Chapter 9: Enthalpy (9.1-9.4) Flashcards
exothermic reactions
- products have less enthalpy than the reactants (negative ΔH)
- chemical energy changed to thermal energy
- chemicals lose energy, the surroundings gain this energy (hence increase in temp)
endothermic reactions
- products have more enthalpy than the reactants (positive ΔH)
- thermal energy changed to chemical energy
- the surroundings lose energy (hence decrease in temp) which goes to the chemicals
standard enthalpy changes are measured using the standard conditions.
what are those conditions?
1 atmosphere pressure / 100 kPa
room temperature of 25 degrees / 298K
solutions must have a concentration of 1mol/dm3
all substances in their standard states (physical state under standard conditions)
equation for the heat energy change
q = m X c X ΔT
q: heat energy change in joules
m: mass of surroundings (the thing that you measure the temp change of) in grams
ΔT: temperature change
common errors when determining enthalpy change of a reaction (experimentally) and how to minimise them
error: heat lost to surroundings
solution: adding a lid, insulating the system
until for ΔH
kJ/mol
make sure you work it out per mole!
common errors when determining enthalpy change of combustion (experimentally) and how to minimise them
errors: heat loss to surroundings, incomplete combustion of reactant in spirit burner, evaporation of reactant from wick, non-standard conditions being used (heated up during combustion)
solutions: adding a lid to water beaker, using draft shields around apparatus
breaking bonds
endothermic
positive ΔH
energy is needed from the surroundings to break the bonds
making bonds
exothermic
negative ΔH
how to work out the enthalpy change of a reaction from average bond enthalpies
ΔrH = ∑(bond enthalpies of reactants) - ∑(bond enthalpies of products)
how activation energy effects reaction rate
Reactions with small activation energies are normally rapid, since energy needed to break bonds is readily available from surroundings.
Higher activation energies may present such a large energy barrier that reactions happen very slowly or not at all.
what is average bond enthalpy
the mean amount of energy required to break 1 mole of a specified type of covalent bond in a gaseous molecule
limitations of average bond enthalpies
they are an average value obtained from many molecules containing the same type of bond, actual value for a bond in a certain molecule may be slightly higher/lower.
what is Hess’ law used for?
to calculate enthalpy changes that are not easy to measure directly in experiments
state Hess’ law
Hess’ law states that, if a reaction can take place by more than one route, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
