1.4 - Energetics Flashcards
What energy change is breaking bonds associated with?
Energy is taken in to break bonds - endothermic reactions
What energy change is making bonds associated with?
Energy released to make bonds - exothermic
What are some uses of thermochemistry?
Measuring and comparing the energy values of fuels.
Calculating energy requirements for industrial processes.
Working out the theoretical amount of energy released/taken in a reactions.
Predicting if a reaction will take place
What is an endothermic reaction?
One with an overall positive enthalpy change -
Energy and breaking bonds > energy out making bonds
What is an exothermic reaction?
One with an overall negative enthalpy change – energy and breaking bonds less than energy out making bonds
If a reversible reaction is endothermic, one way what type of reaction is the other way?
Exothermic
Give two 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 🔺H
What are the standard conditions?
100kPa
298K
What does ‘in standard state’ mean?
The state an element/compound exists at in standard conditions
How would Antheil be change diagrams look for an endothermic reaction and exothermic reaction?
Define standard enthalpy of formation
The envelope change when one mole of a compound is formed from its constituent elements in standard conditions with reactants and products in their standard states
Define standard enthalpy of combustion
The enthalpy change when one mole of a substance is burnt completely in oxygen and standard conditions with reactants 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 main kinetic energy of the particle in the system so is independent of the number of particles present
How can you calculate an enthalpy change from experimental data?
Use the equation Q =MCAT where M is the mass C is specific heat capacity in Delta T is change in temperature
How can you make a calorimeter more accurate?
Add a lid on top of the beaker
Add mineral around the beaker
What is a flame calorimeter? How 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 and enclosed flame and the fuel is burned in pure oxygen not air
What is Hess’s law?
States that the anthropology change for a reaction is the same regardless of the route taken
What is the enthalpy of an element?
Zero
Define bond dissociation enthalpy
The enthalpy change required to break a covalent bond with all species in the gaseous state differs for the same type of bond in different molecules
Define mean bond enthalpy
Average value for the bond dissociation enthalpy of a given bond
Why may experimental methods for an 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 dental peers not be as accurate as using standard anthropology of combustion/formation?
Bond enthalpy are mean for the same bond across different molecules standard enthalpy of combustion and formation applied to that molecule therefore they are more accurate
