Energetics Flashcards
what energy change is breaking bonds associated with?
Energy is taken in to break bonds - endothermic reaction
what energy change is making bonds associated with?
Energy is released to make bonds - exothermic reaction
what are some uses of thermochemistry?
- measuring and comparing energy values of fuels
-calculating the energy requirement for industrial processes
-working out the theoretical amount of energy released/taken in, in a reaction
-predicting if a reaction will take place or not
what is an endothermic reaction?
- positive enthalpy change
energy in breaking bonds > energy in making bonds
what is an exothermic reaction?
- negative enthalpy change
energy in making bonds> energy in breaking bonds
if a reversible reaction is endothermic in one way, what type of reaction is the other way?
exothermic
give two examples if exothermic reactions?
combustion of fuels
neutralisation
give an example of endothermic reaction?
thermal decomposition
define enthalpy change? what symbol is used to represent it?
energy change in system at constant pressure
△H
what are the standard conditions?
pressure - 1 atm
temperature - 25° / 298K
what does ‘in standard state mean’ ?
State an element or compound exist in standard conditions. 1atm or 298K
draw an enthalpy change diagram for an endothermic and exothermic reaction
diagram - labelled w activation energy and change in enthalpy
define standard enthalpy formation?
The enthalpy change when 1 mole of compound is formed from its constituent elements, in standard conditions (100Kpa, 298K), with reactant and products in their standard states.
give an example of an equation which represents standard enthalpy of formation?
H2(g) +0.5O2(g) –> H2O(l)
define standard enthalpy of combustion?
The enthalpy change when 1 mole of substance is burnt completely in oxygen in standard conditions (100Kpa,298K) with reactant and products in their standard states.
What is the difference between heat and temperature?
Heat is the sum of all particles’ energy, therefore it is affected by the amount of substance.
Temperature is related to the mean kinetic energy of the particles in a system, so is independant of the number of particles present.
How can you calculate enthalpy change from experimental data?
Q= mc△t
m = mass
c= SHC
water SHC = 4.18gJK
△t - change in temperature
draw a simple calorimeter?
diagram
what is a flame calorimeter? How does it differ from a simple calorimeter?
reduces heat lost to the surroundings to give more accurate results - has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.
How would you measure the change in enthalpy for a reaction occurring in (aq)?
- use a expanded polystyrene cup as a calorimeter = it is a good insulator and reduced heat lost
- heat is generated in solution
- measure the change in temperature
- take SCH of solution = 4.18 and density as = 1 g cm-3
what is Hess’s Law?
states the enthalpy change of the reaction is the same regardless of the route taken
what is the enthalpy of an element?
the enthalpy of of all elements in their standard states
100KPa 298K
define bond dissociation enthalpy?
the enthalpy change required to break a covalent bond, with all species in their gaseous state. Differs for the same bond in different types of molecule.
define mean bond enthalpy?
average value for the bond dissociation enthalpy of a given bond
why may experimental methods for enthalpy determination may not be very accurate?
- Heat is lost to the surroundings
- Not in standard conditions
- Reaction may not go to completion
why will using bond enthalpies not be as accurate as using standard enthalpy of combustion/fermentation?
Bond enthalpies are the mean for one bond across many different molecules. Standard enthalpy of combustion/ formation apply to just one molecule, therefore they are more accurate.
