While researching energy values of fuels, a chemist designed an experiment to find out the heat of combustion of pentane. Using the known hexane heat of combustion of hexane = 4650kJ/mol and the data below calculate the heat of combustion of pentane.
Set up= copper beaker 42.5g filled with 500ml water and spirit burner underneath
DATA:
Initial temp= 26, final temp = 28.75 mass hexane = 10.11g
Initial temp= 26, final temp = 27.33 mass pentane = 8.15g
Specific heat capacity water= 4.18 and copper=0.39
To calculate the heat of combustion of pentane, we can use the concept of calorimetry. Calorimetry is the process of measuring changes in heat and can be used to determine energy values such as heat of combustion.
In this experiment, the chemist used a calorimeter setup consisting of a copper beaker filled with 500 ml of water. The spirit burner, containing either hexane or pentane, is placed underneath the beaker. By measuring the temperature change of the water, we can calculate the amount of heat released during combustion.
First, let's determine the energy absorbed by the water when the hexane was burnt. The equation for heat transfer is:
q = m * c * ΔT
where q is the heat absorbed (or released), m is the mass of the water, c is the specific heat capacity of water, and ΔT is the change in temperature.
For hexane:
q_hexane = m_water * c_water * ΔT_water
Given:
m_water = 500 g
c_water = 4.18 J/g·°C
ΔT_water = 28.75 - 26 = 2.75°C
Plugging in the values:
q_hexane = 500 g * 4.18 J/g·°C * 2.75°C
Next, let's determine the energy released during the combustion of hexane. The mass of hexane used is 10.11 g. Since the heat of combustion is given to be 4650 kJ/mol, we need to calculate the number of moles of hexane burned.
To do this, we can use the molar mass of hexane (C6H14) to convert the given mass into moles.
Molar mass of hexane(C6H14) = 6*12.01 + 14*1.01 = 86.18 g/mol
Number of moles of hexane burned:
n_hexane = Mass_hexane / Molar mass_hexane
= 10.11 g / 86.18 g/mol
Finally, we can calculate the heat of combustion of hexane per mole:
ΔH_hexane = q_hexane / n_hexane
= (q_hexane / (10.11 g / 86.18 g/mol))
Now, we can use the same process to calculate the energy absorbed by water when pentane was burned and the number of moles of pentane burned.
For pentane:
q_pentane = m_water * c_water * ΔT_water
Given:
m_water = 500 g
c_water = 4.18 J/g·°C
ΔT_water = 27.33 - 26 = 1.33°C
Plugging in the values:
q_pentane = 500 g * 4.18 J/g·°C * 1.33°C
Next, let's determine the energy released during the combustion of pentane. The mass of pentane used is 8.15 g. Similarly, we need to calculate the number of moles of pentane burned using the molar mass of pentane (C5H12).
Molar mass of pentane(C5H12) = 5*12.01 + 12*1.01 = 72.15 g/mol
Number of moles of pentane burned:
n_pentane = Mass_pentane / Molar mass_pentane
= 8.15 g / 72.15 g/mol
Finally, we can calculate the heat of combustion of pentane per mole:
ΔH_pentane = q_pentane / n_pentane
= (q_pentane / (8.15 g / 72.15 g/mol))
By comparing the values of ΔH_hexane and ΔH_pentane, we can determine the heat of combustion of pentane relative to hexane.