1.4 - Energetics Flashcards
What energy change is breaking bonds associated with?
Endothermic Reaction - Energy is taken in.
What energy change is making bonds associated with?
Exothermic Reaction - Energy is released.
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 whether a reaction will take place or not.
What is an endothermic reaction?
It has an overall positive enthalpy change.
Energy in breaking bonds > Energy out making bonds.
What is an exothermic reaction?
It has an overall negative enthalpy change.
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
Examples of Exothermic Reactions.
- Combustion
- Neutralisation
Examples of Endothermic Reactions.
- Photosynthesis
- Decomposition
Define enthalpy change and give the symbol used to represent it.
Energy change of a system at a constant speed. It is represented by the symbol ‘ΔH’.
What are the standard conditions.
100kPa / 1 atm pressure
298K / 25*C Temperature
What does ‘in standard state’ mean?
- The state an element/coumpound 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 rectants and prodcts in their standard states.
Define standard enthalpy change 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.
What is the difference between heat and temperature?
- Heat is the sum of al particles’ energy, therefore it is affected by the amount of substance; temperature is related to the mean kinetix 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?
Use the equation:
q = mc∆T
Where:
- m = the mass of the substance being heated.
- c = specific heat capacity
- ∆T = teperature change.
What is a flame colorimeter; how does it differ to a simple calorimter?
- Reduces heat lost to the sorrounding to give more accurae results; has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air.
What is Hess’s Law?
States that the enthalpy change for a reaction is the same regardless of teh route taken.
Waht is the enthalpy change of an element?
The enthalpy change of all elements in their standard states (the states in which they exist at 100kPa and 298K) is defined as 0.
Define the 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.
