Energetics Flashcards
Define enthalpy change
Heat energy change at a constant pressure
Define enthalpy change of formation
Enthalpy change when 1 mole of compound is formed from its elements under standard conditions, all reactants and products being in their standard states
Define enthalpy of combustion
Enthalpy change when 1 mole of substance is combusted completely in oxygen under standard conditions, all products and reactants being in their standard states
What is an exothermic reaction?
Making bonds
Energy transferred from system to surroundings
Products have less energy than reactants
Rise in temperature
ΔH = negative value
What is an endothermic reaction?
Breaking bonds
Energy transferred from surroundings to system
Products have more energy than reactants
Fall in temperature
ΔH = positive value
What are the standard conditions for enthalpy changes?
100kPa
298K
Solution at 1mol dm-3
What is the equation for finding energy change?
Q = m x c x ΔT
Q = energy change (J)
m = mass of water (g)
c = specific heat capacity (always given)
ΔT = change in temperature (K)
What are some issues with using calorimetry?
1) Heat loss to surroundings
2) Method assumes all solutions have heat capacity of water
3) We ignore energy absorbed by apparatus
4) incomplete reactions
5) Density of solutions are assumed to be same as water
How do you calculate enthalpy change of reaction?
ΔH = q / moles
Issues with measuring enthalpies of combustion using calorimetry
1) Energy loss from calorimeter
2) Incomplete combustion of the fuel
3) Incomplete transfer of energy
4) Fuel evaporates after weighing
5) Heat capacity of calorimeter not included
6) Measurements not carried out under standard conditions
What does Hess’s law state?
Total enthalpy change for a reaction is independent of the route by which the chemical change takes place
What is the equation for enthalpy change of formation?
∑products - ∑reactants
What is the equation for enthalpy change of combustion?
∑Reactants - ∑Products
What is mean bond energy?
Enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
How do you calculate mean bond enthalpy?
∑Bond energies broken - ∑Bond energies made
(Endothermic - exothermic)
How do you calculate enthalpy change?
Reactants - products
