2 Thermochemistry Flashcards
State the ‘first law of thermodynamics.’
the law of conservation of energy; the energy of the universe is constant
define ‘internal energy (E).’
the sum of the kinetic and potential energies of all of the particles in the system
can be changed by flow of work, heat or both
State the formula for ΔE (change in system’s internal energy).
ΔE = q + w
q - heat
w - work
State the ‘second law of thermodynamics.’
entropy will always tend to increase.
heat will always flow from a higher to a lower temperature
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 absolute zero equivalent to?
0°C and -273.15K
Define ‘thermochemistry.’
the study of energy changes accompanying a chemical reaction
the amount of energy taken in and given out (ΔH and ΔS)
Define ‘enthalpy,’ ΔH.
for a reaction, it is a measure of the amount of heat released or consumed by a reaction
State the formula used to thermodynamically define ‘enthalpy.’
H = E + pV H - Enthalpy E - energy of the system (J) p - pressure of the system V - volume of the system
For a ΔH < 0 state the:
a) relative stability of reactant and product bonds
b) the heat change during the reaction
a) products > reactants
b) heat is released
For a ΔH > 0 state the:
a) relative stability of reactant and product bonds
b) the heat change during the reaction
a) reactants > products
b) heat is consumed
For a ΔH = 0 state the:
a) relative stability of reactant and product bonds
b) the heat change during the reaction
a) products = reactants
b) no change in heat
Define an ‘exothermic’ reaction
heat is released by the reaction
Define an ‘endothermic’ reaction
heat is consumed by the reaction
Define the ‘standard enthalpy change of formation, ΔHf .’
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 always the ΔHf of a pure element, in its standard state?
zero
State the value of ΔHf for the two following equations in terms of ΔH.
- 2C(s) + 2H₂(g) → C₂H₄(g)
- N₂(g) + 2O₂(g) → 2NO₂(g)
- ΔH = ΔHf (C₂H₄)
2. ΔH = 2ΔHf (NO₂)
What does a bomb calorimeter do and why?
studies exothermic reactions
because heat is transferred to raise the temperature of surroundings (water)
Fill in labels 1-9 on the figure of a bomb calorimeter.
- Motorised stirrer
- Electrical leads for igniting sample
- Thermometer
- Insulated container
- Oxygen inlet
- Bomb (reaction chamber)
- Fine wire in contact with the sample
- Cup holding sample
- Water
State the formula for measuring enthalpy changes using calorimetry, including units and the final step that needs to be taken.
ΔH = mCpΔT ΔH - enthalpy change (J) m - mass (kg) Cp - specific heat capacity (J/kg/K) Δt - change in temperature Final step - ensure that you don't forget to find the moles and divide by them into questions where substances are given
State ‘Hess’ Law.’
The overall enthalpy change accompanying a chemical reaction is independent of the route taken in going from the reactants to products
(provided in each case the same initial and final states of temperature apply to the reactants and the products)
State the formula for ΔH by Hess’ Law, including units.
ΔH = ΣH(products) - ΣH(reactants)
ΔH - change in enthalpy (J)
ΣH - sum of enthalpies (J)
What is the heat change of a whole reaction equivalent to?
the sum of the heat changes in its steps
What is the ΔH for a process, involving the transformation of reactants into products?
It is not dependent on pathways that are taken
Which 2 things can be used to calculate the ΔH for a reaction?
The heat of formation of all reactants and products
Bond energies of individual bonds
What is breaking bonds and what type of value is it?
endothermic
positive value
What is making bonds and what type of value is it?
exothermic
negative value
State the official formula for ΔH.
ΔH = ΣH(broken) - ΣH(formed)
State the formulas for ΔHr and when to use them.
Generally use:
ΔHr = ΣΔH(reactants) - ΣΔH(products)
But when a combustion reaction (any time O2 is added to a compound):
ΔHr = ΣΔH(products) - ΣΔH(reactants)
Explain the steps taken to draw a Hess’ Law/Born-Haber Cycle from enthalpies of combustion/formation.
- Write a balanced reaction equation for the enthalpy you need to find
- Write the equations for the enthalpies you have been given
- Draw the overall chemical reaction as an enthalpy diagram (with the reactants on one line and the products on another)
- Draw a reaction representing the intermediate step(s) by placing the relevant reactants on a line. (remember to check for state changes/standard states).
- Apply Hess’ Law - add up the enthalpies for each step, taking direction of reaction into account.
State the formula linking ΔHf and ΔHr.
ΔHr = ΣHf(products) - ΣHf(reactants)
Give a common example of a compound used to draw Born-Haber Cycles.
NaCl
State each of the following in basic terms:
a) first law of thermodynamics
b) second law of thermodynamics
c) third law of thermodynamics
a) conservation of energy (energy cannot be created or destroyed)
b) entropy tends to increase (temperature tends to flow from a higher to a lower temperature)
c) entropy is temperature-dependent (if all thermal motion removed from particles state called absolute zero would occur)
