3.1.4 Energetics Flashcards
What is enthalpy change?
Heat energy transferred in a reaction at constant pressure. Units are kJ mol-1
What type of enthalpy reactions are Oxidation Reactions most commonly?
Exothermic (-ve ΔH)
What type of enthalpy reaction is thermal decomposition?
Endothermic (+ve ΔH)
What type of enthalpy reactions occur when bonds are broken?
Endothermic (+ve ΔH)
What happens to energy when bonds are formed?
Energy is released (bond braking exothermic, -ve ΔH)
What is the enthalpy change overall?
Enthalpy change for a reaction is overall effect of bond breaking and formation. If more energy is needed to break bonds than is released when bonds are formed then ΔH is +ve (& vice versa)
Why are bond enthalpies NOT exact?
The energy required to break a bond depends on the environment its in.
In calculations, mean bond enthalpy is used.
How can enthalpy changes be calculated using Mean Bond Enthalpies?
Enthalpy Change = Total energy absorbed - total energy released
What are the 2 main types of ΔH (for AS/Energetics)?
1) Standard enthalpy of formation, ΔfH, enthalpy change when 1 mol of a compound is formed from its elements in standard states under standard conditions
2) Standard enthalpy of combustion, ΔcH, enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions
How can you find out Enthalpy Changes?
Calorimetry
How does Calorimetry work?
1) To find enthalpy of combustion of a flammable liquid, burn it in apparatus (called a calorimeter).
2) As fuel burns, it heats water. Work out heat energy that has been absorbed by the water if you know the temp change and the specific heat capacity of water
3) Ideally all heat given out by fuel is absorbed by water, but it can always be lost to the environment/surroundings making it hard to get an accurate result
4) Some combustion may be incomplete as well (so less energy given out), flammable liquids often volatile so some fuel lost to evaporation
How do you use Calorimetry to measure enthalpy changes in solution in Neutralisation?
1) Add a known volume of acid to an insulated container (e.g. polystyrene cup) and measure the temperature
2) Then add a known volume of alkali and record the temp of the mixture at regular intervals over a period of time (ensure to stir the solution so evenly heated)
3) Find the temp change for the experiment. Use it to calculate enthalpy change of reaction (q=mcΔT)
What is Hess’ Law?
The total enthalpy change of a reaction is independent of the route taken
How can Enthalpy Changes be worked out from Enthalpies of Formation?
ΔH = Sum ΔfH Products - Sum ΔfH Reactants
Only ΔfH of compounds, not elements
How can Enthalpy Changes be worked out from Enthalpies of Combustion?
ΔH = Sum ΔcH Reactants - Sum ΔcH Products
