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 —> endothermic 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 during a reaction
What is an endothermic reaction
One with an overall positive enthalpy change (+^H) —> energy in breaking bonds > energy out making bonds
What is an exothermic reaction
One with an overall negative enthalpy change (-^H) —> energy in breaking bonds < energy out making bonds
If a reversible reaction is endothermic one way, which is it the other way
Exothermic
Give 2 examples of exothermic reactions
Burning fuels
Neutralisation reactions
Give an example of an endothermic reaction
Thermal decomposition
Define enthalpy change
What signal is used to represent it
Energy change of a system at a constant pressure
Represented by delta H
What are the standard conditions
100k Pa
298K
What does ‘in standard state’ mean
The state that an element is in at standard conditions
Define standard enthalpy of formation
The enthalpy change when one mole of a compound is formed from its constituent elements in standard conditions with all elements in their standard states
Give an example of an equation which shows the standard enthalpy of formation
There are many e.g. H2 (g) + 1/2 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 with reactants and products in their standard states
Give an example of an equation which represents the 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 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 = mc^T
Where m is the mass of substance (usually water) , c is the specific heat capacity (waters SHC = 4.18 g/J/K) and ^T is the change in temperature
What is a flame calorimeter and does it differ to a simple calorimeter
Reduces heat loss 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 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 to be 1 g/cm3
What can you use to make experimental determination of enthalpy change of a reaction more accurate?
Cooling curves
What is Hess’s Law
States that the enthalpy change of a reaction is the same regardless of the route taken
What is the enthalpy of an element
The enthalpy of all elements when in their standard states 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 values 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
