Reactivity & Conservation Laws

Chemical Equations

• Word & symbol forms describe reactions.
• General form: $A + B \rightarrow C + D$ (Reactants → Products)
• Example (word): Magnesium + Oxygen → Magnesium oxide
  • Symbol: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$
• Atoms are neither created nor destroyed; they are rearranged.

Law of Conservation of Mass (LO 5.2.1)

• Definition: In an isolated system, mass is neither created nor destroyed during physical/chemical changes.
• Implications:
  • Total number of atoms, hence mass, remains constant.
  • If gas escapes (open system), measured mass may appear to change.
• Sample check: $\text{CaCO}<em>3 + 2\text{HCl} \rightarrow \text{CaCl}</em>2 + \text{H}<em>2\text{O} + \text{CO}</em>2$
  • Closed flask → mass constant
  • Open flask → $\text{CO}_2$ loss decreases measured mass

Law of Conservation of Energy (LO 5.2.1)

• Definition: Energy cannot be created or destroyed; it only transforms (e.g., chemical ↔ thermal) with total amount conserved.
• All reactions involve:
  • Energy absorbed to break bonds
  • Energy released on forming new bonds

Endothermic vs. Exothermic

• Endothermic
  • \text{Heat absorbed} > \text{Heat released}
  • Surroundings cool
  • Example: Sherbet reaction (sodium hydrogencarbonate + citric acid)
• Exothermic
  • \text{Heat released} > \text{Heat absorbed}
  • Surroundings heat up (may emit light/sound)
  • Example: $2\text{K} + 2\text{H}<em>2\text{O} \rightarrow 2\text{KOH} + \text{H}</em>2$
• Regardless of type: total energy before = total energy after

Quick Recap

• Write balanced equations in words & symbols.
• Mass is conserved: no atom gain/loss → constant mass in closed system.
• Energy is conserved: only changes form; net quantity unchanged.
• Reaction energy profile determines endothermic or exothermic behavior.