Enthalpy Definitions
Define the standard enthalpy change of combustion.
The enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions.
State Hess’s Law.
The total enthalpy change for a reaction is the same, regardless of the path taken.
Define bond enthalpy
The energy required to break 1 mole of a bond in gaseous molecules
What is an exothermic reaction?
A chemical reaction that releases energy
What is the enthalpy change of formation?
The enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
How can Hess’s Law be used to determine an unknown enthalpy change?
By combining known enthalpy changes of different steps in a reaction pathway.
What is the difference between average and exact bond enthalpy?
Average bond enthalpy is a mean value across different molecules, while exact bond enthalpy is specific to a given molecule.
How does an endothermic reaction affect its surroundings?
It absorbs heat, causing the surroundings to cool.
What is the standard enthalpy change of neutralization?
The enthalpy change when an acid and a base react to form 1 mole of water under standard conditions.
Calculate the enthalpy change of reaction for 2Al(s) + Fe₂O₃(s) → 2Fe(s) + Al₂O₃(s) using the given enthalpy changes: ΔHf[Fe₂O₃] = -824.2 kJ/mol, ΔHf[Al₂O₃] = -1675.7 kJ/mol
ΔH = (-1675.7) - (-824.2) = -851.5 kJ/mol
Why is bond breaking always endothermic?
Because energy is absorbed by system to overcome the attractive forces holding the atoms together
Give one example of an exothermic reaction.
Combustion of CH₄
Combustion of H₂
Neutralisation of HCl with NaOH
Reaction of Na with water
Name two factors that affect enthalpy change values
Temperature, pressure, and physical states of reactants and products.
In a Hess’s Law cycle, what happens if you reverse a reaction?
The enthalpy change sign must be reversed.
What is the formula for calculating enthalpy change between reactants and products?
ΔHr = ΔH1-ΔH2
r = reaction
1 = products
2 = reactants
How does the enthalpy profile diagram of an endothermic reaction differ from an exothermic one?
Endothermic diagrams have reactants lower than products, while exothermic ones have reactants higher than products.
Why is the enthalpy change of combustion always negative?
Because combustion is an exothermic process, releasing heat energy.
Use Hess’s Law to calculate the enthalpy change for this reaction:
A → C, given A → B = -100 kJ/mol and B → C = +150 kJ/mol.
ΔH = +50 kJ/mol
Calculate ΔH for the following reaction, SHOW YOUR WORKINGS:
CH₄ + 2O₂ --> CO₂ + 2H₂O
Bond Energies:
C-H: 413 C-O: 358
C-C: 348 C=O: 799
O-H: 495 O₂: 495
[1652+990] - [1598+1852]
[2642]-[3450]
=-808 kJ/mol
Why do exothermic reactions generally occur more readily than endothermic reactions?
Exothermic reactions release energy, making them more likely to proceed without continuous energy input, whereas endothermic reactions require an external energy source to occur.