Before and after a reaction, the total "stuff" (mass) stays the same.

• Atoms are the building blocks; they just switch partners in reactions.

• Chemical equations act like a balance, keeping atoms equal on both sides!

The Law of Conservation of Mass

Multiple Choice

According to the Law of Conservation of Mass, what happens to atoms in a chemical reaction?

• Some atoms are destroyed.

• New atoms are created.

• Atoms are rearranged.

• Atoms change their mass.

​​Open Systems: Free Exchange!

• No matter enters or leaves.

• Energy can still be exchanged.

• Shows Conservation of Mass.

​​Closed Systems: All Sealed Up!

Open vs. Closed Systems in Reactions

• Matter and energy exchange freely.

• Example: Campfire smoke escapes.

• Mass may appear to change.

Multiple Choice

Why might a scientist prefer a closed system to study a reaction involving gas production to demonstrate conservation of mass?

• Gases react faster in closed systems.

• Closed systems allow more gas to be produced.

• To prevent the gas from escaping and ensure all atoms are accounted for in mass measurements.

• Open systems are too expensive to set up.

​Correction

• Mass is conserved; atoms rearrange. Total mass remains same.

• Atoms rearrange, not created/destroyed. Atom types stay same.

• Coefficients show molecule numbers, not mass, for conservation.

• Atoms keep identity; they bond differently to form new molecules.
  

​Misconception

Common Misconceptions

• Mass is lost or gained in chemical reactions.

• Chemical reactions create or destroy atoms.

• Coefficients in equations show mass of substances.

• Atoms change identity in a chemical reaction.
  

Summary

• Chemical equations show how reactants become products.

• Chemical formulas (H₂O) show atoms in a molecule.

• Coefficients show how many molecules react.

• Atoms rearrange; total mass stays the same.

Open Ended

Explain in your own words why the total mass stays the same in a chemical reaction.

Poll

In the equation CH₄ + 2O₂ → CO₂ + 2H₂O, identify the product(s).

• CH₄ and O₂

• CO₂ and H₂O

• CH₄ and CO₂

• O₂ and H₂O

Open Ended

If you mix two substances in a sealed plastic bag and a reaction occurs, is this an open or closed system? Why?

Poll

How well can you explain that the total number of atoms stays the same in a chemical reaction? (1=Not well, 4=Very well)

• 1

• 2

• 3

• 4

Poll

Rate your confidence in using a chemical formula (e.g., H₂O) to identify the types and number of atoms in a molecule. (1=Not confident, 4=Very confident)

• 1

• 2

• 3

• 4