3.1.4 Energetics Flashcards
(49 cards)
Standard temperature
298 KELVIN
Standard pressure
100KPa
Standard definition of enthalpy formation
The standard enthalpy of formation is the enthalpy change, when one mole of a compound, is formed from its elements in their standard states and under standard conditions
Standard enthalpy of combustion definition
The standard enthalpy of combustion is the enthalpy change when one mole of a substance, is burned completely in excess oxygen, all substances in normal states under standard conditions.
What is standard concentration
1 mol dm -3
Standard unit for standard enthalpy
KJ mol -1
What is the value of the standard enthalpy change of neutralisation
- 57 KJ mol -1
Enthalpy change for an Exothermic reaction is
Negative
Enthalpy change for an endothermic reaction is
Positive
Define standard enthalpy change of neutralisation
Enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions with all substances in their standard states
Define enthalpy change of a reaction
Enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions and all chemical in their standard states
Equations for energy change of water
Energy change of the water = mass of water (g) x specific heat capacity of water ( J g-q K-1) x temperature change of water (c)
= m x c x temp change
Equation for standard enthalpy change of combustion
Thermal energy (KJ) / number of moles - (mass of fuel / molar mass of ?)
AH = Q/ moles
Reasons why for calorimeters particular value may not be correct
X4
Unlit sprit burned uncapped fuel will evaporate - think burn more fuel then we actually did
Heat energy released does not all pass into water - lots air or metal calorimeter
Not all fuel combusts- incomplete combustion releases less thermal energy
Not carried out under standard conditions
Hydrogen exist as
H2
Where can you add numbers in the equation for standard enthalpy of combustion
This applies for all
Example equation
H2 + O2 —>H2O
You can’t change the initial product
So in the example you can’t change the number of H2
Answer H2 + 1/2O2 —-> H2O
Where can you add number when doing standard enthalpy of formation
Example equation
C + H2 —> C4H10
You can add number of the left side so the reactants but not the right can’t change the products
Answer 4C + 5H2 —-> C4H10
Define standard enthalpy of a reaction
Enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions and standard states
When constructing Hess cycles for standard enthalpy change of formation do you need to know the enthalpy change of a compound or element
Compound
Enthalpy change of an element is always zero
When using data for standard enthalpy change of formation which way do the arrows point
Up
When using formation data both arrows point upwards
When using standard enthalpy of combustion data which way do the arrows go in a Hess cycle
Down
(Trees burn down)
Do you use big numbers in Hess cycles
YES
When doing an enthalpy cycle of formation (using combustion data) what are the molecules that go on the bottom of the cycle
E.g. if you have elements C and H
You would have CO2 + H2O
You don’t need to add oxygen to original equation
When finding the standard enthalpy of combustion do you need the data for the elements or compounds
Both you need to use the enthalpy of formation for all the molecules/elements present
Excluding oxygen
