1.4 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 the energy values of fuels
Calculating the energy requirements 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?
One with an overall positive enthalpy change (+ΔH) → energy in breaking bonds > energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change (-ΔH) → energy in breaking bonds < energy out making bonds
If a reversible reaction is endothermic one way, what type of reaction is the other way?
exothermic
Give 2 examples of exothermic reactions
Combustion of fuels
Neutralisation
Give an example of an endothermic reaction
Thermal decomposition
Define enthalpy change; what symbol is used to
represent it?
Energy change of a system at a constant pressure represented by ΔH
What are the standard conditions?
100kPa / 1atm pressure
298K / 250C temperature
What does “in standard state” mean?
The state an element/compound exists at in standard conditions (100kPa, 298K)
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from it constituent elements in standard conditions (100kPa, 298K), with reactants and products in their standard states.
Give an example of an equation which represents
standard enthalpy of formation
H2 (g) + ½ O2(g) → H2O (l)
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is burnt completely in oxygen in standard conditions (100kPa, 298K), with reactants and products in their standard states.
Give an example of an equation which represents
standard enthalpy of combustion
C (s) + O2 (g) → CO2 (g)
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 independent of the number of particles present.
How can you calculate enthalpy change from experimental data?
Use the equation Q = mcΔT, where m is the mass of the substance being heated (usually water), c is the specific heat capacity of that substance (water’s SHC = 4.18gJ-1K -1) and ΔT is the change in temperature
What is a flame calorimeter; how does it differ to a
simple calorimeter?
Reduces heat lost to the surrounding 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 enthalpy change for a
reaction occurring in (aq)?
Use an expanded polystyrene cup as a calorimeter (good insulator → reduce heat loss).
Heat is generated in the solution; measure this
temperature change.
Take heat capacity of
solution to be 4.18 and density of solution = 1gcm-3
What can you use to make experimental
determination of enthalpy change of reaction more
accurate?
Cooling curves
What is Hess’s Law?
States that the enthalpy change for a reaction is the same regardless of the route taken
What is the enthalpy of an element?
The enthalpy of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0
Define bond dissociation enthalpy.
The enthalpy change required to break a covalent bond, with all species in the gaseous state; differs for the same bond type in different molecules.
Define mean bond enthalpy.
Average value (across different chemical environments) for the bond dissociation enthalpy of a given bond.
Why may experimental methods for enthalpy
determination 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/formation?
Bond enthalpies are a mean for the same bond across different molecules; standard enthalpy of combustion and formation apply just to that molecule, therefore they are more accurate.
