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VICChemistryQuick questions
Unit 3: How can design and innovation help to optimise chemical processes?
Quick questions on Thermochemical equations and enthalpy changes: VCE Chemistry Unit 3
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is a thermochemical equation?Show answer
A thermochemical equation is a balanced chemical equation that includes the enthalpy change (ΔH) for the reaction as written. The equation must include the states of all reactants and products because enthalpy changes depend on phase (vapour vs liquid water release different energies, for example).
What is sign convention for ΔH?Show answer
ΔH is the change in enthalpy (H_products minus H_reactants), measured at constant pressure.
What is reading a thermochemical equation?Show answer
C(s) + O2(g) -> CO2(g) ΔH = -394 kJ mol^-1
What is scaling energy for a given mass?Show answer
To find the energy released or absorbed by a non-stoichiometric amount, use the mole ratio.
What is energy profile diagrams?Show answer
Thermochemical equations are visualised as energy profile diagrams showing reactants, products, and the activation energy barrier.
What is states matter?Show answer
Always include states because the same reaction releases different ΔH depending on phase. For example:
What is forgetting states?Show answer
Markers deduct for missing or wrong state symbols. Combustion uses (l) for water by VCE convention.
What is sign-flipping ΔH after halving coefficients?Show answer
Halving the equation halves the magnitude of ΔH but keeps the sign. Only reversing the equation flips the sign.
What is calling Ea "the enthalpy change"?Show answer
Activation energy is the barrier from reactants to transition state. ΔH is the net difference between reactants and products. They are independent: a reaction can be exothermic with a high Ea or endothermic with a low Ea.
What is quoting ΔH per gram?Show answer
Molar enthalpy is always per mole of the species (usually the fuel for combustion). Energy per gram is a different quantity (energy content).