1/40
Looks like no tags are added yet.
Name | Mastery | Learn | Test | Matching | Spaced |
---|
No study sessions yet.
Enthalpy change (ΔH)
The heat transferred at constant pressure during a chemical reaction.
Standard enthalpy change of reaction (ΔrH°)
The enthalpy change when reactants in their standard states form products in their standard states at 1 bar pressure and 25°C (298 K).
Standard state
The most stable form of a substance at 1 bar and a specified temperature (usually 25°C).
Positive ΔrH°
Indicates an endothermic reaction that absorbs heat from surroundings.
Negative ΔrH°
Indicates an exothermic reaction that releases heat to surroundings.
Reversing a chemical equation
Reverses the sign of ΔrH°.
Scaling the reaction
If you multiply the coefficients in a reaction, multiply ΔrH° by the same factor.
Different physical states
ΔrH° values differ for different physical states of products/reactants.
ΔH calculation step 1
Convert grams to moles using mol=mass (g)/molar mass (g/mol).
ΔH calculation step 2
Use ΔrH° to calculate ΔH for actual amount as ΔH=(mol of substance)×(ΔrH°/mol-rxn).
Given reaction
2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l), ΔrH° = -5755 kJ/mol-rxn.
454 g of butane
Example mass used for energy change calculations in the reaction.
Moles of C₄H₁₀ from grams
454 g of C₄H₁₀ converts to 7.811 mol using 58.12 g/mol as molar mass.
Stoichiometry for ΔH
ΔH = moles × (ΔrH°/mol-rxn) for calculating energy changes.
Result of ΔH calculation
For 7.811 mol of C₄H₁₀, ΔH equals -22,500 kJ using given reaction data.
Check physical states
Important for accurate enthalpy calculation and interpretation.
Coefficients in reactions
Determine how ΔrH° scales and should be accounted for in calculations.
Unit conversion importance
Carefully convert grams to moles and then to kJ in calculations.
ΔrH° applications
Represents energy per mole of the overall reaction, not per mole of a single substance.
Exothermic reaction
A reaction characterized by a negative ΔrH°, indicating heat release.
Endothermic reaction
A reaction characterized by a positive ΔrH°, indicating heat absorption.
Heat Transfer
The exchange of thermal energy during chemical reactions, quantified by ΔH.
Thermodynamic standard conditions
Refers to the 1 bar pressure and 25°C (298 K) under which standard enthalpy changes are measured.
Chemical Reaction Sign Convention
The sign of ΔrH° signifies the heat flow direction in a reaction.
Condensation of water vapor
A specific reaction with ΔrH° of -44.0 kJ/mol.
Reversible reactions
A reaction whose ΔrH° values will change sign when the direction is reversed.
Physical state impact
The physical state (gas, liquid, solid) can significantly affect ΔrH° values.
Reactants in standard states
The form of reactants at 1 bar and 25°C before a reaction occurs.
Molar mass definition
The mass of one mole of a substance, typically in grams per mole (g/mol).
Energy per mole concept
ΔrH° is calculated as energy change per mole of the entire reaction.
Units of enthalpy change
Typically measured in kJ/mol, indicating energy per mole.
ΔH significance
Indicates the energy change associated with chemical reactions under specified conditions.
Stoichiometric coefficients
Represent the proportional amounts of reactants and products in a reaction.
Balanced chemical equation
An equation that shows equal numbers of atoms for both reactants and products.
Heat of formation
The change in enthalpy when one mole of a compound is formed from its elements in their standard states.
Calorimetry
The measurement of heat transfer in chemical reactions.
Enthalpy of vaporization
The heat required to convert a substance from a liquid to a gas at constant temperature and pressure.
Endothermic vs exothermic
Endothermic absorbs heat, exothermic releases heat during reactions.
Molecular interpretation of ΔH
Reflects the energy absorbed or released due to bond breaking and forming in reactions.
Importance of ΔrH° in thermodynamics
Critical for predicting the direction and spontaneity of chemical reactions.
Chemical energy,
Potential energy stored in the bonds of chemical compounds.