Chem Unit 2 (Energetics) Flashcards
What are the standard conditions for enthalpy changes?
100 kPa
298 K
All substances in their normal physical states
Solutions= 1 mol/dm-3
Enthalpy Change?
Heat measured under constant pressure
Standard enthalpy change of reaction?
Enthalpy change when substances react under standard conditions in quantities given by the equation.
Standard enthalpy change of formation?
Enthalpy change when one mole of a substance forms from its elements under standard conditions.
Standard enthalpy change of combustion?
Enthalpy change when one mole of a substance burns completely in oxygen under standard condtions
Hess’s Law?
The total enthalpy change of a reaction is independent of the route at which the reaction is carried out as long as the initial and final conditions are the same.
Mean enthalpy bond change?
The enthalpy change it takes to break a covalent bond averaged out over a wide range of compounds (that contain that bond)
Energy change reactions?
energy change = bonds broken in reactants - bonds made in products
INCLUDE BALANCED EQUATIONS
Hess’s Law
arrows going in the same direction always add together to = the remaining arrow
AH1 + AH2 = AH3
AH2 = AH3 - AH1
If there are 4 arrows = AH1 + AH2 = AH3 + AH4
why are elemts 0?
pure elemts are always 0 by defintion.
formation of ammonia from its elements?
1/2 N2 + 3/2 H2 = NH2
Calimomentry
measures amount of heat energy released or absorbed in a reaction
(usually involves a spirit lamp and a breaker of water).
Equation for measuring enthalpy of combustion?
mass of water x specific heat capacity x temperature change
=enthalpy change (J)/ 1000 = (Kj)
moles = mass/ Mr
enthalpy change/ moles always add - sign since it’s EXOTHERMIC)
Accuracy?
The experiment result is always lower than the data book result because of procedural errors. Heat energy is lost heating the copper calirometer and the air around the apparutus so any experiment involving calimomety is lower than the density experiemnt.
Apparutus erros are so small they are insignificant
Apparutus errors?
Balance = 0.01/reading times 100
Thermometer = 1/temp change (AH) times by 100
Measuring cylinder = 1/volume of water times by 100
Bonds that break first/less exothermic?
Lowest enthalpy value
cycloalkanes- ring structure so they are weaker
combustion products?
Combustion equations = decimal point.
Complete combustion- always CO2 and H20 except for Nitrogen reactants = N2 + H2O or sulfur priducts which are SO2 + H20
What would you do when you have to work out a bond?
AH2 = reactants - products
put the num,bers and the number of unknown bonds in for each thing
Solve it like an algaebraic equatiion
How do you work out out the standard enthalpy change for combustion with known volumes of solutions?
1) Add together the two volumes
2) Times by density (usually 1.99 g/cm-3)
3) do q = mcAT (for the enthalpy change) / 1000 to get kJ.
4) Work out moles (eiher from ratiio or calculate suing conc x volume)
5) Do enthalpy change / moles
6) add - sign for combustiion as its’s EXOTHERMIC!!
Bond Enthalpy
Energy needed to break a particular bond in a gaseous molecule
Why does the bond enthalpy for the same type of bond vary in moleules?
Example: methane CH4
- when you start to pull apart the bonds, they exist in a differnt environemt.
- First you’re breaking the C-H bond from a CH4 molecule and the a CH3, thena CH2,
-each time the size of the molecule is getting smallern so the disoociation (breaking of C-H bonds) will be easier/ takes less energy.
-For CH4, the mean bond enthalpy is an average of these four values.
The data book value for C-H bonds is going to be averaged over a wide range of compounds containing C-H bonds.
