Chemical Reactions

Chemical Reactions

Learning Objectives

• Define chemical equation and identify its components.

• Interpret information from a chemical equation's structure.

• State Law of Conservation of Mass; see its use in balanced equations.

• Differentiate open and closed systems in chemical reactions.

• A chemical equation is a shorthand for chemical reactions.

• It shows reactants (inputs) and products (outputs) of a reaction.

• Rust example: 4Fe + 3O₂ → 2Fe₂O₃ (Iron + Oxygen → Iron Oxide).

• Symbols (like H₂O) are like text abbreviations (like 'jk').

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.

• Formulas show molecules using symbols and subscripts.

• Element symbols are 1 or 2 letters (H for Hydrogen).

• Subscripts show atom counts. No subscript means one atom.

• H₂O: 2 H atoms, 1 O atom. NH₃: 3 H atoms, 1 N atom.

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 (left) form products (right), separated by a 'yields' arrow (→).

• Plus signs (+) separate items. General form: Reactant + Reactant → Product + Product.

• Coefficients (numbers in front) show particle count and balance equations.

• N₂ + 3H₂ → 2NH₃: 1 nitrogen & 3 hydrogen molecules make 2 ammonia.

Structure 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 made or lost in chemical reactions!

• Total mass before a reaction equals total mass after.

• Atoms rearrange to form new stuff; none disappear.

• Balanced equations (e.g., Fe + S → FeS) show this rule.

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

• Nothing gets in or out – it's sealed tight!

• Everything stays locked inside, safe and sound.

• Perfect for scientists to weigh things precisely!

​​Closed Systems

Open and Closed Systems

• Matter & gases can freely move in or out.

• Like a plant in an open pot breathing air.

• Weighing is tricky if gases can escape!

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.

​Here's the Deal!

• Mass is always conserved; it just changes form.

• Coefficients show the number of molecules or atoms.

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

• Atoms only rearrange; none are made or destroyed.
  

​Oops! Not Quite...

Common Misconceptions

• Mass is lost or gained in chemical reactions.

• Coefficients in equations show the mass of substances.

• The reaction arrow (→) simply means 'equals'.

• New atoms are created during chemical reactions.
  

Summary

• Chemical equations use symbols and formulas for reactions.

• Reactants are on the left, products on the right.

• Coefficients balance equations, showing molecule counts.

• Law of Conservation: Matter isn't created or destroyed.

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

Rate your confidence in interpreting chemical equations (1=Low, 4=High).

• 1

• 2

• 3

• 4

Poll

How well do you feel you understand the Law of Conservation of Mass now? (1=Not well, 4=Very well)

• 1

• 2

• 3

• 4