| what is enthalpy | a measure of the chemical energy in a substance (measured in kJ) |
| is bond breaking an exothermic or endothermic reaction | endothermic |
| is bond making an exothermic or endothermic reaction | exothermic |
| what is meant by enthalpy change | the difference in energy between the products and reactants in a reaction |
| do exothermic reactions have a positive or negative enthalpy | negative |
| do endothermic reactions have a positive or negative enthalpy | positive |
| what is meant by enthalpy of combustion | the energy change when one mole of the substance burns completely in oxygen |
| what must be done when writing chemical equations for enthalpy of combustion | they must be balanced for one mole of the substance that is burning and they must show the correct state symbols |
| what formula do you use to calculate enthalpy of combustion | Eh = cmΔT
where Eh is the heat energy gained by water in kJ
c is specific heat capacity of water (4.18 kJ kg-1˚C-1)
m is mass of water in kg
∆T is change in temperature of the water in degrees celcius |
| describe an experiment used to find the enthalpy of combustion of fuel, such as ethanol | set up the following apparatus
take the following measurements: mass of water, mass of ethanol + burner before burning, temp of water before burning, mass of ethanol + burner after burning, highest temperature of water after burning
use Eh=cm∆T to calculate enthalpy of combustion |
| is enthalpy of combustion exothermic or endothermic | exothermic
enthalpy of combustion always has a negative sign |
| name some sources of error in this experiment of calculating enthalpy of combustion | enthalpy of combustion by practical methods in the lab is often much lower than the databook value because of
heat loss to surroundings
loss of ethanol by evaporation
incomplete combustion |
| name improvements that could be made to this experiment | loose fitting lid could have been placed on the copper beaker to reduce heat loss to surroundings
insulation of the copper beaker to reduce heat loss to surroundings
use of a bomb calorimeter would give a more accurate value for enthalpy combustion |
| describe how a bomb calorimeter reduces heat loss to surroundings | insulation and lids are used
water surrounds the calorimeter
ignition of the fuel is done wihout removing the ish burner from the appartatus
the combustion occurs in a closed container reducing any evaporation of ethanol
the reaction is carried out in pure oxygen (under pressure) to reduce any incomplete combustion |
| what is hess' law | states that the enthalpy change for a chemical reaction is independent of route |
| how do you use hess's law to calculate enthalpy change of a reaction | write down the equation for which you want to find the enthalpy
write out the equations and values of the enthalpies of combustion of the other reactants in the equation
compare these with the target equation and rearrange when necessary (swap equation round and change sign of enthalpy of combustion value)
cancel out and calculate enthalpy |
| what is meant by molar bond enthalpy | the energy required to break one mole of a particular bond between a pair of atoms in the gaseous state
OR
the energy required to break one mole of bonds in a diatomic molecule |
| what is mean bond enthalpy | the energy required to break one mole of bonds that occur in compounds (not diatomic)
the mean is used for certain bonds as the energy required differs depending on where the bond is found (position in compound, other atoms around it, etc) |
| how do you use bond enthalpies to calculate enthalpy change of a reaction | find the different types of bonds broken
find the values for bond enthalpies in the data booklet
find the different types of bonds made
find the different values for bond enthalpies in the data booklet
(remember bond breaking requires energy and bond making releases energy)
total the enthalpies |
