2.Energy changes and fuels Flashcards
What is enthalpy?
the heat content that is stored in a chemical system.
Endotheric-
+ve delta H
Exothermic-
-ve delta H
What are self heating cans?
a can which contains a chamber where an exothermic reaction can happen.
What are self cooling beer cans.
water evaporates to use heat energy.
Exothermic reaction example?
Combustion respiration.
Endotheric reaction example?
thermal decomposition of limestone ( CaCO3) photosynthisis
What is a enthalpy profile diagram?
A diagram for a reaction to compare the enthalph of the reactants with the enthalpy of the products. Shows Ea, and Delta H .
Define activation energy?
Ae is the minimum energy required to start a reaction by the breaking of bonds.
What are standard condions.
25 dec c(298K), 1atm, 1 moldm-3
Define the standard enthalpy change of reaction.
The enthalpy change that accompanies a reaction in the molar quantities(the number of moles of reactants as stated), expressed in a chemical equation under standard conditions, with reactants and products in their standard states.
Define the standard enthalpy change of combustion.
is the enthalpy change when one mole of a substance is completely reacted with oxygen under standard conditions, and all the products in their standard states
Define the standard enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Define 1st Ionisation energy
the required for 1 e- to be removed from every atom in 1 mole of gaseous atoms.
define mean bond enthalpy.
The average amount of energy needed to break 1 mole of a specific bond in the gaseous state.
Why might mean bond enthalpy be incorrect.
as its an average not for that specific molecule.
what is specfic heat capacity?
the energy required to raise the temp of 1g of a substance by 1 deg C
Q(heat exchange)=
m cp deltaT
How is enthalphy change calculated?
-Q(in KJ)/mol
What is the mass used in calorometry?
the mass of the thing being heated- usually water
How do you find delta Hc
Burn a know mass of substance in air. measure the volume of water being heated. measure the waters temp change.
Why might measure enthalpy of combustion be different?
Incomplete combustion? Heat loss to surroundings. heat lost to calorometer. mass of fuel lost as vapour
Why might measure enthalpy of formation/reaction be different?
heat lost to surroundings heat lost to calorimeter heat lost to solid reactants.
whats the relationship between carbon atoms in a alcohol and enthalph of combustion.
more c atoms = higher enthalpy of combustion.
ENthalpy change using bond enthalpies=
Energy released when broken - energy requred to form product bonds.
What is hess’ law?
if a reaction can take place by more then one route and the initial and final conditions are the same, the total enthalpy change is the same for each route.
Why isnt it always possible to directly measure enthalphy change?
More then one reaction takes place a high Ae a very slow rate.
Work our enthalpy change of reaction from their combustion energies.
construct a hess cycles/diagram. work out the reaction routes. (energy to combust left- energy to combust right = enthaply of reaction)
Work our enthalpy change of reaction from their formation energies.
construct a hess cycles/diagram. work out the reaction routes. (energy to form right- energy to form left right = enthaply of reaction)
Two hese cycles for formation and combustion (not andys way)
Standarf for for anys hess cycle.
Elecments
fuel XCO2+yH2+O2
Xco2+yH20
Why are some cyclic thigns more exothermic
the ring structure has been squashed from 109.5 to 60 in a tri, 90 in a but, etc, this requires energy which is released when its burnt.
Why does enthalpy of combustionn increase linearly for homologeos series.
a regualr amour more of the same bond is being broken each increase= more energy released.
What is a procedural error?
a fault with the method
eg incomplete combustion, non standard condtitions, cp of equipment ignored,head lost to environment…
