Chapter 6: Enthalpy Changes Flashcards
Heat of reaction:
Amount of heat evolved or absorbed in a reaction, quantities of reactants and products being sane as represented by chemical equation
Energy change during reaction is mainly due to
Chnage in bond energy ie potential energy resulting from breaking and making of new bonds in reactants and products
Distinguish between system and surrounding
System: area of interest
Surrounding: in theory, everything else in the universe
Energy lost by system is gained by surroundings and vice versa
Distinguish between open and closed systems
Open: can exchange energy and matter with surroundings
Closed: only exchange energy but not matter
Heat content of a system at _ is _
Constant pressure
It’s enthalpy (heat inside)
System is reservoir of
Heat
According to law of conservation of energy total energy doesn’t chnage during process but
Can be transferred between system and surrounding
ΔH is
Used to denote enthalpy change
ΔH is +0 when
Heat is added to system
Chemical system ie _ consists of _ and _ of it’s particles. These add up to _
Substance
Kinetic(motion) and potential energy(position)
Internal energy
Internal energy of substance depends on
Physical state
Structure
Mole of a substance has _ heat content
Characteristic
If reaction happens at constant pressure, _ and _ are the same
Heat change
Enthalpy change
Enthalpy is heat content. We want to know _ ssincewe can’t really know the enthalpy as such
Enthalpy change
In exothermic reaction…
- heat is given out to surrounding
- heat content of reactants> products.
- excess given out in forms of heat
- then falls to room temp as heat is lost
- net decrease in potential energy/heat content of system
- ΔH is -ve
Endothermic reaction…
- heat gained from surrounding
- total heat content of reactants less than products
- products of energy falls below room at first then rises to room temp becahse heat lost is gained from surrounding
- ΔH postive as heat gained
From an energy profile diagram how to identify Ea and ΔH?
Ea: from reactants line to threshold peak
ΔH: reactants line to products line (+/-)
Differentiate between energy level and profile diagrams
Profile has threshold for Ea along with the 2 lines
Level only has 2 lines to get ΔH
Standard conditions for measuring enthalpy changes. And why?
Temperature : 298K (25°C)
Pressure: 100kPa
Physical state: All solutions conc 1mol/dm3
To compare enthalpy changes, conditions must be fixes
Standard state is
Most stable physical share of substance at 100kPa at given temp ie 298K
Standard enthalpy change denoted by
ΔHø
Kelvin scale:
Emphasizes relationship between average KE and temperate
Absolute temp in Ke directly proportional to KE/movement
Absolute zero
-273°C
Lowest possible temp where all movement stops
Increase in temp depends on :
- mass of object (smaller will have greater temp increase, as sane energy shared amongst less particles)
- heat added
- nature of substance
Specific heat capacity:
Amount of heat needed to increase unit mass by 1K
Specific heat capacity depends on
No of particles present in unit mass, which in turn depends on mass of individual particles
Heat/energy change(^Hr)=
-mcΔt
Standard molar enthalpy change ΔHrø=
ΔH÷n
Amount of energy released depends on
Amount of fuel burnt
Heat released by exo reaction is absorbed by _ and the temp of it _. Assuming all of it is absorbed by it
Water
Increases
As water gains heat as enthalpy chnage of reaction is _ when temp of water increases
Negative
Heat absorbed by reaction in an _ reaction from _, so temp of _. Enthalpy chnage of reaction is _
Endo
Water
Water decreases
Postive
Why can we not accurately say water has indeed gotten all the heat content released from a combustion reaction?
- some heat needed for copper calorimeter and some has gone to surrounding
- combustion of fuel likely to be incomplete owing to limited oxygen
- experiment maybe not performed under standard conditions
Finding enthalpy change of reaction in solution using cooling correction:
Plot a cooling curve to improve result of experiment.
Keep recording temp of a volume of solution, in intervals. Add excess reagent and keep recording for temp rise and approx linear fall.
Thermometer takes time to respond to temp chnage of solution, even if reaction is instantaneous.
During that tine heat is lost.
To estimate amount lost and work out max temp, temp is recorded at series of times and a temp time graph is plotted.
Line of best fit through falling temp drawn and extrapolated. Another line drawn from time at which reagent was added.
Meeting point is actual max temp rise
Largest error in experiment conducted in polystyrene cup
Heat lost to environment soon as temp rises above room temp
Max temp recorded is lower than true value obtained in perfectly insulated system
(We make some allowance by extrapolating cooling section of graph to start tike of reaction)
Assumptions to proceed with calculation of enthalpy change in solution:
1) no heat loss from system
2) all heat goes from reaction to water
3) solution is dilute V(s)= V(H2O)
4) water has density of 1g/cm3 hence mass of sol=mass of water
Define standard enthalpy change of reaction
Enthalpy change when the amounts of reactants shown in equation react to give products under standard conditions. Reactants and products must be in their standard states.
Define standard enthalpy change of formation
Enthalpy change when one mole of a compound is formed from it’s elements under standard conditions. Reactants and products must be in standard states
Define standard enthalpy change of conbustion
Enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. Reactants and products must be in their standard states
Define standard enthalpy change of neutralisation
ΔHnø is enthalpy change when one mole of water is formed by reaction of an acid with an alkali under standard conditions
Define standard enthalpy change of solution
ΔHsolø is enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions
Define standard enthalpy change of atomisation
^Hatø is enthalpy change when one mole of gaseous atoms is formed from it’s element under standard conditions
Define standard enthalpy change of hydration of an anhydrous salt
Enthalpy change when one mole of a hydrated salt is formed from one mole of the anhydrous salt under standard conditions
Hess’s law:
Enthalpy change for any chemical change is independent of route provided the starting conditions and final conditions, and reactants and products are same
(because is 1 is large and exo, and 2 is small and exo, P->Q could give a lot of energy, and take reverse route (2, small and endo) only taking in small amount of energy. This could repeat and be perpetual source of energy. This doesn’t happen)
Hence energy can neither be created nor destroyed (first law of thermodynamics)
Hess’s law equation
Q=Q1+Q2
Why are standard enthalpy measurements important
- give a measure of stability of a substance relative to it’s elements
- used to calculate enthalpy changes of all reactions especially ones we can’t measure in laboratory (hypothetical/real reactions)
Use of enthalpy of formation
Used to calculate standard enthalpy change on any reaction
General expression for ΔH reaction of any reaction
ΔHr=totalΔHproducts - totalΔHreactants
bcos reactants+reaction=products, from a formation cycle ie elements–>reactants or products, and reactants–>products
ΔHrø measured in _ and written _
kJ/mol
At end of reaction
Enthalpy of substance is equal to it’s
Stranded enthalpy of formation
Enthalpy of all elements in their normal states under standard conditions is
Zero
Bond energy:
Energy needed to break one mole of particular kind of covakent bond under standard conditions.
Aka bond dissociation energy
For diatomic molecules in _, dissociation energy…
Gaseous state
Twice enthalpy of atomization
Bond breaking is -
Bond making is -
Endo
Exo
Another way to express ΔHrø in terms of bond energy
^Hrø= sum of bond enthalpy of bonds broken - sum on bond enthalpy of bonds formed
