Chapter 2 Homework Packet 1. The first people to attempt to explain why chemical changes occur were (see page 40 of text—early history) a) metallurgists b) physicians c) the Greeks d) physicists e) alchemists The Greeks were the first in recorded history to conceive of the idea of matter as being composed of elements. Around 450 B.C., Empedocles first suggested that various proportions of the elements, “earth, wind, fire and air,” constituted all matter. Aristotle subsequently expanded these ideas, suggesting that the four basic elements were characterized by varying degrees of the qualities, “hot, cold, moist and dry.” Changes (what we know as chemical changes) occurred by the addition of these qualities to one of the basic elements. Also in the 5th century B.C., Leucippus and his student Democritus proposed that matter was composed of tiny indivisible particles. They reasoned that if a bar of gold could be continuously divided, a particle would eventually be reached that, if divided further, would cease to exhibit the properties of gold. They called these theoretical particles atoms (Gr. atomos). Aristotle rejected this line of reasoning and the concept that matter was composed of these indivisible particles was abandoned until the scientists of the 1600-1800’s developed the atomic theory. Answer C 2. The Greeks proposed that matter consisted of four fundamental substances: (see page 40 of text—early history) a) atoms, fire, water, air b) atoms, metal, fire, air c) fire, earth, water, air d) fire, metal, water, air c) earth, metal, water, air See explanation to question 1. Answer C 3. The first chemist to perform truly quantitative experiments was a) Paracelsus b) Boyle c) Priestly d) Bauer e) Lavoisier Robert Boyle was one of the first truly quantitative scientists. He made observations of the relationship between pressure and volume of gases, leading to the understanding that pressure is inversely related volume, and providing one of the important clues to the development of atomic theory. He hypothesized that in order for the relationship he observed to exist, gases must be made of particles in motion. The motion of the particles crashing against the side of the container would be what caused the pressure of a volume of gas. If a volume of gas was compressed, the particles would be closer together, there would be more collisions of particles with the wall, and the pressure would increase. His ideas significantly influenced the idea Copyright © Cengage Learning. All rights reserved. Chapter Error! Unknown document property name.: Error! Unknown document property name. 22 that not only are gases made of particles, but all matter is made of particles. Answer B 4. The scientist who discovered the law of conservation of mass and is also called the father of modern chemistry is a) Priestly b) Bauer c) Lavoisier d) Proust e) Boyle In the late 1700’s, Antoine Lavoisier perfected techniques of precise measurements of mass and for the first time, demonstrated that the amount of mass in reactants of a reaction consistently equaled the amount of mass in the resulting products. Because of the consistency with which this observation is made (that is, we always observe this) the absence of a change of mass during a normal chemical equation became known as the law of conservation of mass. Answer C 5. Which of the following pairs of compounds can be used to illustrate the law of multiple proportions? a) NH4 and NH4Cl b) ZnO2 and ZnCl2 c) H2O and HCl d) NO and NO2 e) CH4 and CO2 According to the law of multiple proportions, if two elements (for example, elements A and B) can form two different compounds, , if you determine the ratio of mass fractions (mass percents) of A to B in each compound, and then determine the ratio of these ratios (“the ratio of the ratios”) this ratio will be a ratio of small whole numbers. Because of the conditions of this natural law, the compounds involved must both be composed of the same two elements. In this case, the only compounds that meet this requirement are NO and NO2. Answer D 6. Which of the following pairs can be used to illustrate the law of multiple proportions? a) H2O and C12H22O11 b) H2SO4 and H2S c) SO and SO2 d) CO and CaCO3 e) KCl and KClO2 See explanation for question 5. The only compounds that meet the requirements of this law are SO and SO2. Answer C 7. According to the law of multiple proportions: a) If the same two elements form two different compounds, they do so in the same ratio. b) It is not possible for the same two elements to form more than one compound. Copyright © Cengage Learning. All rights reserved. Chapter Error! Unknown document property name.: Error! Unknown document property name. 23 c) The ratio of the masses of the elements in a compound is always the same. d) The total mass after a chemical change is the same as before the change. e) None of these. The law of multiple proportions states that if elements A and B react to form two different compounds, the different masses of B that combine with a fixed mass of A can be expressed as a ratio of small whole numbers. (A) False—this law states that the elements combine not in the same ratio but in the ratio of small whole numbers. (B) False—we know through observations that two elements commonly form more than one compound. (C) False—While it is true that the ratio of masses of elements in a compound is always the same, this is a statement of the law of definite proportions, not the law of multiple proportions. (D) False—While the total mass after a chemical change is the same as before the change, this is a statement of the law of conservation of mass, not the law of multiple proportions. (E) True—noe of the above relates to the law of multiple proportions. Answer E 8. A sample of chemical X is found to contain 5.0 grams of oxygen, 10.0 grams of carbon, and 20.0 grams of nitrogen. The law of definite proportion would predict that a 67 gram sample of chemical X should contain how many grams of carbon? Remember the strategy/show the work: gC = (mass fraction of C)(sample mass) = (.2857)(67.0g) = 19 g C 10.0 g C mass fraction of C = = .2857 5.0 g O + 10.0 g C + 20.0 g N Think—I want the grams of C in a 67 gram sample of chemical X—do I know a formula that would provide me with this? Yes—I simply need to multiply sample mass by the mass fraction of C, and this will give me the number of grams of carbon in the sample. Write out this formula. Then, inspect the formula to realize that you already have the sample mass but you do not have the mass fraction. Then realize that you are given data for the masses of oxygen, carbon and nitrogen for a different sample of X, so you should be able to use this data to calculate the mass fraction of carbon. You quickly realize that a formula that would give you this would be to divide the mass of carbon, by the sum of the masses of all of the components. If you label the calculation, the calculation is simple enough that you would not need to write out the formula, just show the calculation. Then take this value and plug it back in to the original formula. There are 2 sig figs in the final result, limited by 5.0 g in the given values. Answer 19 g C 9. Consider the following two compounds: H2O and H2O2 . According to the law of multiple proportions, the ratio of hydrogen atoms per gram of oxygen in H2O to hydrogen atoms per gram of oxygen in H2O2 is a) 2:2 b) 4:1 c) 1:1 d) 2:1 e) 1:2 Copyright © Cengage Learning. All rights reserved. Chapter Error! Unknown document property name.: Error! Unknown document property name. 24 The fact that there is the same mass of H in each compound but twice the mass of O in H2O2, when the question is asking for the amount of H per gram of O, makes this question a little confusing. It would be much easier to find the ratio of O atoms per gram of H. But, think of it in the following way—how many hydrogen atoms are there in each compound for every one O atom? In H2O, for every 1 O atom, there are 2 H atoms. In H2O2, for every 1 O atom, there is only 1 H atom. So, for H2O compared to H2O2, for every gram of oxygen, the ratio of atoms of hydrogen is 2:1. Answer D 10. Which of the following statements from Dalton's atomic theory is no longer true, according to modern atomic theory? a) Atoms are not created or destroyed in chemical reactions. b) All atoms of a given element are identical. c) Atoms are indivisible in chemical reactions. d) Elements are made up of tiny particles called atoms. e) All of these statements are true according to modern atomic theory. At first glance it might appear that all of these statements are still true. However, at the time of the first statement of the atomic theory in the early 1800’s, investigators did not know about isotopes—they truly thought all atoms of a given element would be identical.