chemistry module 3 enthalpy changes
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111 Terms
Enthalpy
Total chemical energy within a substance.
Enthalpy Change (ΔH)
Symbol representing change in enthalpy.
Exothermic Reaction
Products have less energy than reactants.
Heat Energy Release
Heat is given off to surroundings.
Temperature Increase
Environment temperature rises during exothermic reactions.
Energy Decrease
System's energy decreases in exothermic reactions.
Negative ΔH
Indicates enthalpy decrease in exothermic reactions.
Endothermic Reaction
Products have more energy than reactants.
Heat Energy Absorption
Heat is absorbed from surroundings.
Temperature Decrease
Environment temperature falls during endothermic reactions.
Energy Increase
System's energy increases in endothermic reactions.
Positive ΔH
Indicates enthalpy increase in endothermic reactions.
Enthalpy Profile Diagram
Shows energy levels of reactants and products.
Activation Energy (Ea)
Minimum energy needed for successful collisions.
Transition State
Unstable intermediate state in a reaction.
Energy Level Diagram
Visual representation of energy changes in reactions.
Combustion of Methane
Exothermic reaction with ΔH of -890 kJ/mol.
ΔH for Combustion
Enthalpy change for methane combustion reaction.
Kinetically Controlled Reaction
Reaction may not occur if rate is slow.
Thermodynamically Possible
Exothermic reactions can occur under favorable conditions.
Worked Example
Illustration of drawing energy level diagrams.
Labeling Axes
Important step in drawing energy level diagrams.
ΔH
Standard enthalpy change of a reaction.
E_a
Activation energy for a reaction.
Forward Reaction E_a
Energy needed for the forward reaction.
Reverse Reaction E_a
Energy needed for the reverse reaction.
Standard Conditions
100 kPa pressure, 298 K temperature.
Standard Physical State
Substance in solid, liquid, or gas form.
ΔH_f
Enthalpy change for formation reactions.
ΔH_r
Enthalpy change for reaction processes.
ΔH_c
Enthalpy change for combustion reactions.
ΔH_neut
Enthalpy change for neutralization reactions.
Enthalpy Change Calculation
Energy released or absorbed during a reaction.
ΔH for Water Formation
ΔH = -286 kJ mol for water.
Total ΔH for Reaction
Sum of enthalpy changes for all reactants.
ΔH for 2H₂ + O₂
ΔH = -572 kJ for two moles of water.
ΔH for Fe₂O₃ Formation
ΔH = -1648 kJ for two moles of Fe₂O₃.
Hump in Reaction Pathway
Represents activation energy barrier in reactions.
ΔH of Elements
ΔH of an element in standard state is zero.
O₂ Standard State
ΔH_f of O₂ is 0 kJ mol.
Energy Difference Calculation
Ea = ΔHforward + ΔH_reverse.
Standard Enthalpy Symbol
Symbol 𝚫 indicates standard conditions.
Pressure Conversion
101 kPa equals 101325 Pa for precision.
Calorimetry
Measurement of enthalpy changes in reactions.
Specific Heat Capacity
Energy needed to raise 1 g by 1 °C.
Specific Heat Capacity of Water
4.18 J g⁻¹ K⁻¹.
Energy Transfer Equation
q = m x c x ΔT.
ΔT
Change in temperature, final - initial.
Bond Dissociation Energy
Energy to break one mole of covalent bond.
Bond Energy
Simplified term for bond dissociation energy.
Average Bond Energy
Average energy of bonds in different environments.
Enthalpy Change Calculation
ΔH = q/n or m × c × ΔT.
Exothermic Reaction
Reaction with negative ΔH, releases heat.
Endothermic Reaction
Reaction with positive ΔH, absorbs heat.
Combustion of Methane
Burning methane releases energy in calorimetry.
Energy Released Calculation
Total energy = 299,567 J for 2.50 g methane.
Energy per Gram of Methane
120,000 J g⁻¹ or 120 kJ g⁻¹.
Polystyrene Cup
Simple calorimeter for measuring enthalpy changes.
Vacuum Flask
Insulated container for calorimetry experiments.
Metal Can Calorimeter
Alternative calorimeter using metal can.
Heat Absorption Percentage
30% of energy absorbed by water in experiment.
Enthalpy Cycles
Used to calculate average bond energy indirectly.
Data Tables for Bond Energies
List average bond enthalpies for calculations.
Temperature Rise
Indicates exothermic reaction in calorimetry.
Temperature Fall
Indicates endothermic reaction in calorimetry.
Standard Enthalpy Change
Change in enthalpy during a chemical reaction.
Bond Breaking
Endothermic process requiring energy input.
Bond Forming
Exothermic process releasing energy.
ΔH
Symbol representing change in enthalpy.
Exothermic Reaction
Reaction releasing energy, ΔH is negative.
Endothermic Reaction
Reaction absorbing energy, ΔH is positive.
Bond Energies
Energy required to break chemical bonds.
Haber Process
Reaction producing ammonia from nitrogen and hydrogen.
Enthalpy Change Calculation
Difference between bonds broken and formed.
Combustion Enthalpy
Enthalpy change when one mole combusts in oxygen.
Average Bond Enthalpy
Mean energy required to break bonds in molecules.
Hess's Law
Total enthalpy change independent of reaction path.
Indirect Route
Alternative pathway for a chemical reaction.
Direct Route
Straightforward pathway from reactants to products.
Calorimetry
Experimental method to measure heat changes.
Balance Sheet Calculation
Organized method to calculate enthalpy changes.
Ethyne Combustion
Complete combustion of ethyne producing CO2 and H2O.
Mole of Substance
Amount of substance containing 6.022 x 10²³ entities.
Energy Release
Energy emitted during bond formation.
Energy Requirement
Energy needed to break chemical bonds.
ΔH for Bonds Broken
Positive value representing energy absorbed.
ΔH for Bonds Formed
Negative value representing energy released.
Enthalpy Change
Heat change during a chemical reaction.
Direct Route
Enthalpy change from elements to products directly.
Indirect Route
Enthalpy change via reactants to products.
ΔH
Symbol for enthalpy change.
ΔH_f
Enthalpy change of formation from elements.
ΔH_c
Enthalpy change of combustion.
Hess's Law
Total enthalpy change is constant regardless of route.
Energy Conservation
Total energy remains constant in reactions.
Formation Reaction
Reaction forming a compound from its elements.
Combustion Reaction
Reaction of a substance with oxygen producing heat.
ΔH = ∑ΔH(products) - ∑ΔH(reactants)
Formula for calculating enthalpy of formation.
ΔH = ∑ΔH(reactants) - ∑ΔH(products)
Formula for calculating enthalpy of combustion.
Balanced Equation
Chemical equation with equal atom counts on both sides.
Note 
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