A physical change is associated with the change in physical state of substance but the chemical composition remains same. For example melting of ice and crushing of paper represent the physical change.
A chemical change involves change in chemical composition of substances by breaking or making of chemical bonds. It results the formation of new substances which exhibit new chemical and physical properties different from starting substances.
A chemical change is also called as chemical reaction and can be easily represented with the help of chemical equation. A chemical equation is symbolic representation of a chemical reaction which involves all substances as their molecular formulae.
The starting substances are called as reactant and the new substances formed are called as products. Reactants and products are separated by an arrow where reactants are at left side and products are at right side of arrow.
The physical state of reactant and product molecules must be written as subscripts with their molecular formulae like solid as (s), liquid (l), gas as (g) and aqueous as (aq).
On the basis of products formed; chemical reactions can be classified in different types.
- Synthesis reactions – These reactions are also called as combination reaction as two or more reactant molecules combine to form one product. For example reaction of magnesium with oxygen forms magnesium oxide.
- Decomposition reactions- These are just opposite reactions of synthesis reactions as they involve the decomposition of one substance to two or more components. It usually occurs in the presence of heat or some catalyst.
- Displacement reaction- Such reactions usually occur with salts in which one component of salt is displaced by other. For example reaction of Fe with CuSO4 turns the blue color solution of CuSO4 to pale green due to formation of FeSO4 and Cu as solid. Displacement reactions can be two types; single displacement and double displacement reactions. Neutralization and precipitation reactions are good examples of double displacement reactions.
- Redox reactions- These reactions are combination of oxidation and reduction reactions.
Combustion reactions can be defined as the burning of fuels in the presence of oxygen to release energy in different forms such as light or flame. The product of combustion always depends on amount of oxygen present during combustion. Complete combustion occurs in a plentiful supply of air whereas incomplete combustion occurs in limited supply of air. In case of complete combustion, more energy is released compare to incomplete combustion. For example complete combustion of hydrocarbons give carbon dioxide and water as final product with ample amount of energy in the form of heat and light. On the contrary, incomplete combustion results the formation of carbon monoxide with sooty flame. The substance which reacts with oxygen during combustion is known as fuel. Most of the energy during combustion reactions is released in the form of heat and light energy. Since there is around 21% of oxygen in air therefore fuel can receive enough oxygen for complete combustion that results release of large amount of energy. For example; complete combustion of methane produces carbon dioxide and water as final product and release a good amount of energy.
Here methane acts as fuel and it is called as complete combustion of methane.
Other good examples of fuel are natural gas and petrol which mainly contains hydrocarbons. Hydrocarbons are organic compound composed of carbon and hydrogen mainly. Burning or combustion is an example of oxidation reaction in which carbon present in fuel oxidized to carbon dioxide and oxidation of hydrogen produces water. The general oxidation reaction of hydrocarbons can be written as;
Hydrocarbon + oxygen $\rightarrow$ carbon dioxide + water
The incomplete combustion occurs due to poor supply of oxygen or air.
This combustion also produces water as product with carbon monoxide instead of carbon dioxide with soot.
Hydrocarbon + oxygen $\rightarrow$ Carbon monoxide + Carbon + water
Here carbon monoxide is a poisonous gas and incomplete combustion releases lesser amount of energy therefore complete combustion is preferred to incomplete combustion. Some common combustion reactions are shown below.
We can plug different values of x and y to get the combustion reaction of desired chemical compound. For example the balanced combustion reaction equation for methane is;
Hence we can say that 1 mole of methane reacts with 2 moles of oxygen to form 1 mole of carbon dioxide with 2 moles of water. As the number of carbon atoms increases in hydrocarbons, the number of oxygen and both products also increases. To balance such reactions we have to follow certain steps of balancing the chemical equation. Always write the molecular equation for combustion of given compound with CO2 and H2O as final products. Start the balancing the equation with C and H atoms on both sides of the equation. After balancing C and H atoms, balance O atoms this is because O2 stands alone in equation so we have to balance it at the end. After balancing O atoms, recheck C and H atoms again. Letâs take an example of combustion of propane. The skeleton molecular equation for combustion of propane is;
C3H8 + O2 $\rightarrow$ CO2 + H2O
To balance H atoms at both sides, add 4 as coefficient with water molecule at right side.
C3H8 + O2 $\rightarrow$ CO2 + 4 H2O
Next step will be balancing the carbon atoms at both sides. There are 3 C atoms on the left and only 1 on the right side therefore we need to add a 3 coefficient with CO2 on the right.
C3H8 + O2 $\rightarrow$ 3CO2 + 4 H2O
Now recheck the number of oxygen atoms at both sides. There are 10 O atoms on the right side with 6 in the 3 CO2 and four in the 4 H2O. Therefore to balance it at both sides, we need to put 5 coefficients with O2 on the left.
C3H8 + 5 O2 $\rightarrow$ 3CO2 + 4 H2O
Complete combustion of hydrocarbons occurs in the presence of excess of oxygen to produce CO2(g) and water vapor (H2O(g)). The complete combustion of hydrocarbons produces a clean flame whereas an incomplete combustion of hydrocarbons occurs with insufficient O2(g) and excess of hydrocarbon. So we can say that oxygen is limiting reagent in such combustion reactions. It produces carbon monoxide with water vapor and carbon as soot with sooty flame. The complete combustion of methane can be written as;
CH4 + 2 O2 $\rightarrow$ CO2 + 2 H2O
On the contrary the incomplete combustion of ethane, C2H6 produces CO, water vapor with carbon (soot).
C2H6 + 2O2 $\rightarrow$ CO + C + 3H2O
Alkanes, alcohols and alkenes are most common hydrocarbons which can be used as fuel for different combustion reactions. The complete combustion of alkane produces carbon dioxide (CO2(g)) and water (H2O(g) in the presence of excess of oxygen gas. Later water vapor condenses to H2O(l) at room temperature and pressure.
Alkane + O2(g) $\rightarrow$ Carbon dioxide gas + Water vapor
The amount of heat energy released when one mole of the alkane combusts in excess oxygen gas is called as the molar heat of combustion of the alkane or molar enthalpy of combustion. Similarly alkanol also produces carbon dioxide (CO2(g)) and water (H2O(g)) in excess oxygen gas.
Alkanol + O2(g) $\rightarrow$ carbon dioxide gas + water vapor
For the determination of heat of combustion, we have to determine the number of moles of the substance consumed during the combustion reaction. The molar heat of combustion or molar enthalpy of combustion is expressed in units of kilojoules per mole (kJ mol-1). We always check the heat of combustion for the combustion of 1 mole of given organic compound.
Hence we can say that combustion of one mole of methane gas in excess oxygen gas releases 890 kJ of heat so the molar heat of combustion of methane gas is 890 kJ mol-1 with positive sign as heat releases during the process. The enthalpy change must be written as ΔH = -890 kJ mol-1 because the reaction produces energy that makes it an exothermic reaction.
The complete thermo-chemical combustion equation for methane can be written as;
CH4(g) + 2O2(g) $\rightarrow$ CO2(g) + 2H2O(g) ΔH = -890 kJ mol-1
The heat of combustion for 2 moles of methane can be calculated by multiply every term by two;
2 × CH4(g) + 2 × 2O2(g) $\rightarrow$ 2 × CO2(g) + 2 × 2H2O(g) ΔH = 2 × -890 kJ mol-1
2 CH4(g) + 4O2(g) $\rightarrow$ 2CO2(g) + 4H2O(g) ΔH = -1780 kJ mol-1
Hence combustion of two moles of methane releases 1780 kJ of heat whereas half mole of methane releases only 445 kJ as given below.
½ × CH4(g) + ½ × 2O2(g) $\rightarrow$ ½ × CO2(g) + ½ × 2H2O(g) ΔH = ½ × -890 kJmol-1
½CH4(g) + O2(g) $\rightarrow$ ½CO2(g) + H2O(g) ΔH = -445 kJ mol-1
In other words the heat of combustion of n moles of given hydrocarbon will be equal to the value of the molar heat of combustion of the fuel multiplied by n-factor.
Heat released (kJ) = n (mol) × Molar enthalpy of combustion (kJ mol-1)
Let’s summarize the combustion reactions. Combustion reactions are chemical reactions which releases a large amount of heat and overall its an oxidation reaction of fuel with oxygen.
Complete combustion is more efficient compare to incomplete combustion as incomplete leads with limited supply of oxygen. Combustion of hydrocarbons like methane, ethane produces carbon dioxide with water vapor. All combustion reactions are exothermic and releases energy in the form of heat and flame. The combustion reaction of methane is as given below.
CH4(g) + 2O2(g) $\rightarrow$ CO2(g) + 2H2O(g) ΔH = -890 kJ mol-1