Enthalpy Review

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ENTHALPY

Learning Objectives

• Define thermodynamics and thermochemistry.

• Explain energy conservation: system and surroundings.

• Compare heat and temperature; convert °C and K.

• Define enthalpy (ΔH) and endo/exothermic reactions.

• The 'system' is the specific part we are studying.

• 'Surroundings' is everything outside the system.

• Energy cannot be made or lost, only moved or changed.

• Energy change in the system equals the opposite change in surroundings.

Energy: System, Surroundings & Conservation

Multiple Choice

According to the law of conservation of energy, if a system releases 50 J of energy, what happens to the surroundings?

• The surroundings lose 50 J of energy.

• The surroundings gain 50 J of energy.

• The surroundings' energy remains unchanged.

• The total energy of the universe decreases by 50 J.

• Heat is the transfer of energy between objects. Heat always flows from warmer to cooler things.

• We measure heat in Joules (J) or Calories (cal).

• Temperature measures average particle energy in something.

• We measure temperature in Celsius (°C) or Kelvin (K).

Heat vs. Temperature: What's the Difference?

• Enthalpy (H) is a substance's total internal energy at constant pressure.

• Enthalpy change (ΔH) is the energy difference between products and reactants.

• ΔH is like the heat absorbed or released in a reaction.

• ΔH is like distance between places, not their exact GPS location.

​Enthalpy (H) and Enthalpy Change (ΔH)

Multiple Choice

What does the enthalpy change (ΔH) represent in a process?

• The total internal energy of the system

• The heat change at constant volume

• The heat change at constant pressure

• The change in temperature of the system

• Chemical reactions break old bonds and form new ones.

• Breaking bonds uses energy; forming bonds releases energy.

• Exothermic reactions release net energy (feel warm, ΔH is negative).

• Endothermic reactions absorb net energy (feel cool, ΔH is positive).

Exothermic vs. Endothermic Reactions

Multiple Choice

In an endothermic reaction, which statement is true?

• Energy is released to the surroundings.

• More energy is released forming bonds than is absorbed breaking bonds.

• The enthalpy of the products is lower than the enthalpy of the reactants.

• More energy is absorbed breaking bonds than is released forming bonds.

• Enthalpy change of a reaction is shown as ΔHrxn.

• We can show ΔHrxn in three main ways.

• Thermochemical equation: Heat is shown as product (exo) or reactant (endo).

• ΔH notation: Reaction shown with its ΔHrxn value (+ endo, - exo).

Representing Enthalpy Changes (ΔHrxn)

Multiple Choice

Consider the reaction: N₂(g) + O₂(g) → 2NO(g) ΔHrxn = +180.6 kJ. How would this be written as a thermochemical equation with the heat term included?

• N₂(g) + O₂(g) → 2NO(g) + 180.6 kJ

• N₂(g) + O₂(g) + 180.6 kJ → 2NO(g)

• N₂(g) + O₂(g) - 180.6 kJ → 2NO(g)

• 180.6 kJ + N₂(g) + O₂(g) → 2NO(g)

• Standard Enthalpy (ΔH˚): Heat change measured at standard conditions.

• Standard Conditions (SATP) are 25°C and 100 kPa.

• The ˚ symbol means standard conditions are used.

• Standard values help compare different reactions fairly.

Standard Enthalpy of Reaction (ΔH˚)

Multiple Choice

What does the symbol '˚' in ΔH˚rxn signify?

• The reaction is reversible.

• The reaction occurs at absolute zero.

• The reaction occurs under standard conditions (SATP).

• The enthalpy value is an approximation.

• Diagrams show heat changes in reactions.

• The y-axis shows enthalpy (heat). Levels show reactants and products.

• Exothermic: Reactants higher, arrow down, negative ΔH (heat released).

• Endothermic: Reactants lower, arrow up, positive ΔH (heat absorbed).

Enthalpy Diagrams

Multiple Choice

In an enthalpy diagram for an exothermic reaction, where are the reactants located relative to the products?

• At a lower enthalpy level

• At the same enthalpy level

• At a higher enthalpy level

• The relative levels depend on temperature

• Thermochemical equations link moles of substances to heat change (ΔH).

• The ΔH value shown matches the moles in the balanced equation.

• Change the amount of reactants? The heat change (ΔH) changes proportionally.

• Example: Double the MgCO₃ doubles the ΔH (+117.3 kJ to +234.6 kJ).

Linking Moles and Heat Change

Multiple Choice

For the reaction 2Al(s) + 3Cl₂(g) → 2AlCl₃(s), the energy change (ΔH˚) is -1408 kJ. What's the energy change for reacting only 1 mole of Al(s)?

• -1408 kJ

• -704 kJ

• -2816 kJ

• +1408 kJ

• Energy changes when matter changes its state (like melting or boiling).

• Adding heat causes phase changes like melting or boiling.

• Temperature stays constant while a substance changes phase.

• Melting/boiling absorb energy (endothermic); freezing/condensing release energy (exothermic).

Phase Changes and Energy

Multiple Choice

Which phase change process is endothermic and involves a substance changing from solid to liquid?

• Vaporization

• Condensation

• Melting

• Freezing

• Dissolving something changes its heat energy (ΔHsoln).

• Endothermic dissolving absorbs heat and feels cold.

• Exothermic dissolving releases heat and feels hot.

• Cold/hot packs use these heat changes!

Heat Changes When Things Dissolve

Multiple Choice

A cold pack often works based on which type of process?

• Exothermic dissolution

• Endothermic dissolution

• Exothermic phase change

• Endothermic phase change

• Nuclear reactions change the nucleus, or core, of atoms.

• They release much more energy than chemical reactions.

• Einstein's E = mc² links mass (m) and energy (E).

• This equation shows tiny mass can become huge energy.

Nuclear Reactions & Energy (E=mc²)

Multiple Choice

What fundamental principle explains the large energy release in nuclear reactions?

• Law of Conservation of Energy

• Law of Conservation of Mass

• The interconversion of mass and energy (E=mc²)

• The formation and breaking of chemical bonds

• Mass defect is the missing mass in an atom's nucleus.

• A nucleus weighs *less* than its protons and neutrons separately.

• This missing mass becomes energy holding the nucleus together (binding energy).

• More binding energy means a stronger, more stable nucleus.

Mass Defect & Nuclear Binding Energy

Multiple Choice

What is the 'mass defect'?

• The mass lost during a chemical reaction.

• The difference between the mass of a nucleus and the sum of its nucleons' masses.

• The mass gained when a nucleus forms.

• The mass converted to heat in physical changes.

• Nuclear fission splits a heavy atom's nucleus (like Uranium).

• A neutron collision starts this splitting process.

• Fission releases huge amounts of energy and more neutrons.

• Used in nuclear power plants and atomic bombs.

Nuclear Fission: Splitting Atoms

Multiple Choice

Which statement best describes nuclear fission?

• Two light nuclei combine to form a heavier nucleus.

• A heavy nucleus splits into lighter nuclei.

• An atom loses electrons.

• A chemical bond is broken.

• Tiny atoms join to make a bigger atom.

• ​Joining atoms releases lots of energy.

• This process powers our Sun and other stars.

• Fusion needs extremely high heat to start.

Nuclear Fusion

Multiple Choice

What conditions are required for nuclear fusion to occur?

• Very low temperatures and high pressure

• Very high temperatures and high pressure

• Presence of heavy nuclei

• Complete absence of energy

• Energy changes vary a lot between different processes.

• Physical changes (like melting ice) involve small energy changes.

• Chemical reactions (like burning fuel) involve bigger energy changes.

• Nuclear reactions release the most massive amounts of energy.

​Comparing Energy Changes

Multiple Choice

Rank the following processes from smallest to largest typical energy change per mole:

• Nuclear, Chemical, Physical

• Chemical, Physical, Nuclear

• Physical, Nuclear, Chemical

• Physical, Chemical, Nuclear

• Specific heat (c): Energy to heat one gram of a substance by 1°C.

• Heat capacity (C): Energy to heat an entire object by 1°C.

• Specific heat (c) is unique to the substance itself (like water).

• Heat capacity (C) depends on mass and substance (teacup vs tub).

Specific Heat vs. Heat Capacity

Multiple Choice

What is the definition of specific heat capacity (c)?

• Energy to raise 1 mole by 1°C

• Energy to raise 1 gram by 1°C

• Energy to raise any amount by 1°C

• Total heat content of a substance

• Calculate heat (Q) using mass (m), specific heat (c), and temperature change (ΔT).

• Formula: Q = m • c • ΔT.

• Q=Heat(J), m=mass(g), c=specific heat(J/g•°C), ΔT=change in temp (°C or K).

• Positive Q means heat absorbed (temp rises); Negative Q means heat released (temp falls).

Heat Calculation: Q = mcΔT

Multiple Choice

How much heat (Q) is needed to raise the temperature of 10.0 g of water (c = 4.184 J/g•°C) from 20.0°C to 30.0°C?

• 41.84 J

• 418.4 J

• 836.8 J

• 1255.2 J

• Calorimetry is how we measure heat changes in reactions.

• A calorimeter is a special tool used to trap heat.

• A simple coffee-cup calorimeter keeps the reaction separate.

• Heat lost by the reaction warms up the solution inside.

What is Calorimetry?

Multiple Choice

What is the main principle behind coffee-cup calorimetry for determining the heat of reaction?

• Heat lost by reaction = Heat gained by calorimeter walls

• Heat lost by reaction = Heat gained by solution

• Heat gained by reaction = Heat lost by solution

• The volume of the system remains constant

​Correction

• Temperature measures particle speed; Heat is energy transfer.

• Energy only changes form; total energy is conserved.

• Releasing heat means system loses energy (ΔH is negative).

• Breaking bonds needs energy; forming bonds releases energy.
  

​Misconception

Common Misconceptions

• Heat and temperature are the same thing.

• Energy can be created or destroyed.

• Reactions releasing heat have positive ΔH.

• Breaking chemical bonds releases energy.

Summary

• Thermochemistry studies energy changes during chemical reactions.

• Energy is always conserved; it only changes form or location.

• Exothermic reactions release heat; endothermic reactions absorb heat.

• Energy changes vary: Physical < Chemical << Nuclear reactions.

Open Ended

Is a reaction with ΔH = +50 kJ exothermic or endothermic?

Multiple Choice

How do you calculate Enthalpy of a reaction?

• ΔH = ΔH_products - ΔH_reactants

• ΔT = q / mC

• ΔG = ΔH -TΔS

• E = mc²

Multiple Choice

Calculate the enthalpy of the following reaction:
Ca(OH)₂ -> CaO + H₂O
ΔHf in kJ/mol:  
      Ca(OH)₂  -983.2
        CaO         -634.9
        H₂O         -285.5

• 62.8 kJ
• -62.8 kJ
• -1840.6
• 1840.6

Multiple Choice

Using the equations below:

C_(s) + O_2(g) → CO_2(g) ∆H = –390 kJ

Mn_(s) + O_2(g) → MnO_2(s) ∆H = –520 kJ

what is ∆H (in kJ) for the following reaction?

MnO_2(s) + C_(s) → Mn_(s) + CO_2(g)

• 910

• 130

• -130

• -910

Multiple Choice

Consider the following equations.

Mg_(s) + O_2(g) → MgO_(s) ∆H = –602 kJ

H_2(g) + O_2(g) → H₂O_(g) ∆H = –242 kJ

What is the ∆H value (in kJ) for the following reaction?

MgO_(s) + H_2(g) → Mg_(s) + H₂O_(g)

• -844

• -360

• +360

• +844

Multiple Choice

The standard enthalpy changes of formation of carbon dioxide and of methanoic acid are −394 kJ/mol and −409 kJ/mol respectively. Calculate the enthalpy change for the reaction

H_{2 (g)} + CO_{2 (g)} → HCOOH _(l)

• −803 kJ/mol

• −15 kJ/mol

• +803 kJ/mol

• +15 kJ/mol

Multiple Choice

The standard enthalpy change for the combustion of graphite is −393.5 kJ/mol and that of diamond is −395.4 kJ/mol.

What is the enthalpy change for the reaction below, in kJ/mol?

C _{(s, graphite)} → C _{(s, diamond)}

• −1.9

• +1.9

• −788.9

• +788.9

Open Ended

What quantity relates the heat absorbed or released by a substance to its mass and temperature change?

Poll

Which process generally releases more energy per mole: chemical combustion or nuclear fission?

• Chemical Combustion

• Nuclear Fission

• About the same

• Depends on the substance

Poll

How confident are you in explaining the difference between exothermic and endothermic reactions? (1=Not confident, 4=Very confident)

• 1

• 2

• 3

• 4

Poll

Rate your understanding of how calorimetry is used to measure enthalpy change. (1=Very unclear, 4=Very clear)

• 1

• 2

• 3

• 4