5. Energetics & Thermochemistry Flashcards
Define ‘bond enthalpy’.
The energy required to break one mole of bonds by homolytic fission, in one mole of gaseous covalent molecules under standard conditions.
ΔH= ?
-Q/ n
Q= ?
mcΔT
Define ‘specific heat capacity’.
The energy required to heat 1g of a substance by 1°C
State Hess’ Law.
The enthalpy change for a chemical reaction will always be equal, regardless of which route is taken.
What is enthalpy change?
The heat energy transferred at constant pressure.
Enthalpy change= ?
Enthalpy of products - enthalpy of reactants
Define ‘heat’.
- Thermal potential energy.
- Heat will transfer from a warmer species to a cooler species down a potential energy gradient.
Define ‘temperature’.
A measure of the kinetic energy of a particle.
What is an endothermic reaction?
- Involve a transfer of heat from the surroundings to the system.
- The enthalpy of the system increases, so is positive.
What is an exothermic reaction?
- Involve a transfer of heat from the system to the surroundings.
- The enthalpy of the system decreases, so is negative.
What are the x and y axis of an energy profile graph of a reaction?
- X= reaction coordinate
- Y= potential energy
What four factors affect enthalpy change?
- Mass of the surroundings
- Amount of heat the surroundings will store
- The temperature change during the reaction
- The moles of the system
Describe metal oxides.
- Basic as an OH- ion is released.
- Dissociates when dissolved in water (if soluble).
Describe non-metal oxides.
- Acidic as H+ ion released
Describe how global warming is caused.
- The earth’s surface absorbs the IR and reradiates it a lower frequency.
- Greenhouse gases absorb lower frequency IR, which is used to stretch covalent bonds like a spring.
Where do fossil fuels come from?
- Oil from fossilised marine organisms
- Coal from fossilised plants in swamps.
Name the 5 steps of creating biofuels.
- Photosynthesis
- Harvest crops
- Transportation
- Fermentation
- Purification
Define ‘standard enthalpy change of combustion’.
The enthalpy change when one mole of a substance is completely burnt in oxygen under standard conditions.
Define ‘average bond enthalpy’.
- The average amount of energy required to break one mole of covalent bonds in a gaseous molecule under standard conditions.
- In diatomic molecule only.
- Average refers to the fact that the bond enthalpy is different in different molecules.
Define ‘standard enthalpy change of formation’.
The enthalpy change when one mole of a substance is formed from its elements in their standard states under standard conditions.
Define ‘standard enthalpy change of atomisation’.
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state under standard conditions.
Define ‘first electron affinity’.
The enthalpy change when one electron is added to each atom in one mole of gaseous atoms under standard conditions.
Give the generic equation representing first electron affinity.
X(g) + e- –> X-(g)
Why will the data book value for the standard enthalpy of combustion differ from the calculated value?
- Bond enthalpy values are average values deduced from across a range of molecules and are not entirely accurate.
- Standard enthalpy of combustion refers to substances in their standard states, so there may be differences in the states of products formed.
Define ‘lattice enthalpy’
The enthalpy change when one mole of ionic solid forms from its constituent gaseous ions.
What is the enthalpy of formation of an element?
Zero
What is entropy?
- A measure of disorder.
- The number of ways that particles can be arranged and the number of ways that energy can be distributed between those particles.
How can entropy be increased?
- Increase the number of moles.
- Solid –> liquid –> gas.
What is a spontaneous reaction?
A reaction that takes place continually without intervention.
What must the entropy be for a spontaneous reaction?
Greater than 0.
What are the units of entropy?
J/mol/K
What happens to entropy between phase changes? Why?
- There is a sharp increase.
- Bonds are weakened or broken so there are suddenly more ways to arrange molecules.
Why is there a gradual increase of entropy when heating occurs?
There is a larger distribution of the molecular energies.
Total entropy change = ?
Entropy change of the system + entropy change of the surroundings.
Entropy change of the system= ?
Entropy of products- entropy of reactants
∆G= ?
∆H - T∆S
What are the units of Gibbs energy?
kJ/mol
What is Gibbs energy dependent on?
Temperature
Is the reaction feasible if ∆H is positive, ∆Ssys is negative, and ∆G is positive?
Never
Is the reaction feasible if ∆H is positive, ∆Ssys is positive, and ∆G is negative?
Only if the temperature is high.
Is the reaction feasible if ∆H is negative, ∆Ssys is positive, and ∆G is negative?
Always
Is the reaction feasible if ∆H is negative, ∆Ssys is negative, and ∆G is negative?
Yes, depending on the temperature being low.
What is free energy?
A suitable measure of a reaction’s feasibility as it indicates how far a reaction will proceed towards completion.
What is free energy proportional to for gas phase systems?
The equilibrium constant.
∆G= ?
-RTlnK
What does kinetic inertness refer to?
A compound or mixture that does not undergo a reaction, despite the reaction appearing to be feasible.
What is the principal reason for feasible reactions being inert?
High activation energy
What do ∆G and the activation energy have to be for a reaction to tale place?
Negative ∆G and low activation energy
