#AP Chemistry: Enthalpy & Energy - The Night Before Review 🚀

Hey! Let's make sure you're feeling confident about enthalpy and energy for tomorrow's AP Chem exam. This guide is designed to be quick, clear, and super helpful for your last-minute review. Let's get started! 💪

#Thermodynamics: Enthalpy (H) and Energy (E)

#What is Enthalpy?

Enthalpy (H) is like the total energy package of a system. It includes all the internal energy plus the energy related to pressure and volume. We usually focus on the change in enthalpy (ΔH), which tells us about heat flow in reactions or phase changes.

Image Courtesy of Chemistry LibreTexts

#Enthalpy of Reaction (ΔH_rxn)

ΔH_rxn is the heat absorbed or released during a chemical reaction. It's our key to figuring out if a reaction is exothermic or endothermic.

• Exothermic Reactions: ΔH is negative (-). Heat is released by the system. Think of a campfire 🔥 – it gives off heat.
• Endothermic Reactions: ΔH is positive (+). Heat is absorbed by the system. Think of an ice pack getting cold as it melts 🧊 – it absorbs heat.

#Examples of Exothermic and Endothermic Reactions:

• Exothermic:
  • Combustion (burning) of fuels like propane (C₃H₈) or methane (CH₄)
  • Neutralization reactions (acid + base)
• Endothermic:
  • Dissolving ammonium nitrate (NH₄NO₃) in water
  • Melting ice (H₂O(s) → H₂O(l))

#Negative vs. Positive ΔH

• Negative ΔH: Heat flows out of the system (exothermic). The surroundings get warmer.
• Positive ΔH: Heat flows into the system (endothermic). The surroundings get cooler.

Image Courtesy of Mr. Lowe

#Other Key Terms

Let's clarify some related terms:

• Energy (E): The ability to do work or transfer heat. Measured in joules (J) or calories (cal).
• Internal Energy (E or U): The total kinetic and potential energy within a system. A state function. ΔE = q + w
• Heat (q): Energy transfer due to a temperature difference. Flows from hot to cold. Measured in joules (J).
• Work (w): Energy transfer due to a force acting over a distance. Especially important for gases (expansion/compression). w = -PΔV

#Quick Summary of Energy Terms:

• Energy: Ability to do work or transfer heat.
• Internal Energy (ΔE): Total energy within a system. ΔE = q + w
• Heat (q): Energy transfer due to temperature difference.
• Work (w): Energy transfer due to force over a distance (especially gas expansion/compression).

#Examples of Energy, Enthalpy, Heat, and Work:

1. Heating water: Heat (q) is transferred from the stove to the water, increasing its temperature.
2. Lifting weights: Work (w) is done by the person as they exert force over a distance.
3. Chemical reaction: Enthalpy change (ΔH) is measured by heat absorbed or released.
4. Gas compression: Work (w) is done on the gas, increasing its internal energy.
5. Car engine: Chemical energy is converted to mechanical energy and heat.

#Important Note:

#Final Exam Focus

• Key Topics:
  • Understanding exothermic vs. endothermic reactions.
  • Calculating ΔH using Hess's Law and bond energies (covered in next section).
  • Relating ΔH to heat flow and temperature changes.
  • Applying the concepts of energy, heat, and work in different scenarios.
• Common Question Types:
  • Multiple-choice questions asking about the sign of ΔH.
  • Free-response questions involving calorimetry and heat transfer calculations.
  • Questions combining enthalpy with other thermodynamic concepts (like Gibbs Free Energy).
• Last-Minute Tips:
  • Time Management: Don't get bogged down on one question. Move on and come back if you have time.
  • Common Pitfalls: Watch out for sign errors (+/-) in calculations. Double-check your units.
  • Strategies: Read questions carefully. Underline key words. Draw diagrams if it helps.

#

Practice Question

Practice Questions

#Multiple Choice Questions

1. Which of the following statements is true regarding an exothermic reaction? (A) The enthalpy change (ΔH) is positive, and heat is absorbed by the system. (B) The enthalpy change (ΔH) is negative, and heat is released by the system. (C) The enthalpy change (ΔH) is positive, and heat is released by the system. (D) The enthalpy change (ΔH) is negative, and heat is absorbed by the system.

2. A reaction has a positive ΔH. Which of the following must be true about the reaction? (A) The reaction is exothermic. (B) The reaction is endothermic. (C) The reaction releases heat into the surroundings. (D) The reaction is spontaneous.

3. When a gas expands against a constant external pressure, the work (w) done by the gas is: (A) Positive (B) Negative (C) Zero (D) Cannot be determined

#Free Response Question

Consider the reaction:

N₂(g) + 3H₂(g) → 2NH₃(g) ΔH = -92 kJ

(a) Is this reaction exothermic or endothermic? Explain. (b) Calculate the heat released when 14.0 grams of N₂ reacts completely with excess H₂. (c) If the reaction is carried out in a closed container and the volume of the container is allowed to change, is work done by the system or on the system? Explain. (d) Draw an energy diagram for this reaction, including the reactants, products, and the activation energy.

#Scoring Breakdown:

(a) (1 point)

• The reaction is exothermic because ΔH is negative. (1 point)

(b) (2 points)

• Convert grams of N₂ to moles: 14.0 g N₂ x (1 mol N₂ / 28.0 g N₂) = 0.50 mol N₂ (1 point)
• Calculate heat released: 0.50 mol N₂ x (-92 kJ / 1 mol N₂) = -46 kJ (1 point)

(c) (2 points)

• Work is done by the system because the number of moles of gas decreases (4 moles of gas to 2 moles of gas), causing a decrease in the volume of the system. (1 point)
• Since the volume decreases, the system does work on the surroundings to maintain the pressure. (1 point)

(d) (2 points)

• Correctly labeled axes (Energy vs. Reaction Progress) (1 point)
• Correctly shows reactants higher in energy than products, with the activation energy barrier. (1 point)

You've got this! Go get that 5! 🌟