Chapter 5: Chemical Energetics Flashcards
(18 cards)
Define enthalpy change (ΔH)
Enthalpy change is the heat energy change at constant pressure, measured in kJ mol⁻¹.
Enthalpy change is a crucial concept in thermodynamics, reflecting the energy exchange in chemical reactions.
What characterizes an exothermic reaction?
Energy is released (ΔH is negative), e.g., combustion.
Exothermic reactions often release heat, resulting in a temperature increase in the surroundings.
What characterizes an endothermic reaction?
Energy is absorbed (ΔH is positive), e.g., thermal decomposition.
Endothermic reactions usually result in a temperature decrease in the surroundings.
Describe the energy profile diagram for an exothermic reaction.
Products are lower than reactants; energy is given out.
This diagram visually represents the energy changes during the reaction process.
Describe the energy profile diagram for an endothermic reaction.
Products are higher; energy is taken in.
The diagram illustrates that energy must be supplied for the reaction to occur.
Write a thermochemical equation for the combustion of methane.
CH₄ + 2O₂ → CO₂ + 2H₂O ΔH = –890 kJ mol⁻¹
This equation shows the enthalpy change associated with the reaction.
What is ΔH°ₒₜ (formation)?
Enthalpy change when 1 mole of compound is formed from its elements in standard states.
Standard enthalpy of formation is essential for calculating thermodynamic properties of compounds.
What is ΔH°ₒc (combustion)?
Enthalpy change when 1 mole of a substance is burned completely in oxygen.
This value is important for understanding energy release in combustion reactions.
What is ΔH°ₒn (neutralisation)?
Enthalpy change when acid and base react to form 1 mole of water.
Neutralisation reactions are key in acid-base chemistry.
What is ΔH°ₒa (atomisation)?
Enthalpy change to produce 1 mole of gaseous atoms from an element in its standard state.
Atomisation enthalpies are useful for calculating the energy required to form gaseous atoms.
How do you calculate ΔH using known data?
ΔH = ΣΔHf products – ΣΔHf reactants.
This relationship is fundamental in thermochemistry for determining reaction enthalpies.
State Hess’s Law.
The total enthalpy change in a reaction is the same, no matter the route taken.
Hess’s Law allows for the calculation of enthalpy changes through alternative pathways.
What elements can be used in constructing energy cycles with Hess’s Law?
- Enthalpy of formation
- Enthalpy of combustion
- Bond enthalpies
These elements help in visualizing and calculating unknown enthalpy changes.
Define bond enthalpy.
Energy required to break one mole of a specific bond in the gas phase (mean bond enthalpy = average over many compounds).
Bond enthalpy is critical for understanding the stability of molecules.
How do you calculate approximate ΔH using bond enthalpies?
ΔH = Σ(bond energies of reactants) – Σ(bond energies of products)
This method provides an estimate of the enthalpy change based on bond strengths.
What does a positive ΔH value indicate?
Endothermic reaction.
A positive ΔH suggests that energy is absorbed from the surroundings.
What does a negative ΔH value indicate?
Exothermic reaction.
A negative ΔH indicates that energy is released into the surroundings.
What are limitations of using average bond enthalpies?
- They are averages, not specific to individual molecules
- May not accurately reflect ΔH in all reactions
These limitations can affect the accuracy of thermodynamic calculations.
