L2 - Thermochemistry Flashcards
State the ‘First Law of Thermodynamics’ / Law of Conservation of Energy.
“The energy of the universe is constant.”
Define ‘internal energy’.
The sum of the kinetic and potential energies of all the particles in the system. It can be changed by a flow of work, heat or both.
What is the equation for ΔE?
ΔE = q + w
ΔE - change in a system’s internal energy
q - heat
w - work
State the ‘Second Law of Thermodynamics’.
“Entropy will always tend to increase. Heat will always flow from higher temperatures to low temperatures.”
State the ‘Third Law of Thermodynamics’.
“Entropy is temperature dependent. If all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would occur.”
What is the value of absolute zero?
0 K = -273.15°C
Define ‘thermochemistry’.
The study of energy changes accompanying a chemical reaction.
Define ‘enthalpy’ (of a reaction).
A measure of the amount of heat released or consumed by a reaction.
What is the equation for H?
H = E + PV
H - energy of the system
P - pressure of the system
V - volume of the system
When ΔH < 0, what is the relative stability of bonds and the heat change during reaction?
Relative Stability of Bonds: Products > Reactants
Heat Change during Reaction: Heat is released
When ΔH > 0, what is the relative stability of bonds and the heat change during reaction?
Relative Stability of Bonds: Products < Reactants
Heat Change during Reaction: Heat is consumed
When ΔH = 0, what is the relative stability of bonds and the heat change during reaction?
Relative Stability of Bonds: Products = Reactants
Heat Change during Reaction: No change
What happens during an exothermic reaction?
Heat is released by the reaction
What happens during an endothermic reaction?
Heat is consumed by the reaction
Define the ‘Standard Enthalpy of Formation’.
The change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states.
What is the standard enthalpy of formation of a pure element in its standard state defined as?
Zero
What studies exothermic reactions?
Bomb calorimeters
What happens in bomb calorimetry?
Heat energy is transferred to raise the temperature of the surroundings (water)
What is the equation for ΔH?
heat capacity
ΔH = m C.p ΔT
enthalpy change = mass x heat capacity x temperature change
State ‘Hess’ Law’.
“The overall enthalpy change accompanying a chemical reaction is independent of the route taken in going from reactants to products (provided that in each case, the same initial and final states of temperature and pressure apply to the reactants and products).”
What is the equation for ΔH?
products & reactants
ΔH = ΣH.products - ΣH.reactants
What is the heat change of a whole reaction equivalent to?
The sum of heat changes in its steps.
ΔH for a process involving the transformation of reactants into products is not dependant on _______.
pathway
What can ΔH for a reaction be calculated from?
- Heat of formation of all reactants and products
- Bond energies of individual bonds
What is the equation for ΔH.r?
ΔH.r = ΣH.broken + ΣH.formed
What types of reactions are:
- bond breaking
- bond making
- bond breaking: endothermic (positive values)
- bond making: exothermic (negative values)
What are the steps for forming a Born-Haber cycle?
1) Write a balanced reaction equation for the enthalpy you need to find
2) Write equations for the enthalpies you have been given
3) Draw the overall chemical reaction as an enthalpy diagram (with the reactants on one line, and the products on the other line)
4) Draw a reaction representing the intermediate step(s) by placing the relevant reactants on a line
(Remember to check for state changes/standard states)
5) Apply Hess’ law – add up the enthalpies for each step, taking direction of reaction into account.
