DF defg: Energetics Flashcards
What is thermochemistry?
The study of the energy and heat associated with chemical reactions.
Draw an enthalpy level diagram for an exothermic reaction.
Consider enthalpy in terms of total energy stored in system (bonds of reactants + products). Reactants store more energy than products, so energy is lost, so ΔH is -ve.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/292/588/846/a_image_thumb.png?1573068179)
Enthalpy change is measured in what units?
kJ mol-1
How is “enthalpy change of a reaction at standard conditions” expressed symbolically?
ΔrH⦵298
What are the standard conditions of temperature, pressure and concentration used to compare enthalpy changes?
- 298 K (25oC)
- 100 kPa (1 atm)
- 1 mol dm-3 (molar)
How are enthalpy changes often measured in the laboratory?
- Energy involved in reaction transferred to/from water surrounding reaction vessel
- ΔT of water measured
- ΔH calculated using ΔE = mcΔT
Explain how a bomb calorimeter accurately measures enthalpy changes.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/292/590/928/q_image_thumb.png?1577203923)
- Fuel burned in oxygen in pressurised vessel
- Energy transferred to/from surrounding water; ΔT measured
- ΔH calculated using ΔE = mcΔT
- When calculating ΔH, pressure variations caused by constant volume of closed vessel are accounted for
Give the definition and symbolic representation of the standard enthalpy change of formation.
ΔfH⦵298
The enthalpy change when one mole of a compound is formed from its elements, under standard conditions and in standard states.
Give the definition and symbolic representation of the standard enthalpy change of reaction.
ΔrH⦵298
The enthalpy change when molar quantities of reactants, of proportions stated in the equation, react under standard conditions and in standard states.
Give 2 formulae which can be used to calculate ΔrH⦵.
ΣΔfH⦵products - ΣΔfH⦵reactants
ΣHbonds formed - ΣHbonds broken
What is the definition and symbolic representation of the standard enthalpy change of combustion?
ΔcH⦵298
The enthalpy change when one mole of a substance is combusted completely in oxygen, in standard conditions and states.
Standard conditions + states are impossible in practice, so the substance is burnt in non-standard conditions, then adjustments are made to account for this
What 3 things should you always do when writing the equation to represent the standard enthalpy change of combustion?
- Balance it
- Show one mole of substance combusting
- Include standard state symbols
Give the definition and symbolic representation of the standard enthalpy change of neutralisation.
ΔneutH⦵298
The enthalpy change when one mole of hydrogen ions react with one mole of hydroxide ions to form one mole of water, under standard conditions and in solutions of molar concentration.
![](https://s3.amazonaws.com/brainscape-prod/system/cm/330/883/976/q_image_thumb.png?1608576690)
A
2NaOH + H2SO4 → Na2SO4 + 2H2O
Mol NaOH = mol H2O = 2 x 10 x 10-3 = 0.02 mol
Energy to form 0.02 mol H2O = 20 x 4.18 x y
Energy to form 1 mol H2O (as in definition) = 4180 y J = 4.18 y kJ
Define specific heat capacity.
The amount of energy required to raise the temperature of 1 g of a substance by 1oC.