Reacting volumes of gases

Let us begin by looking again at the kinds of evidence we already have for the existence of atoms—the evidence from chemistry. We shall consider, in turn, the definite composition of compounds, the simple weight relations among compounds, and the reacting volumes of gases. Each behavior provides experimental support for the atomic theory. 

In order to work out reacting volumes of gases, we can use Gay-Lussac s law of combining volumes When gases react they do so in volumes which bear a simple ratio to each other and to the volumes of the products if they are gases, all measurements of volume being at the same temperature and pressure . 

Since one moie of gas occupies 22.4 dm at s.t.p., rlicgus moUt volume is 22.4 at s.t p. This makes calculations on reacting volumes of gases very simple. An equation which shows how many moles of different gases react together also shows the ratio of the volumes of the different gases that react together. For example, the equation... 

Avogadro s Hypothesis provides a method for identifying the molecules present in a gas. Also, it explains why the volumes of gases that react with each other are in the same simple ratio as are the moles in the balanced equation. The importance of these results makes the explana-... 

There may be more than a thousandfold increase in volume when liquids or solids react to form a gas. The molar volumes of gases are close to 25 L-mol 1 under normal conditions (room temperature and pressure), whereas liquids and solids occupy only about a few tens of milliliters per mole. The molar volume of liquid water, for instance, is only 18 mL-mol 1. In other words, 1 mol of gas molecules at 25°C and 1 atm occupies as much as 1000 times the volume of 1 mol of molecules in a typical liquid or solid. 

Boyle (1662) observed that at constant temperature the volume of a sample of gas varies inversely with pressme, but Boyle did not explain why this was so. Somewhat later. Charles (1787) refined the observation to the effect that the volume of any sample of a gas vanes directly with the absolute temperature provided thal the pressure is held constant. A few years later, Gay-Lussac (1808), in reporting the results of his experiments with reacting gases, observed that volumes of gases that are used or produced in a chemical reaction can be expressed in ratios of small whole numbers—a concept to become known as Gay-Lussac s law of combining volumes. It should be noted that tlie foregoing concepts proposed by Boyle, Charles, and Gay-Lussac were based upon experimental observations, not on theory. 

In 1808 Gay-Lussac published his Law of Combining Volumes of Gases. He determined that when different gases reacted, they would always do so in small whole number ratios (e.g., two volumes of hydrogen would react with one volume of oxygen in forming H20). This was one of the greatest advancements of its time and helped form the basis for later atomic theory and how chemical reactions occur. 

Gay-Lussac s Law of Combining Volumes. The measurement of the volumes of gases which enter into chemical reaction led Gay-Lussac to perceive the existence of an extremely simple relationship which is known by the above title and which may be stated as follows The volumes of gases which react chemically are in the ratio of small whole numbers. Furthermore, if the products of the reaction are also gaseous their volumes are also in the relation of small whole numbers to each other and to the volumes of the original gases. 

From the balanced chemical equation, you know that 2 mol of butane reacts with 13 mol of oxygen, producing 8 mol of carbon dioxide and 10 mol of water vapor. By examining this balanced equation, you are able to find mole ratios of substances in this reaction. Avogadro s principle states that equal volumes of gases at the same temperature and pressure contain equal numbers of particles. Thus, when gases are involved, the coefficients in a balanced chemical equation represent not only molar amounts but also relative volumes. For example, if 2 L of butane reacts, the reaction involves 13 L of oxygen and produces 8 L of carbon dioxide and 10 L of water vapor. 

At this time, French chemist Joseph Gay-Lussac conducted some important experiments on the volumes of gases that react with one another to form new gases. He discovered the law of combining volumes ... 

Model Boyle s law and Charles s law using kinetic theory. Predict the effect of changes In pressure and temperature on the volume of a gas. Relate how volumes of gases react in terms of the kinetic theory of gases. 

Experiments with many other gas reactions show that volumes of gases always react in ratios of small whole numbers. Figure 11.15 presents the combining volumes for a synthesis reaction and a decomposition reaction. The observation that at the same temperature and pressure, volumes of gases combine or decompose in ratios of small whole numbers is called the law of combining gas volumes. 

Avogadro s law states that equal volumes of gases (at the same pressure and temperature) contain the same number of molecules. Therefore, If (within experimental error) equal volumes of H2 and CI2 react together, producing twice that volume of HOI, the ratio In which the molecules react must be 1 1 and the ratio of HOI product moleculesto H2 orCl2 molecules must be 2 1. This confirms the ratio contained In the equation. 

At constant temperamre and pressure, the volumes of gases that react or are formed can be expressed as a ratio of simple whole numbers. 

The heat of reaction at 25°C and 1.00 atm is —890.2 kJ. What is the change in volume when 1.00 mol CH4 reacts with 2.00 mol O2 (You can ignore the volume of liquid water, which is insignificant compared with volumes of gases.) What is w for this change Calculate AU for the change indicated by the chemical equation. 

In this section, you will study the relationships between the volumes of gases that react with each other. You will also learn about the relationship between molar amount of gas and volume, and a single gas law that unifies all the basic gas laws into a single equation. 

As a result of his experiments on the volumes of gases involved in chemical changes, Joseph Louis Gay-Lussac concluded (1808) that when gases react, the volumes consumed and produced, measured at the same temperature and pressure, are in ratios of small whole numbers. For example, at the same temperature and pressure,... 

Atoms combine in ratios of small whole numbers. Volumes of gases react and are produced in ratios of small whole numbers. These results suggest strongly that gas volumes have something to do with whole numbers of atoms or molecules. 

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