Avogadro's Hypothesis

Avogadro's hypothesis Why is the ratio of the volumes of gases in a reaction a simple number? Many experts, including Dalton and Gay Lussac, failed to explain the law of volume ratios discovered by Gay Lussac. Dalton's inability because he assumed that elementary particles were always single atoms (monatomic). In 1811, Amedeo Avogadro described Gay Lussac's experiments. According to Avogadro, elementary particles are not always single atoms (monatomic), but in the form of 2 atoms (diatomic) or more (polyatomic). Avogadro called these particles molecules. Gay Lussac: 2 volumes of hydrogen gas + 1 volume of oxygen gas Avogadro: 2 molecules of hydrogen gas + 1 molecule of oxygen gas 2 volumes of water vapor 2 molecules of water vapor From here Avogadro put forward his hypothesis known as the Avogadro hypothesis which reads: "At the same temperature and pressure, all gases with the same volume will contain the same number of molecules." Thus, the ratio of the volumes of the gases is also the ratio of the number of molecules involved in the reaction. In other words, the volume ratio of the reacting gases is the same as the reaction coefficient. Let's see how Avogadro's hypothesis can explain the law of ratios of volumes and at the same time determine the molecular formulas of various elements and compounds. Example 1 1. The reaction between hydrogen gas and chlorine gas forms hydrogen chloride gas. According to the experiment, the volume ratio of hydrogen gas : chlorine : hydrogen chloride is 1 : 1 : 2. This means that the ratio of the number of molecules of hydrogen : chlorine : hydrogen chloride involved in the reaction is 1 : 1 : 2. For example, the molecular formula of hydrogen gas is Hx , chlorine Cly , and hydrogen chloride HaClb ( x, y, a, b must be integers), then the reaction equation can be written: 1 Hx (g) + 1 Cly (g) 2 Ha Clb (g) The simplest values for x and y that make the equations equivalent are x = 2 and y = 2 (it is impossible for the values x = 1 and y = 1 because if x = 1 and y = 1, then the values for a and b are fractions, i.e. 0.5). For x = 2 then the value of a = 1 and for y = 2 then the value of b = 1. So, the molecular formula of hydrogen is H2 , chlorine is Cl2 , and hydrogen chloride is HCl. The reaction equation above becomes: H2 (g) + Cl2 (g) 2 HCl(g) 2. The reaction between hydrogen gas and oxygen gas forms water vapor. According to the experiment, the volume ratio of hydrogen gas : oxygen : water vapor is 2 : 1 : 2. This means that the ratio of the number of molecules of hydrogen gas : oxygen : water vapor involved in the reaction is 2 : 1 : 2. Suppose the formula for hydrogen gas is Hx , oxygen is Oy , and water Ha Ob , then the reaction equation can be written as follows. 2 Hx (g) + 1 Oy (g) 2 Ha Ob (g) With the molecular formula for hydrogen H2 (x = 2) then the value of a = 2. The simplest value for y is 2, thus b = 1. So the molecular formula for hydrogen is H2 and oxygen is O2 , so the molecular formula for water is H2 O. Example 2 1. Determining the Molecular Formulas of Gas Compounds 1. Two liters of nitrogen gas (N2 ) reacts with 3 liters of oxygen gas (O2 ) to form 2 liters of NaOb gas , all measured at the same temperature ( T) and pressure (P) . Determine the molecular formula of the gas! Answer: Because the ratio of gas volumes is a reaction coefficient, the reaction equation can be written as follows. 2 N2 (g) + 3 O2 (g) 2 NaOb (g) The number of atoms on the left = the number of atoms on the right So, the molecular formula for gas NaOb = N2O3 .