IB Chemistry: Reactivity 1 Mastery

IB Chemistry Course Companion

REACTIVITY 1

What Drives Chemical Reactions?

Enthalpy • Calorimetry • Hess's Law • Born-Haber • Entropy • Gibbs Energy

CORE

1.1 Measuring Enthalpy

1. The Language of Thermodynamics

System vs Surroundings

System: The reaction itself (reactants & products).
Surroundings: Everything else (water, thermometer, universe).

Temp vs Heat

Temp ($T$): Measure of average kinetic energy.
Heat ($q$): Transfer of energy due to $\Delta T$.

2. Exothermic vs. Endothermic

Chemical reactions involve a trade-off: Breaking bonds (Endo) vs. Forming bonds (Exo).

Feature	Exothermic ($\Delta H < 0$)	Endothermic ($\Delta H > 0$)
Bond Process	Bond Forming (Releases E)	Bond Breaking (Requires E)
Observation	Temp RISES	Temp FALLS
Stability	Products more stable	Products less stable

Calorimetry (The Math)

We measure enthalpy changes indirectly by measuring the temperature change of water.

Step 1: Calculate Heat ($q$)

q = mc\Delta T

$m$: Mass of the substance changing temp (usually water).
$c$: Specific Heat Capacity ($4.18$).
$\Delta T$: $T_{final} - T_{initial}$.

Step 2: Molar Enthalpy ($\Delta H$)

\Delta H = \frac{-q}{n}

$q$: Heat from Step 1.
$n$: Moles of the limiting reactant.
Negative Sign: Essential logic check. (Hot water = Exothermic).

Fatal Trap Alert

The Unit Trap: $q$ is in Joules. $\Delta H$ is usually kJ/mol.
You MUST divide q by 1000 before finishing the calculation.
The Mass Trap: Do NOT use the mass of the fuel burned in $mc\Delta T$. Use the mass of the water that heated up.

CORE

1.2 Hess's Law & Bond Enthalpy

1. Hess's Law

The Law: The enthalpy change for a chemical reaction is independent of the pathway taken.

The "Cheat Code" Formulas

Given Formation Data ($\Delta H_f$):

\Delta H = \sum H_f (Prod) - \sum H_f (React)

$\Delta H_f$ of elements ($O_2, Fe$) is ZERO.

Given Combustion Data ($\Delta H_c$):

\Delta H = \sum H_c (React) - \sum H_c (Prod)

Mnemonic: CRaP (Combustion: Reactants - Products).

2. Bond Enthalpy

Energy required to break 1 mole of bonds in gaseous state.

\Delta H = \sum (\text{Bonds Broken}) - \sum (\text{Bonds Formed})

"Break Reactants (Endo) - Form Products (Exo)"

1.3 Energy from Fuels

Incomplete Combustion

Limited Oxygen. Produces CO (Toxic) or Soot. Releases less energy.

Biofuels

From biomass. Carbon Neutral (CO2 released was absorbed by plants).

Hydrogen Fuel Cells

Converts chemical $\to$ electrical directly.

2H₂ + O₂ → 2H₂O

Advantage: Zero emissions.

⛔️ STOP HERE IF YOU ARE SL

Advanced Theory

The following section is for HL Students ONLY.
Born-Haber Cycles • Entropy • Gibbs Energy

AHL

Born-Haber Cycles

A special application of Hess's Law to calculate Lattice Enthalpy.

1. The Cycle Steps (Memorize Definitions)

Atomization ($\Delta H_{at}$)

Element $\rightarrow$ Gas Atom. (Endo)

Na(s) → Na(g)

Ionization Energy (IE)

Gas Atom $\rightarrow$ Cation. (Endo)

Na(g) → Na⁺(g) + e^-

Electron Affinity (EA)

Gas Atom $\rightarrow$ Anion. (1st is Exo)

Cl(g) + e^- → Cl^-(g)

Lattice Enthalpy ($\Delta H_{lat}$)

Gas Ions $\rightarrow$ Solid Lattice. (Highly Exo)

Na⁺(g) + Cl^-(g) → NaCl(s)

2. Trends in Strength

Ionic Charge: Higher charge = Stronger attraction. ($MgO > NaCl$).
Ionic Radius: Smaller radius = Closer packing = Stronger attraction.

AHL

Entropy & Gibbs Energy

1. Entropy ($S$)

Measure of disorder (dispersal of energy).

$S_{gas} \gg S_{liquid} > S_{solid}$

2. Gibbs Free Energy ($\Delta G$)

The ultimate judge of spontaneity.

$\Delta G < 0$
YES $\Delta G > 0$
NO $\Delta G = 0$
Equil.

\Delta G^{\ominus} = \Delta H^{\ominus} - T\Delta S^{\ominus}

The Fatal Unit Trap

$\Delta H$ is in kJ. $\Delta S$ is in J.
You MUST divide $\Delta S$ by 1000 to convert to kJ before plugging into the equation!

3. Equilibrium Link ($K$)

\Delta G^{\ominus} = -RT \ln K

Negative $\Delta G \rightarrow$ Positive $\ln K \rightarrow$ $K > 1$ (Products Favored).

The Examiner's Vault

Strictly assessed on Reactivity 1 content.

1. Hess's Law Logic CORE

(1) $A + 2B \rightarrow C \quad \Delta H = -100$
(2) $C \rightarrow D \quad \Delta H = +50$
Find $\Delta H$ for $A + 2B \rightarrow D$.

2. Bond Enthalpy Calculation CORE

Bond E ($kJ/mol$): $H-H (436)$, $Cl-Cl (242)$, $H-Cl (431)$.
Calc $\Delta H$ for $H_2 + Cl_2 \rightarrow 2HCl$.

3. Spontaneity Logic

Reaction is Endothermic ($\Delta H > 0$) and Order Increases ($\Delta S < 0$). Spontaneous?

4. Calorimetry Calculation

Combusting $0.50g$ Methanol ($M_r=32.04$) heats $200g$ water by $10.0^\circ C$. Calculate $\Delta H_c$.

5. Born-Haber Cycle ($CaCl_2$)

Using standard data ($\Delta H_f = -796$, Lattice = -2253, etc.), calculate the Electron Affinity (EA) for Chlorine.

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Ib chemistry – Reactivity 1