enthalpy Flashcards
what is enthalpy
enthalpy change is the amount of heat change in a reaction at constant pressure.
what are the standard conditions?
100kPa pressure and 298K temperature
endothermic reactions
energy is transferred from the surroundings to the system (chemicals). They require an input of
heat energy e.g. thermal decomposition of calcium carbonate. The products have more energy than the reactants. the ∆H is positive.
exothermic reactions
Energy is transferred from the system (chemicals) to the surroundings. The products have less energy than the reactants. ∆H is negative.
Standard enthalpy change of formation
the enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
standard enthalpy change of reaction
the enthalpy change of a reaction according to the molar quantities in the equation under standard conditions.
standard enthalpy change of neutralisation
the enthalpy change when an acid and alkali react to form one mole of water, under standard conditions.
standard enthalpy change of combustion
the enthalpy change when one mole of a substance is completely burned in oxygen to make CO2 and H2O, under standard conditions.
bond enthalpy
the amount of energy needed to break one mole of a bond type in a molecule in the gaseous state.
mean bond enthalpy
the enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules.
calorimetry
[used to work out the enthalpy change of combustion]. the fuel is burnt to raise the temperature of the water by a specific amount. generally we weigh the fuel before and after burning to work out the mass of fuel burned. energy from the fuel is is transferred into the water although some is lost to the surroundings too. a lid is placed on the top to prevent heat loss and wind shields placed by the flame to prevent a draught moving the flame.
energy transferred can be calculated using - q=mcΔT
q- heat energy lost or gained (J)
m- mass of water or solution (g)
c- specific heat capacity of water (4.18Jg-1k-1)
ΔT- temperature change (K)
substances are mixed in an insulated container and the temperature
change is measured PART 1
General method
washes the equipment (cup and pipettes etc) with the solutions to be used
dry the cup after washing
put polystyrene cup in a beaker for insulation and support Measure out desired volumes of solutions with volumetric pipettes and transfer to insulated cup
clamp thermometer into place making sure the thermometer bulb is immersed in solution
measure the initial temperatures of the solution or both solutions if 2 are used. Do this every minute for 2-3 minutes
At minute 3 transfer second reagent to cup. If a solid reagent is used then add the solution to the cup first and then add the solid weighed out on a balance.
If using a solid reagent then use ‘before and after’ weighing method
stirs mixture (ensures that all of the solution is at the same temperature) Record temperature every minute after addition for several minutes
substances are mixed in an insulated container and the temperature
change is measured PART 2
Errors in this method
* energy transfer from surroundings (usually loss)
* approximation in specific heat capacity of solution. The method assumes all solutions have the heat capacity of water.
* neglecting the specific heat capacity of the calorimeter- we ignore any
energy absorbed by the apparatus.
* reaction or dissolving may be incomplete or slow.
* density of solution is taken to be the same as water
enthalpy of combustion is measured, such as using a series of alcohols in a
spirit burner
ERRORS IN THIS METHOD
* Energy losses from calorimeter
* Incomplete combustion of fuel
* Incomplete transfer of energy
* Evaporation of fuel after weighing
* Heat capacity of calorimeter not included
* Measurements not carried out under standard conditions as H2O is gas, not liquid, in this experiment
HESS’S LAW
THE TOTAL ENTHALPY CHANGE OF A REACTION IS INDEPENDENT OF THE ROUTE TAKEN
