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Why heat of combustion is always negative?

2 Answer(s) Available
Answer # 1 #

The enthalpy change of combustion will always have a negative value, of course, because burning always releases heat.

Fatima lagmp Arxal
Answer # 2 #

Lavoisier’s theory that combustion was caused by a reaction between the burning substance and the gas oxygen, which is present only to a limited extent in the atmosphere, was based on scientific principles. The most important of these was the law of the conservation of matter, which says that the total amount of matter in the universe is always the same. Even ancient philosophers had guessed about this law, and it was proven in the 1800s. “Element” was also defined by Lavoisier as something that couldn’t be broken down. This helped support his theory as well. The English chemist John Dalton, who made the first table of atomic weights, began studying gases soon after. Dalton’s work, as well as many new gases discovered by other scientists, helped support not only Lavoisier’s theory of combustion but his whole new system of chemistry that was based on accurate measurement. Lavoisier’s theory was backed up by the discovery of nitrogen and hydrogen in the latter half of the 18th century, as well as the discovery that the composition of air is very stable even though it is a mixture. The right way to explain combustion, which is thought to be one of the oldest chemical reactions known, is said to have been important in the development of modern science.

In combustion reactions, some things will give off more energy than others.

Enthalpies of combustion can be used to figure out which fuels or substances release the most energy when they are burned. This can help people figure out which fuels or substances are the best to use. They can be found out with the help of a bomb calorimeter.

In this picture, you can see what a bomb calorimeter looks like.

There is a simple way to make this happen in the lab with a spirit burner and a metal can. A simple bomb calorimeter is made up of a copper can filled with water and a thermometer. Over a flame, the can is held. A number of measurements must be made before the job can be done:

Fires are started, and the temperature rises as the fuel is used up. Enthalpy change can be calculated by figuring out the mass of fuel that changed with temperature. This can then be used to figure out the enthalpy of that fuel when it is burned.

It’s called the “enthalpy of combustion” when one mole of a substance completely burns in oxygen. This is the amount of heat energy that comes out.

A negative value for the enthalpy change (H) always means that a combustion reaction is exothermic, which means that the value is always negative.

It’s called the Molar Heat of Combustion, and it’s the heat that comes out when one mole of a certain compound completely burns with oxygen at 100 kPa pressure and 25°C. The final products are carbon dioxide gas and liquid water.

They say that the heat of combustion is a positive number, but the changes in enthalpy of combustion reactions (H) are a negative number because combustion reactions are always exothermic.

It is common to say how many kilojoules of heat are in each mole of something that is burned (kJ/mol. or kJ mol-1).

ethanol has a heat of combustion of 1360 kJ mol-1, which is what most people use to figure this out.

To figure out how much heat ethanol releases when it burns, follow these steps:

It is important to measure and record the weight of a burner that is filled with ethanol.

A beaker with 100 mL of water in it is the best way to measure how much water there is and how hot the water is.

Place the beaker of water on top of the burner and light it up, then stir the water.

Allow the burner to heat the water for a minute, then put it out and turn it off.

Measure and write down the weight of the burner and the temperature of the water right away, then.

Make sure you figure out how much mass has changed and how much temperature has changed!

To figure out how much enthalpy changed, put in the experiment’s results into the formula below.

ΔH = -mCT


ΔH= It means the amount of heat that changes in joules.

m= the mass of water

C= is the thermal capacity (4.2 for water)

T = the change in temperature in Celsius degrees.

To account for heat loss to the outside world, the molar heat of combustion for ethanol will be significantly lower than the value that people usually use.

If you burn one mole of a compound in oxygen with all the reactants and products in their standard state, the standard enthalpy of combustion is defined as the enthalpy change (298K and 1 bar pressure).

Abigail Ravichandra