Energetics Flashcards

Interactive active recall cards for enthalpy definitions, calorimetry, Hess's Law, and bond enthalpies.

Enthalpy Changes: read all 5 questions and answers

Define enthalpy change (ΔH).
The heat energy change measured under conditions of constant pressure.
What are the standard conditions for measuring enthalpy changes?
A pressure of 100 kPa and a temperature of 298 K (25 °C).
Define standard enthalpy of formation (\(\Delta_f H^{\ominus}\)).
The enthalpy change when one mole of a compound is formed from its constituent elements in their standard states under standard conditions.
Define standard enthalpy of combustion (\(\Delta_c H^{\ominus}\)).
The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions, with all reactants and products in their standard states.
Why is the standard enthalpy of formation of elements in their standard states zero?
Because no change occurs when forming an element from its constituent element (itself) under standard conditions.

Calorimetry: read all 5 questions and answers

State the equation used to calculate heat energy change (\(q\)) in calorimetry.
\(q = mc\Delta T\), where \(q\) is heat in J, \(m\) is mass of substance being heated in g, \(c\) is specific heat capacity (4.18 J g⁻¹ K⁻¹ for water), and ΔT is temperature change in K.
How is molar enthalpy change (ΔH) calculated from heat energy (\(q\))?
\(\Delta H = -q / (1000 \times n)\), where \(n\) is the number of moles of the limiting reactant, giving ΔH in kJ mol⁻¹.
State three sources of error in a simple cup calorimeter experiment.
Heat loss to the surroundings, incomplete combustion (if using a spirit burner), and the heat capacity of the calorimeter itself.
How can heat loss be minimised in a calorimetry experiment?
Use a polystyrene cup as a calorimeter, add a lid to reduce evaporation/convection, and place the cup inside a beaker for insulation.
Explain why cooling curve correction is used in graphical calorimetry analysis.
To estimate the theoretical maximum temperature rise if the reaction had occurred instantaneously before any heat was lost to the surroundings.

Hess's Law: read all 3 questions and answers

State Hess's Law.
The enthalpy change for a chemical reaction is independent of the route taken, provided the initial and final states are the same.
Write the mathematical expression to calculate ΔH using enthalpies of formation (\(\Delta_f H\)).
\(\Delta H = \sum(\Delta_f H\text{ of products}) - \sum(\Delta_f H\text{ of reactants})\).
Write the mathematical expression to calculate ΔH using enthalpies of combustion (\(\Delta_c H\)).
\(\Delta H = \sum(\Delta_c H\text{ of reactants}) - \sum(\Delta_c H\text{ of products})\).

Bond Enthalpies: read all 5 questions and answers

Define mean bond enthalpy.
The average enthalpy change required to break one mole of a specific covalent bond in the gaseous state, averaged over a range of different compounds.
Write the formula to calculate enthalpy change of reaction using mean bond enthalpies.
\(\Delta H = \sum(\text{bond enthalpies of bonds broken}) - \sum(\text{bond enthalpies of bonds formed})\).
Why is bond breaking endothermic and bond forming exothermic?
Breaking bonds requires energy to overcome electrostatic attraction between nuclei and electrons (endothermic), whereas forming bonds releases energy (exothermic).
Why are calculations of ΔH using mean bond enthalpies often less accurate than those using Hess's cycles?
Mean bond enthalpies are averages from many different compounds, whereas enthalpies of formation are specific to the actual reactants and products in the reaction.
Why are bond enthalpies defined only for gaseous species?
Because intermolecular forces in solids and liquids would also need to be broken, which would affect the measured enthalpy value.