Standard Enthalpy Change of Combustion Flashcards
what is the standard enthalpy change of combustion
- the enthalpy change at 100kPa at a specified temperature
- when one mole of a substance is completely burned in oxygen
what is the symbol for the standard change of combustion
delta cH-
if you need to specify one mole of the substance that is being burned, what are the two ways you can write the combustion of hydrogen
- H2 + 1/2O2 = H20
- 2H2 + O2 = 2H2O
what does the first equation show and what symbol represents this
- it shows one mole of hydrogen undergoing combustion
- it is represented by delta cH-
what is the enthalpy change of the second equation and why
- 2 x delta cH-
- because you are burning two moles of hydrogen
what practical is done to find the enthalpy change of combustion of a liquid
- a known mass of the liquid is burned
- and the heat energy produced is used to heat a known volume of water
what is the procedure of the experiment
- a spirit burner containing the liquid under test is weighed
- a known volume of water is added to a copper can
- the temp of the water is measured
- the burner is then lit
- the mixture is stirred with the thermometer
what is done after the water reaches a temperature approximately 20C above its initial temperature
- the flame is extinguished
- and the burner is immediately weighed
what is the last step of the practical
- the final temperature of the water is measured
- the final temp is subtracted from the initial temp to calculate the temperature change
what apparatus are used in this experiment
- spirit burner to put liquid in
- draught shield
- copper can to put water in
- lid for copper can
- thermometer
the liquid that was burned was ethanol with a molar mass of 46mgol-1. the volume of water heated was 100cm^3, the mass of ethanol burned was 0.42g and the change in temperature was +24.5C or K. what is the first step in calculating the enthalpy change of combustion
calculate the heat energy transferred to the water
what equation would you use to calculate the heat energy transferred to the water
- Q=mc delta T
- or Q= m-cat (just to remember it dont write this)
what do all those variables in the equation stand for
- Q = heat energy transferred
- m = mass of water
- c = specific heat capacity (of water)
- delta T is the change in temp
if the mass of water is in grams, what would the value of c be for water
4.18Jg-1K-1
if the density of water is 1gcm-3, what is the mass of water in this practical if 100cm^3 was used
- density = mass / volume
- so mass = density x volume
- 1 x 100cm^3 = 100g
what is then the value of Q (no need to actually calculate it right now as long as you know how)
- if Q = mc delta T
- Q = 100 x 4.18 x 24.5
- Q = 10241J or 10.24kJ
what is the second step in this calculation
calculating the amount, n, of ethanol burned
how would you calculate the amount of ethanol burned
- you would need to calculate the moles of it burned
- moles = mass / Mr
- so 0.42g / 46 = 9.13x10^-3 mols
what is the final step to this calculation
calculating delta cH
what is the equation used to calculate delta cH
delta H = (- Q / n)
what would then be the value of delta cH
delta cH- = -(10.24kJ / 9.13x10^-3)
- delta cH- = -1120kJmol-1
what is important to keep in mind when measuring the temperature change before any calculation is even done
- if the reaction is exothermic, the change in T should be positive
- if it is endothermic the change in T should be negative
- as an increase in temp for exo reactions leads to an upwards change in temp
- whereas a decrease in temp for endo leads to a downwards change in temp
- this will change your sign of Q and therefore whether the calculated delta His positive or negative
what are some possible sources of error in this practical that may cause your calculated value to not be perfect
- some of the heat energy when burning is transferred to the air or copper can and not the water
- some of the ethanol might not burn completely to from carbon dioxide and water
- conditions may not be standards, such as water vapour, not liquid water, being produced
