What does “enthalpy change (ΔH)” mean?
The heat energy change in a reaction measured under conditions of constant pressure.
What are standard conditions in enthalpy measurements?
pressure: 100 kPa,
temp: 98K
substances: in their standard states.
Define the standard enthalpy of combustion (ΔcH°).
enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions, with all reactants and products in their standard states
Define the standard enthalpy of formation (ΔfH°).
The enthalpy change when 1 mol of a compound is formed from its elements in their standard states under standard conditions.
What equation is used to calculate heat energy change in a reaction?
𝑞=𝑚𝑐Δ𝑇
What is the standard unit for energy in calorimetry calculations?
joules
How is molar enthalpy change (ΔH) calculated from calorimetry data?
calculate Q
covert KJ (DIVIDE BY 1000)
divde by no. of moles
State Hess’s Law.
The total enthalpy change of a reaction is the same regardless of the route taken, provided the initial and final conditions are the same.
What does Hess’s Law show about energy conservation?
the overall energy change depends on the intial and final states
How can Hess’s Law be used to find an unknown enthalpy change?
ΔH1=ΔH2+ΔH3
How can enthalpy changes of combustion be used with Hess’s Law?
ΔHr=∑ΔHc(reactants)−∑ΔHc(products)
How can enthalpy changes of formation be used with Hess’s Law?
ΔHr=∑ΔHf(products)−∑ΔHf(reactants)
Why is Hess’s Law useful in energetics experiments?
allows calculations of enthalpy changes that cannot be measured directly.
Define mean bond enthalpy.
The average enthalpy change when one mole of a specific type of bond is broken by homolytic fission in the gaseous state, averaged over many compounds.
What is meant by homolytic fission?
The breaking of a covalent bond where each atom receives one electron, forming two radicals.
What are the units of bond enthalpy?
Kjmol-1
How can mean bond enthalpies be used to estimate the enthalpy change (ΔH) of a reaction?
ΔH=∑(bondenthalpiesofbondsbroken)−∑(bondenthalpiesofbondsformed)
Why must all substances be in the gaseous state when using mean bond enthalpies?
Because bond enthalpies are defined for gaseous atoms and molecules, ensuring consistency of comparison.
Why can enthalpy changes calculated using mean bond enthalpies differ from experimental or Hess’s Law values?
mean bond enthalpies are averages from many compounds
2. real bonds have slightly diff strengths in diff chemical envirnoments
3. some reactions may involve non gaseous substances
What type of value is a mean bond enthalpy?
It is a mean value for a bond across a range of compounds.
whats the reaction profile for exothermic
reactants line is above products line
what is the reaction profile for endothermic
products line is above reactants line
what is the transition stage
the peak, when chemical bonds are partially broken and formed. its very unstable.
what are the factors that bond energy depends on
size of atom
electronegativity
bond length
