Concept 7 - Writing Reactions

What is a physical change?

A change in a substance that doesn’t change its identity (e.g., boiling, melting, grinding).

What is a chemical change?

A change that forms a new substance (e.g., burning, oxidizing, rotting).

What is a chemical reaction?

A process where substances collide with enough energy to form new bonds and new substances; matter doesn't change, but energy does.

What does collision theory state?

Reacting particles must collide with enough energy for a reaction; not all collisions cause reactions.

What affects the rate of a chemical reaction?

The frequency of collisions between particles.

Evidence of a chemical reaction?

• Energy change (light, heat)

• Color change

• Odor change

• Gas release

• Formation of a precipitate (solid)

What is the Law of Conservation of Mass?

Matter cannot be created or destroyed, only changed; mass of reactants = mass of products.

How are chemical reactions represented?

• Word form (e.g., hydrogen gas reacts with oxygen gas to form water)

• Formula form (e.g., 2H₂ (g) + O₂ (g) → 2H₂O (l))

In a chemical equation, what are reactants and products?

• Reactants: Starting substances

• Products: Substances formed

Meaning of → and + in chemical equations?

• → = yields/produces

• + = separates multiple reactants or products

What is a coefficient in a chemical equation?

A number that shows how many molecules/atoms are involved; it can change, but subscripts cannot.

What do the state symbols (s), (l), (g), and (aq) mean?

• (s) = solid

• (l) = liquid

• (g) = gas

• (aq) = aqueous (dissolved in water)

How to read: 2Na (s) + Cl₂ (g) → 2NaCl (s)?

2 solid sodium atoms react with 1 chlorine gas molecule to yield 2 solid sodium chlorides.

What is the first step when balancing equations?

Write the unbalanced equation.

Steps to balance a chemical equation?

1. Write the equation

2. List each element below reactants/products

3. Count atoms

4. Adjust coefficients to balance

5. Reduce coefficients if needed

Why can't you change subscripts when balancing equations?

Changing subscripts changes the identity of the substance.

Example of balanced reaction: NaOH + CaBr₂ → Ca(OH)₂ + NaBr

Balanced with coefficients: 2NaOH + CaBr₂ → Ca(OH)₂ + 2NaBr