Why is combustion of methane exothermic

We can show energy changes during a chemical reaction using energy profiles. The energy profile of the reaction between nitrogen dioxide and carbon monoxide is shown on the left while the equation is given below.

NO2(g) + CO(g) => CO2(g) + NO(g) ΔH= –226 kJ mol–1

The symbol ΔH is the difference between H(products) - H (reactants)The negative sign indicates that energy is given out.

As you can see from the energy profile, the chemical energy of the reactants is greater than the products. The difference in energy is given out, during the reaction, as heat. Initially, when mixed together, the reactants lack sufficient energy to react. Their collisions are not strong enough to initiate a reaction by breaking bonds.

After a small amount of energy is supplied, known as activation energy, the reaction proceeds. In this case the activation energy for this reaction is shown on the energy profile as 132 kJ mol–1. An activated complex forms which then splits to form the products.

At completion of the reaction 226 kJ of energy is given out for every mol of NO2 that reacts.

Click to see the Flash animation(source Youtube)

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Thermochemical equations

Such chemical equations show the mole ratio of reactants and products, their states and the enthalpy change(energy change)

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Calculate the energy released when 150g of octane burn in oxygen according to the equation above.

C = 12, H =1, O =16.

Step 1 Calculate the mole of octane.

150/114 = 1.316 mole

Step 2 Two mole of octane produce 10108kj of energy

so 1.316 mole of octane will produce (1.316 / 2) X 10108 = 6650kj

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C8H18(g) + 12.5O2(g) => 8CO2(g)+ 9H2O(g) ΔH = -5054kj/mol

1) Calculate the energy released when 34.5 litres of octane gas react with oxygen at STP according to the reaction above.
C = 12, H =1, O =16.

Solution

2) Calculate the amount of energy released when 40 grams of glucose burn in oxygen according to the equation below?

C6H12O6(aq) + 6O2(g) => 6CO2(g)+ 6H2O(l) ΔH = -2803kj/mol

Solution

3) Calculate the amount of energy absorbed when 40 grams of carbon dioxide are used in photosynthesis to produce glucose according to the equation below.

6CO2(g)+ 6H2O(l) => C6H12O6(aq) + 6O2(g) ΔH = +2803kj/mol

Solution

4) Calculate the mass of octane needed to produce 760kj of energy when it reacts with oxygen according to the equation below.

2C8H18(g) + 25O2(g) => 16CO2(g)+ 18H2O(g) ΔH = -10108kj/mol

Solution

5) Calculate the mass of glucose needed by a plant in order to produce 340kj of energy during respiration.

C6H12O6(aq) + 6O2(g) => 6CO2(g)+ 6H2O(l) ΔH = -2803kj/mol

Solution

Breaking bonds between atoms requires energy. Creating new bonds releases it. The enthalpy of a reaction is equal to the energy required to break the bonds between reactants minus the energy released by the formation of new bonds in the products. So, if a reaction releases more energy than it absorbs, the reaction is exothermic and enthalpy will be negative. Think of this as an amount of heat leaving (or being subtracted from) the reaction. If a reaction absorbs or uses more energy than it releases, the reaction is endothermic, and enthalpy will be positive. Let's look at the enthalpy changes in the combustion of methane. In this reaction, the bonds between the hydrogens and the carbon and the bonds between the oxygens are broken. Breaking these bonds requires energy to be absorbed by the reaction. But then, new bonds form between hydrogen and oxygen and between carbon and oxygen. For this reaction, the energy released is larger than the energy absorbed.

This means combustion has an overall negative enthalpy and is an exothermic reaction.

Updated March 28, 2018

By Bert Markgraf

Combustion is an oxidation reaction that produces heat, and it is therefore always exothermic. All chemical reactions first break bonds and then make new ones to form new materials. Breaking bonds takes energy while making new bonds releases energy. If the energy released by the new bonds is greater than the energy needed to break the original bonds, the reaction is exothermic.

Common combustion reactions break the bonds of hydrocarbon molecules, and the resulting water and carbon dioxide bonds always release more energy than was used to break the original hydrocarbon bonds. That's why burning materials mainly made up of hydrocarbons produces energy and is exothermic.

Combustion is an exothermic oxidation reaction, with materials such as hydrocarbons reacting with oxygen to form combustion products such as water and carbon dioxide. The chemical bonds of the hydrocarbons break and are replaced by the bonds of water and carbon dioxide. Creation of the latter releases more energy than is required to break the former, so energy is produced overall. In many cases a small amount of energy such as heat is required to break some of the hydrocarbon bonds, allowing some new bonds to form, energy to be released and the reaction to become self-sustaining.

In general terms, oxidation is the part of a chemical reaction in which the atoms or molecules of a substance lose electrons. It is normally accompanied by a process called reduction. Reduction is the second part of the chemical reaction in which a substance gains electrons. In an oxidation-reduction or redox reaction, electrons are exchanged between two substances.

Oxidation was originally used for chemical reactions in which oxygen combined with other materials and oxidized them. When iron is oxidized, it loses electrons to oxygen to form rust or iron oxide. Two iron atoms lose three electrons each and form ferric ions with a positive charge. Three oxygen atoms gain two electrons each and form oxygen ions with a negative charge. The positively and negatively charged ions are attracted to each other and form ionic bonds, creating iron oxide, Fe2O3.

Reactions not involving oxygen are also called oxidation or redox reactions as long as the mechanism of electron transfer is present. For example, when carbon and hydrogen combine to form methane, CH4, the hydrogen atoms each lose an electron to the carbon atom, which gains four electrons. Hydrogen is oxidized while carbon is reduced.

Combustion is a special case of an oxidation chemical reaction in which enough heat is produced to make the reaction self-sustaining, in other words, as a fire. Fires in general have to be started, but they burn by themselves until they run out of fuel.

In a fire, materials that contain hydrocarbons, such as wood, propane or gasoline, burn to produce carbon dioxide and water vapor. The hydrocarbon bonds first have to be broken for the hydrogen and carbon atoms to combine with oxygen. To start a fire means providing the initial energy, in the form of a flame or a spark, to break a few of the hydrocarbon bonds.

Once the initial starting energy results in broken bonds and free hydrogen and carbon, the atoms react with oxygen in the air to form carbon dioxide, CO2, and water vapor, H2O. The energy released by the formation of these new bonds heats the remaining hydrocarbons and breaks more bonds. At this point the fire will keep burning. The resulting combustion reaction is highly exothermic, with the exact amount of heat given off depending on the fuel and how much energy it takes to break its bonds.