enthalpy

what is enthalpy

a measure of heat energy in a chemical system

what is enthalpy change

the heat energy exchange with its surroundings at constant pressure

standard conditions

100kPa

'the system' definition

reactants and products

'the surroundings' definition

containing equipment and immediate surroundings

'the universe' definition

both the surroundings and the system

exothermic

from system to surroundings (heat released)

endothermic

from surroundings to system (heat absorbed)

activation energy definition

minimum amount of energy for a reaction to occur

enthalpy change of formation (ΔfHo)

takes place when one mole of a compound is formed from its constituent elements

enthalpy change of combustion (ΔcHo)

takes place when one mole of substance reacts completely with oxygen

enthalpy change of neutralisation (ΔneutHo)

an energy change that accompanies the reaction of an acid with a base to form one mole of water

enthalpy change of reaction (ΔrHo)

the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation

bond enthalpy definition

the amount of energy required to break 1 mole of bonds within a gaseous molecule under standard conditions (always endothermic/positive enthalpy value)

bond dissociation enthalpy definition

the enthalpy change that takes place when breaking by homolytic fission

energy change equation

q = mcΔT

spirit burner method

1. record temp of xcm^3 of water in beaker

2. add (methanol) to the spirit burner and weigh

3. place it under the beaker and light for three minutes

stirring the water

4. extinguish after three minutes and immediately record the temperature of the water

5. re-weigh spirit burner

determination of ΔcHo (from spirit burner)

1. use q=mcΔt to find energy change of water (m= mass of water

2. find moles of fuel burnt using moles=mass/mr (mass= mass of spirit burner after experiment- mass before)

3. convert energy found in first step from J to kJ (/1000) and divide by number of moles

4. combustion is exothermic so must be negative

accuracy of experimental ΔcH value

• heat loss to surroundings (beaker

• incomplete combustion of methanol (see black soot)

• evaporation of methanol from the wick

• non-standard conditions

determination of ΔrH for a solid

1. use q=mcΔt to find the energy change of the solution

2. use n=cxv to find moles of solid not in excess

3. ensure equation is balanced and find moles

4. convert energy to kJ (/1000) and divide by moles

cooling curve

• a diagram that illustrates temperature changes and changes of state for a substance as heat is removed

• extrapolate the downward curve back to the y-axis and the initial horizontal like

and add a vertical line from the time the solutions were mixed to the extrapolated curve. temperature value of the line is the Δt

determination of ΔneutH

1. use q=mcΔt to find the energy change of the solution

2. use n=cxv to find moles of solution

3. ensure equation is balanced and find moles

4. convert energy to kJ (/1000) and divide by moles

bond breaking

endothermic

bond making

exothermic

hess' law

if a reaction can take place by two routes