Heat of Combustion Flashcards
What is a combustion reaction?
A reaction in which a substance is burned
Define heat of combustion.
The heat of combustion of a substance is the heat change when one mole of the substance is completely burned in excess oxygen
Why is the phrase ‘completely burned in excess oxygen’ important in the definition for the heat of combustion?
This is because some elements have more than one oxide
How are heats of combustion accurately measured?
They are accurately measured using an instrument called a bomb calorimeter
What is a calorimeter?
A calorimeter is any container used for the determination of heat changes
Describe a bomb calorimeter.
A bomb calorimeter consists of a small steel container (the bomb) with a screw-on cap and various other items
What are the steps involved in using a bomb calorimeter to measure the heat of combustion of a compound?
(6)
Known mass of substance placed in the crucible inside the bomb
Bomb is filled with oxygen under pressure (O in excess = substance completely burned)
The bomb is placed in a known quantity of water contained in the calorimeter
Substance is ignited electrically using an ignition coil of wire
Rise in temp of water is measured - heat produced measured using formula…
The heat produced when one Mole of the substance is completely burned in excess oxygen is then calculated
What is placed in the crucible inside the bomb?
A known mass of the substance whose heat of combustion is being measured
Where is the known mass of the substance whose heat of combustion is being measured?
In the crucible inside the bomb
What is the bomb filled with?
Oxygen under pressure
Why is the bomb filled with oxygen under pressure?
To ensure the oxygen is in excess to make sure that the substance will be completely burned in oxygen
What is filled with oxygen under pressure?
The bomb is filled with oxygen under pressure
What is the bomb placed in?
The bomb is placed in a known quantity of water contained in the calorimeter
What is placed in a known quantity of water contained in the calorimeter?
The bomb
How is the substance ignited?
Using an ignition coil of wire
What is the ignition coil of wire used for?
To ignite the substance
How is the heat produced measured?
Using the formula:
Heat produced = mass x specific heat capacity x rise in temperature
What is measured using the following temperature?
Heat produced = mass x specific heat capacity x rise in temperature
The heat produced is measured using the formula
What is finally calculated after using the bomb calorimeter?
The heat produced when one mole of the substance is completely burned in excess oxygen
What is the secondary function of the bomb calorimeter?
To compare the efficiencies of various fuels
Other than in the number of moles, how may chemists wish to measure the heat given out by a fuel?
In terms of the mass of a fuel rather than the number of moles
Define kilogram calorific value.
Kilogram calorific value of a fuel is the heat energy produced when 1 kg of the fuel is completely burned in oxygen
What can kilogram calorific values be calculated from?
Heats of combustion for fuels that have a definite chemical formula such as methane and butane
How is the calculation for kilogram calorific values carried out?
(2)
The heat of combustion is divided by the relative molecular mass of the fuel (which has a definite chemical formula)
Multiply by 1000 to get the kilogram calorific value
When calculating the kilogram calorific value, why do you need to divide your answer (heat of combustion divided by RMM) by 1000?
As the heat of combustion divided by the RMM is only the heat given out when one gram of the compound is burned (you want Kg not g)
What are kilogram calorific values used for?
To compare the efficiencies of various fuels
According to kilogram calorific values, methane is a good fuel explain this.
Methane has a very high kilogram calorific value
Other than its high kilogram calorific value, explain why methane is a good fuel.
It burns with a clean flame and can be easily distributed by being piped to homes
How are the kilogram calorific values of variable compositions measured?
They have to be measured using a bomb calorimeter
