3.1.4 Energetics Flashcards
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
Why do you never use the standard enthalpy of combustion for oxygen
Oxygen cannot be combusted
Oxygen is used to combust chemicals
Breaking bonds is an ….. reaction
Endothermic reaction
Putting energy in
Forming bonds is an ….. reaction
Exothermic
Energy released
Define average bond enthalpy
The mean bond enthalpy is the standard enthalpy change associated with breaking one mole of covalent bonds in a gaseous substance into individual gaseous atoms averaged over several compounds.
Different molecules have the same/different bond enthalpy?
Different
That why it’s called average bond enthalpy
When do the kangaroos finally come out of there mothers pouch
When they become an adult
Nitrogen exists as
N2
Bromines standard state is
Liquid
What are these standard states
CH3Br
Na2O2
Liquid
Solid
When writing combustion equations what is usually produced
Carbon dioxide and water
(Sometimes oxides as 1 product)
Hydrocarbons eg. C6H14 in their standards states are
Gases
Na + O2 =
Na2O
(Not balanced)
When writing neutralisation equations what are the products + state symbols
X (aq)+ H2O (l)
State the meaning of the term enthalpy change as applied to a chemical reaction
Heat energy change at a constant pressure
What does oxygen always exist as
O2
Exothermic sign
-
Endothermic sign
+
Units for enthalpy change
KJ mol-1
Enthalpy equation for breaking and forming bonds
Enthalpy change = breaking bonds - forming bonds
Do you use big numbers when finding bond enthalpy
YES
Equation to find energy transferred equation
q=mc[delta]t
Important this is energy not enthalpy
q=mc(delta)t
What each part means and values
q= heat energy joules
M= mass in grams
C=specific heat capacity (given)
T= temperature kelvin
How to find enthalpy change once found energy changed
Enthalpy is KJmol-1 aka kJ/mol
So find the moles and divide by the energy found in KJ
The cycle you draw is based off,….
What data your given
E.g. formation data = formation cycle
How to improve accuracy without changing equipment to reduce percentage uncertainty
Increase concentrations
