Standard Enthalpy Change of Combustion

Question 1

what is the standard enthalpy change of combustion

Answer 1

• the enthalpy change at 100kPa at a specified temperature

- when one mole of a substance is completely burned in oxygen

Question 2

what is the symbol for the standard change of combustion

Answer 2

delta cH-

Question 3

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

Answer 3

• H2 + 1/2O2 = H20

- 2H2 + O2 = 2H2O

Question 4

what does the first equation show and what symbol represents this

Answer 4

• it shows one mole of hydrogen undergoing combustion

- it is represented by delta cH-

Question 5

what is the enthalpy change of the second equation and why

Answer 5

• 2 x delta cH-

- because you are burning two moles of hydrogen

Question 6

what practical is done to find the enthalpy change of combustion of a liquid

Answer 6

• a known mass of the liquid is burned

- and the heat energy produced is used to heat a known volume of water

Question 7

what is the procedure of the experiment

Answer 7

• 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

Question 8

what is done after the water reaches a temperature approximately 20C above its initial temperature

Answer 8

• the flame is extinguished

- and the burner is immediately weighed

Question 9

what is the last step of the practical

Answer 9

• the final temperature of the water is measured

- the final temp is subtracted from the initial temp to calculate the temperature change

Question 10

what apparatus are used in this experiment

Answer 10

• spirit burner to put liquid in
• draught shield
• copper can to put water in
• lid for copper can
• thermometer

Question 11

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

Answer 11

calculate the heat energy transferred to the water

Question 12

what equation would you use to calculate the heat energy transferred to the water

Answer 12

• Q=mc delta T

- or Q= m-cat (just to remember it dont write this)

Question 13

what do all those variables in the equation stand for

Answer 13

• Q = heat energy transferred
• m = mass of water
• c = specific heat capacity (of water)
• delta T is the change in temp

Question 14

if the mass of water is in grams, what would the value of c be for water

Answer 14

4.18Jg-1K-1

Question 15

if the density of water is 1gcm-3, what is the mass of water in this practical if 100cm^3 was used

Answer 15

• density = mass / volume
• so mass = density x volume
• 1 x 100cm^3 = 100g

Question 16

what is then the value of Q (no need to actually calculate it right now as long as you know how)

Answer 16

• if Q = mc delta T
• Q = 100 x 4.18 x 24.5
• Q = 10241J or 10.24kJ

Question 17

what is the second step in this calculation

Answer 17

calculating the amount, n, of ethanol burned

Question 18

how would you calculate the amount of ethanol burned

Answer 18

• you would need to calculate the moles of it burned
• moles = mass / Mr
• so 0.42g / 46 = 9.13x10^-3 mols

Question 19

what is the final step to this calculation

Answer 19

calculating delta cH

Question 20

what is the equation used to calculate delta cH

Answer 20

delta H = (- Q / n)

Question 21

what would then be the value of delta cH

Answer 21

delta cH- = -(10.24kJ / 9.13x10^-3)

- delta cH- = -1120kJmol-1

Question 22

what is important to keep in mind when measuring the temperature change before any calculation is even done

Answer 22

• 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

Question 23

what are some possible sources of error in this practical that may cause your calculated value to not be perfect

Answer 23

• 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