Chemical Reactions Reference Lessons

Chemical Reactions

Learning Objectives

• Spot the clues that a chemical reaction has happened!

• Understand chemical recipes: Define and read equations and their parts.

• Know the rule: Explain Conservation of Mass and why equations balance.

• Open or Shut? Compare open and closed systems in chemical reactions.

• Abracadabra! Reactions make new things. Watch for these cool clues!

• Is that gas? Look for bubbles, fizz, foam, smoke, or an odor!

• Solid surprise! A new solid (precipitate) can pop up in solutions.

• Hot or not? See unexpected color changes or feel a temperature shift!

Signs of a Chemical Reaction

Multiple Choice

Which of the following is a common sign that a chemical reaction might be happening?

• Melting ice

• Formation of bubbles

• Dissolving sugar in water

• Boiling water

• Reactions can be described with words or symbols.

• Word equation: "Iron reacts with oxygen to form iron oxide."

• A chemical equation symbolically shows reactants and products.

• Example: 4Fe + 3O₂ → 2Fe₂O₃ shows rusting iron.

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 use symbols and numbers to show molecules.

• Element symbols are short codes, like H for Hydrogen or O for Oxygen.

• Subscripts (small numbers) show how many atoms of each element.

• Example: Water (H₂O) means 2 Hydrogen atoms and 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 (starters) on the left; products (new stuff) on the right!

• Arrow (→) shows 'yields'! Plus (+) signs separate different chemicals.

• Coefficients: front numbers show molecule amounts, key for balancing equations!

• State symbols (s,l,g,aq) show form! E.g., 2H2(g) + O2(g) → 2H2O(l).

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 isn't created or destroyed in reactions, just rearranged.

• Total mass of reactants equals total mass of products.

• Atoms rearrange; their number and type stay the same.

• This law is why chemical equations must be balanced.

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.

• Balance equations to follow the Law of Conservation of Mass: atoms stay equal.

• Adjust coefficients (big numbers in front). Never change subscripts (small numbers).

• List elements, count atoms. Adjust coefficients until all elements are balanced.

• H₂ + O₂ → H₂O balances to 2H₂ + O₂ → 2H₂O. (4H, 2O both sides).

Balancing Chemical Equations

Multiple Choice

When balancing a chemical equation, which numbers can you change?

• Subscripts only

• Coefficients only

• Both subscripts and coefficients

• Neither, equations are always balanced

​​Open Systems

• ​Sealed: No matter enters or leaves.

• ​​Energy can still be exchanged with surroundings.

• ​Perfect for showing Conservation of Mass.

​​Closed Systems

Open vs. Closed Systems in Reactions

• ​Matter & energy exchange freely with surroundings.

• ​​Example: Campfire smoke escapes, heat is released.

• ​Mass may seem lost if gas escapes.

Multiple Choice

Why might a scientist prefer a closed system to study a reaction involving gas production?

• Gases react faster in closed systems.

• Closed systems allow more gas to be produced.

• To prevent the gas from escaping and ensure accurate mass measurements.

• Open systems are too expensive to set up.

​Correction

• Mass is conserved; apparent changes are due to gases.

• Coefficients represent the number of particles.

• The arrow (→) means 'yields' or 'reacts to form'.

• Some color changes are physical, not all chemical.
  

​Misconception

Common Misconceptions

• Mass is lost or gained in chemical reactions.

• Coefficients represent the mass of substances.

• The arrow (→) in an equation means 'equals'.

• All color changes indicate a chemical reaction.
  

Summary

• Look for clues! Gas, color change, or energy shifts often mean a reaction.

• Equations show reactions: Reactants → Products. State symbols tell their physical state.

• Mass is conserved! Atoms just rearrange; total mass stays the same in reactions.

• Balance equations with coefficients so atoms match on both sides, proving mass conservation.

Open Ended

Briefly explain the Law of Conservation of Mass in your own words.

Poll

In the equation 2H₂ + O₂ → 2H₂O, identify the product(s).

• H₂

• O₂

• H₂ and O₂

• H₂O

Open Ended

Is a burning campfire an example of an open or closed system? Why?

Poll

How confident are you in identifying signs of a chemical reaction? (1=Not confident, 4=Very confident)

• 1

• 2

• 3

• 4

Poll

Rate your understanding of balancing chemical equations after this review. (1=Still confused, 4=Understand well)

• 1

• 2

• 3

• 4