Fundamental Concepts in Chemistry

1. Science • Chemistry: Matter, Properties, • Changes, Energy • Scientific Laws Fundamental Concepts in Chemistry Dr. Yau (loosely based on Chap. 1.1, 1.2, 1.3, 2.1 in Jespersen, Brady & Hyslop, 6th edition)

2. What is Science? “Science” comes from the Latin word “scientia” which means knowledge. Science can be defined as the study of a body of knowledge that is based on natural phenomenon, organized in a systematic way to offer logical, testable explanations and predictions.

3. What is Chemistry? Why are you required by your major to take chemistry courses? Enrollment in other college courses is expected to have dropped but CHEMISTRY courses are almost all full with students waiting for a spot to appear. WHY? Chemistry is often referred to as the “central science.” Few of you are actually chemistry majors.

4. What is Chemistry?

5. What is Chemistry? Chemistry is a branch of science that studies matter, its properties, changes it undergoes, and the energy released or absorbed during these changes. We will now examine each of these keyword. Matter is defined as anything that has mass and occupies space. Is air considered matter? Why or why not?

6. Mass vs. Weight Mass is a measure of the amount of matter that is in a given sample. Weight is the force with which something is attracted to the earth by gravity. An astronaut has the same mass whether on earth or on the moon. An astronaut has more weight on earth than on the moon because the earth has a larger gravitational pull.

7. Mass vs. Weight In this class, we are going to stay on earth and consider gravity to be essentially the same throughout the semester. “Mass” and “weight” will be used interchangeably even though their definitions are different. You will hear “atomic mass” and “atomic weight.” Know that they mean the same thing.

8. Physical vs. Chemical Properties Chemistry is a branch of science that studies matter, its properties, changes it undergoes, and the energy released or absorbed during these changes. In chemistry we often classify “properties” into two categories: • Physical properties • Chemical properties

9. Physical vs. Chemical Properties Physical properties: characteristics of a substance that can be observed without changing its chemical composition. e.g. physical state, boiling point (bp), melting point (mp), freezing point (fp) density, solubility, viscosity Chemical properties: characteristics of a substance that becomes apparent as it interacts with another substance and the substance undergoes a change in chemical composition. e.g. flammability, corrosive, reactivity with acids

10. Physical Properties: Note: mp and fp should be the same temperature. Why? Viscosity (How viscous is it? How easy is it to pour?) Honey has a high viscosity. It is viscous. Learn to use these terms correctly. Which is more viscous?.... Water or Elmer’s glue? 10

11. Learning Check: Chemical or Physical Property? Which of the following is a chemical property? A) water is colorless B) water reacts violently with solid Na metal C) water dissolves table salt D) all of these E) none of these Ans. Only (b)

12. Learning Check: Chemical or Physical Property?

13. Extensive vs. Intensive Properties Properties can also be categorized in other ways: Intensive properties are ones that are not affected by the size of the sample. e.g. melting point, density, color, texture, temp Extensive properties are ones that are affected by the size of the sample. e.g. mass, volume Note: Properties used to identify substances are always intensive. Density, color, and texture are often helpful in identification; however, temperature is not.

14. Example of entries on caffeine in the CRC Handbook of Chemistry & Physics: Synonyms & Formula: 1,3,7-trimethylxanthine Mol.wt. = 194.19 Crystalline form, color= wh nd (white needles) m.p. oC = 237 b.p. oC = sub 178, sub 8915 Density = 1.2319 (1.23 g/cm3 at 19 oC) Solubility: w =  , v (h) (slightly; very soluble if hot) al =  eth = i ace = … bz = … other solvents = chl s (h) (soluble in hot chloroform) Ref. = B26, 266 (Beilstein page reference)

15. Changes Chemistry is a branch of science that studies matter, its properties, changes it undergoes, and the energy released or absorbed during these changes. Changescan be classified as… • physical changes • chemical changes (chemical reactions)

16. Physical vs. Chemical Changes Physical change- a process that results in no new substance (chemical composition of the sample does not change) • Change in the size of the sample e.g. cutting a piece of paper in half • Change in the physical state of the sample e.g. evaporation, melting, freezing • Change in the concentration of the sample e.g. adding more solvent, removing all or some of the solvent. Note: If a change does not fit one of the above categories, it is a chemical change.

17. Changes in Physical States

18. Changes in Physical States

19. Physical vs. Chemical Changes Chemical change- a change that results in the formation of a new substance Examples: • Rusting of iron • Burning of paper (Burning is ALWAYS a chem rxn with O2.) • Plants growing taller • You growing fatter • Respiration: Inhale O2 and exhale CO2

20. Evidence of Chemical Change Chemical change involves… formation of a new solid, new liquid, new gas. Do not confuse this with JUST physical changes as in freezing, melting, boiling, etc. Just production of a gas does not always means there is a chemical change. It could be just a liquid boiling. Not always but often proof of new substance formed & therefore showing a chem change occurred: • temperature change • an unexpected color change • a new smell

21. Learning Check: Chemical Or Physical Change?

22. Learning Check: Chemical Or Physical Change? Which of the following is not a chemical change? • A) a match burns in air • B) ice melts in air • C) an aluminum door whitens in air • D) solid sugar + water makes a liquid • E) sweating helps cool us down • F) dry ice disappearing at room temperature. Answer: B, D, E and F are physical changes.

23. Energy Accompanying Changes During chemical & physical changes, energy (E) is usually either absorbed or evolved (released). Endothermic Reactions are ones where E is absorbed. Exothermic Reactions are ones where E is evolved (emitted, released).

24. Learning Check: When you light a gas stove, natural gas (mostly methane) reacts with oxygen in the air to produce carbon dioxide and water: CH4(g) + 2O2(g) CO2(g) + 2H2O (g) 1. This is an… A. endothermic reaction. B. exothermic reaction. 2. The process of water boiling is an… A. endothermic reaction. B. exothermic reaction. C. neither because it is not reaction. Ans. 1B, 2A

25. 3 Important Scientific Laws • Law of Conservation of Mass(or Law of Conservation of Matter) • Law of Definite Proportions • Law of Multiple Proportions

26. Law of Conservation of Mass • No detectable gain or loss of mass occurs in chemical reactions. • Mass is conserved. Implication: • Reactions involve reorganization of materials.

27. Law of Definite Proportions A given compound is made of elements present in a definite proportion (fixed ratio) by mass. e.g. Water always contain H and O in the ratio of 1 : 8 by mass. (regardless of where it came from, as long as it is pure water).

28. Law of Multiple Proportions When 2 elements combine to form a compound, they may combine in more than one ratio. e.g. H and O can combine in a ratio of 1:8 to form water. H and O can also combine in a ratio of 1:16 to form a different compound. Each compound has its own fixed ratio (still obeying the Law of Definite Proportions). (What is the compound containing H and O in1:16 ratio?) Hint: What compound other than water do you know of that contains only H and O?