Thermochemistry Review
How are temperature and heat related? What are units for each?
What is the heat of reaction for NaOH and HCl of 30 mL NaOH and 70 mL HCl go from 21.2 C to 29.5 C?
Compare and contrast Heat of Reaction and Heat of Formation.
Calculate the Heat of Formation – problem #3 page 524.
What is the relationship between heat of reaction and stability of a product?
Draw an energy diagram for a reaction (2NH3 + H2 -> 2 NH4) which requires 196 kJ/mol to react and absorbs
124 kJ. Show the other two ways to give the change in energy from the reaction. Is this reaction endothermic or
exothermic?
Define enthalpy, entropy, and free energy.
What is the relationship between enthalpy, entropy, and free energy?
List three phase changes and tell what is happening to enthalpy and entropy in each.
What is free energy and how is it calculated?
If ΔH is -34 kJ/mol, ΔS is +.98 kJ/mol T, and temp is 189 C, is the reaction spontaneous? Show your work.
What is a reaction mechanism?
Write out a possible reaction mechanism for 2Mg + O2 -> 2 MgO
How are activation energy and activated complex related?
What two things can happen to an activated complex?
What is Hess’s law and what is it used for?
Explain the Collision Theory.
Draw energy diagrams for both an exothermic and endothermic reactions. Label E, E’, Ea, Ea’, and where the
activated complex would exist.
Name the four factors of reaction rate we can control and explain how/why they impact reaction rate.
Write the heat of formation reaction for the following (use pg 902 for the energies)
Fe2O3
NaOH
CaCO3
NaNO3
Thermochemistry Review Key
How are temperature and heat related? What are units for each?
Temperature is a measure of the average kinetic energy in an object. Heat is the transfer of energy due to
differences in temperature. Temp = F, C, K. Energy = Joules
What is the heat of reaction for NaOH and HCl of 30 mL NaOH and 70 mL HCl go from 21.2 C to 29.5 C?
ΔE = (4.18j/g C)(100 g)(8.3 C) = 3469.4 J /1000 = 3.4694kJ
ΔE = -3.4694 kJ
The energy the water absorbs is the energy the reaction releases.
Compare and contrast Heat of Reaction and Heat of Formation.
Heat of Reaction is the change in energy for any reaction. Heat of Formation is the change in energy in
the formation of 1 mole of product at a specific temperature and pressure.
Calculate the Heat of Formation – problem #3 page 524.
Find Heat of Formation of S + O2 -> SO2
SO3 -> ½O2 + SO2
S + 3/2O2 -> SO3
S + O2 -> SO2
+99.1 kJ
-395.2 kJ
-296.1 kJ
What is the relationship between heat of reaction and stability of a product?
The more negative the heat of reaction (or formation), the more stable the product(s) will be
Draw an energy diagram for a reaction (2NH3 + H2 -> 2 NH4) which requires 196 kJ/mol to react and absorbs
124 kJ. Show the other two ways to give the change in energy from the reaction. Is this reaction endothermic or
exothermic?
Ea =
196 kJ
ΔH = 124 kJ
2NH3 + H2 -> 2 NH4 – 124 kJ
2NH3 + H2 -> 2 NH4 ΔH = 124 kJ
Define enthalpy, entropy, and free energy.
Ethalpy ΔH – Heat energy in a sample of matter
Entropy ΔS – the degree of randomness or chaos in a sample of matter
Free energy ΔG – a summation of the possible change due to potential in both Enthalpy and Entropy
What is the relationship between enthalpy, entropy, and free energy?
ΔG = ΔH –TΔS, where T is temperature in K.
A negative ΔG is a spontaneous reaction
List three phase changes and tell what is happening to enthalpy and entropy in each.
Melting both entropy and enthalpy are increasing
Evaporating both entropy and enthalpy are increasing
Sublimating both entropy and enthalpy are increasing
Freezing both entropy and enthalpy are decreasing
Condensing both entropy and enthalpy are decreasing
Solidifying both entropy and enthalpy are decreasing
What is free energy and how is it calculated?
Free energy is a measure of both the entropy and enthalpy of a reaction.
It is calculated using the formula ΔG = ΔH –TΔS, where T is temperature in K.
If ΔH is -34 kJ/mol, ΔS is +.98 kJ/mol T, and temp is 189 C, is the reaction spontaneous? Show your work.
ΔG = ΔH –TΔS
ΔG = (-34 kJ/mol) – (462 k)(.98 kJ/mol K)
ΔG = (-34 kJ/mol) – 452.76 kJ/mol
ΔG = -486.76kJ/mol
What is a reaction mechanism?
The possible method of which bonds of reactants can be broken and bonds of products formed
Write out a possible reaction mechanism for 2Mg + O2 -> 2 MgO
2 Mg + O2 -> 2Mg + 2O
2Mg + O2 ->Mg + MgO2
2Mg + 2O -> 2MgO
Mg + MgO2 -> MgO + O + Mg
Mg + O -> MgO
Mg + Mg + O=O
Mg + Mg + O=O
Mg + Mg + O + O
Mg + Mg-O=O
MgO + Mg + O
Mg + Mg-O + O
MgO + MgO
MgO + MgO
How are activation energy and activated complex related?
Activation energy is the energy needed to start the reaction, or to bring the reactants together to form an
activated complex.
What two things can happen to an activated complex?
Form product or break back apart into reactants
What is Hess’s Law and what is it used for?
Hess’s law states that
It allows us to take reactions that we know the heat of formation or reaction and then combine them to
make a reaction we do not know the heat of formation or reaction. If we use the energies from the known
reactions, we can calculate the heat of formation/reaction for the new reaction.
Explain the Collision Theory.
Reactions happen because reactants collide with enough energy and at the right angle to form new bonds
Draw energy diagrams for both an exothermic and endothermic reactions. Label E, Ea, Ea’, and where the
activated complex would exist.
Endothermic
Ea’
Ea
ΔH
or
ΔE
ΔH’
or
ΔE’
Exothermic
Ea
Ea’
ΔH
or
ΔE
ΔH’
or
ΔE’
Name the four factors of reaction rate we can control and explain how/why they impact reaction rate.
Collision Theory – atoms/molecules must collide with enough energy and the right angle to react.
Heat – more collisions and more energy available
Concentration – increases number of collisions
Surface area – reactions happen at the surface, so increasing surface area increases the number of atoms/molecules
that can react at a given time
Catalyst – reduces the activation energy needed to perform a reaction, thereby speeding up the reaction
Write the heat of formation reaction for the following (use pg 902 for the energies)
Fe2O3
2Fe + 3/2O2 -> Fe2O3
ΔHf = -824.2 kJ/mol
NaOH
Na + 1/2O2 + 1/2H2 -> NaOH
ΔHf = -425.9 kJ/mol
CaCO3
Ca + C + 2/2O2 + 1/2H2 -> CaCO3
ΔHf = -1207.6 kJ/mol
NaNO3
Na + 1/2N2 + 3/2O2 -> NaNO3
ΔHf = -467.9 kJ/mol