Law of Combining Volumes Section 4.2 pg. 163-168 Curriculum Objectives: Describe and compare the behaviour of real and ideal gases in terms of the kinetic molecular theory. Explain the law of combining volumes.

Background Information: The Kinetic Molecular Theory The central idea of the kinetic molecular theory is that the smallest entities of a substance are in continuous motion. These entities may be atoms, ions or molecules. As they move about, the entities collide with each other and with objects in their path. Observation of microscopic particles, such as a pollen grain, shows a continuous, random motion. This is known as Brownian motion, named for Scottish scientist, Robert Brown.

Kinetic Molecular Theory According to Kinetic molecular theory, the motion of molecules is different in solids, liquids and gases. Solids - primarily vibrational motion. Liquids - vibrational, rotational and some translational motion Gases – the most important form of motion is translational

Kinetic Molecular Theory Kinetic Molecular Theory explains: Gases are compressible (due to most of a sample of gas being unoccupied space, thus particles can be forced closer together) Gas pressure (due to pressure being the result of particle collisions distributed over walls of a container causing a force per unit area) Boyle’s Law (due to reduced volume, there is a shorter distance between walls thus more frequent collisions, causing increased pressure) Charles’ Law (due to increase in temperature, there is an increase in particle speed causing more collisions with the wall. The wall moves outward, thus volume increases)

Kinetic Molecular Theory - Practice Try pg. 164 #1 a), d), e) a) According to the k.m.t., as the temperature increases, the average speed of the gas particles increases. If the volume is kept constant, then faster-moving particles will collide more often with the sides of the container. More collisions mean a greater pressure. d) According to the k.m.t., gases such as air are very compressible because most of the volume is empty space. The fact that there is very little empty space between the molecules of a liquid, such as oil, makes liquids not compressible. In a hydraulic system, the pressure applied at one end (e.g., brake pedal) needs to be transmitted to the other end (e.g., brakes). This will work only if the medium inside the system is not compressible. e) A bullet moves in a straight line over a long distance before it hits its target. According to the k.m.t., a gas molecule moves only a very short distance before colliding with another gas molecule, and thus changing its direction.

The Law of Combining Volumes Section 4.2 Pg. 164 - 166 Joseph Gay-Lussac Amedeo Avogadro

Law of Combining Volumes The kinetic molecular theory explains many physical properties of gases. But what about their chemical properties? In 1809, Joseph Gay-Lussac, a colleague of Jacques Charles, measured the relative volumes of gases involved in chemical reactions. His observations led to the Law of Combining Volumes, which states that: “When measured at the same temperature and pressure, volumes of gaseous reactants and products of chemical reactions are always in simple ratios of whole numbers” This is also known as the Gay-Lussac’s Law

Gay-Lussac’s Law of Combining Volumes A simple example of this is the decomposition of liquid water, in which the volumes of hydrogen and oxygen gas are always produced in a 2:1 ratio Which side is Hydrogen? 2H2O(l)  2H2(g) + O2(g)

Avogadro’s Theory Two years after Gay-Lussac’s Law, Avogadro proposed an new explanation in terms of numbers of molecules Avogadro proposed: “equal volumes of gases at the same temperature and pressure contain equal numbers of molecules” This means the mole ratios provided by a balanced equation are also the volume ratios. This in now best called Avogadro’s Theory

Avogadro’s Theory When all gases are at the same temperature and pressure, the law of combining volumes provides an efficient way of predicting the volumes of gases in a chemical reaction. Coefficients: 1 3 2 Chemical Amounts: 1 mol 3 mol 2mol Volumes: 1 L 3 L 2 L Example: 2 mL 6 mL 4 mL VH2: 2 ml x ( 3 ) = 6 mL 1 VNH3: 2 ml x ( 2 ) = 4 mL

Example: Law of Combining Volumes Use the law of combining volumes to predict the volume of oxygen required for the complete combustion of 120 mL of butane gas from a lighter. 1) The first step is to write the balanced chemical equation, including what you are given and what you need to find: 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g) 120 mL V = ? 2) From this chemical equation you can see that 13 mol of oxygen is required for every 2 mol of butane. Therefore, the volume of oxygen has to be greater than 120mL by a factor of 13/2. VO2: 120 ml C4H10 x ( 13 mol O2) = 780 mL 2 mol C4H10 To make sure that the ratio is used in the correct order, you could include the chemical formula with each quantity as shown above. Note the cancellation of the units and chemical formulas

Example #2: Law of Combining Volumes A catalytic converter in the exhaust system of a car uses oxygen (from the air) to convert carbon monoxide to carbon dioxide, which is released through the tailpipe. If we assume the same temperature and pressure, what volume of oxygen is required to react with 125L of carbon monoxide during a 100 km trip? 1) The first step is to write the balanced chemical equation, including what you are given and what you need to find: 2CO(g) + O2(g) → 2CO2(g) 125 L V = ? 2) From this chemical equation you can see that 1 mol of oxygen is required for every 2 mol of carbon monoxide. Therefore, the volume of oxygen has to be less than 125L by a factor of 1/2. VO2: 125 L CO x ( 1 mol O2) = 62.5 L O2 2 mol CO According to the law of combining volumes, 62.5L of oxygen is required.

Learning Tip This equivalence between the chemical amounts (coefficients) and the volumes only works for gases, and only if they are at the same temperature and pressure.

Homework – due tomorrow Pg. 166 #5-6 Pg. 168 #1, 2, 4, 5 Law of combining volumes WS