Chapter 1: Introduction to Chemistry

Chemistry: the study of matter, its composition, and its changes (Hint: Since chemistry is about matter and its changes, think about atoms and molecules)

Substance: (element or compound) has a constant composition and constant properties throughout a given sample, and from sample to sample

A Tale of Two Substances

Ozone Earth’s atmosphere: troposphere, stratosphere, mesosphere, thermosphere, exosphere Ozone occurs in the stratosphere Ozone protects life on Earth by blocking UVB

1920s G.M.B. Dobson began monitoring ozone in the stratosphere 1981 and 1983 British Antarctic Survey noticed thinning of ozone, especially in October

Chlorofluorocarbons (CFCs) 1928 Thomas Migdley synthesized the first CFC CFCs were used as refrigerants (refrigerators and air conditioners) and propellants CFCs were thought to be safe and inert 1970s to 1990 increasing amounts of CFCs were found in the stratosphere

1970s scientists (Molina and Rowland) wondered if there was a connection between CFC increases and ozone depletion

Chemistry and Matter

Matter: anything that has mass and takes up space

Mass: a measurement that reflects the amount of matter

Weight: the effect of Earth’s gravitational pull on a mass

As chemists consider the composition of matter and study the changes matter undergoes, remember that it pays to consider the submicroscopic events that affect our macroscopic environment

Models: visual, verbal, or mathematical explanations of experimental data

Scientific methods (remember Robert Boyle and his systematic approach)

Purpose: a statement of the problem or area of interest Hypothesis: tentative explanation (Molina and Rowland) Experimentation: controlled observations that test a hypothesis

Independent variable: chosen to be changed systematically in the experiment Dependent variable: measured response to changes in the independent variable Control: baseline or standard for comparison of changes in the dependent variable Observation

Qualitative: sensory or descriptive data or information Quantitative: numerical data or information (Antoine Lavoisier)

Analysis: determination of the results of an experiment (including weaknesses in data collection or instrumentation) and any decisions regarding the next actions to take

Results: the product of the analysis including any mathematical determinations – the results should not include any deductions

Conclusion: a deduction based on the evidence in an experiment (empirical, so not proven) Reporting (or peer review): allows other scientists to evaluate the findings for themselves and

it also allows other researchers to confirm the results

Theory: an explanation of a natural phenomenon (based on many observations over time) Scientific Law: a relationship in nature that has been supported by many experiments (and

agreed by peer review to be a useful model)

Scientific research

Pure research: gain knowledge for its own sake (Molina and Rowland) Applied research: solve a specific problem (replacements for CFCs)

A Tale of Two Substances (the rest of the story)

Molina and Rowland’s model predicting the interaction of CFCs and ozone

1987 The Montreal Protocol countries agreed to phase out CFCs

Figure 1.17 on page 20 shows that global use of CFCs began to decline almost immediately after the 1987 Montreal Protocol, yet CFC concentrations in the stratosphere continued to climb. Why?

Answer: It takes a long time for CFCs to move up from the troposphere to the stratosphere.

Note the mechanism shown in Figure 1.13 (partially reproduced as Molina and Rowlands model on previous page)

If CFCs react with the UVB, why doesn’t that keep Earth safe? Answer: The CFC + UVB reaction is very slow and does not occur fast enough to

reduce UVB to safe levels. Ozone reacts very quickly with UVB to produce O2 and atomic oxygen, O which then quickly reforms O3 which quickly reacts with more UVB.

Why doesn’t atomic chlorine react with ozone until August? Answer: Atomic chlorine does not react until there is enough sunlight (spring is in

August in Antarctica) to warm the temperature. Note the graph on the bottom of page 21 in the Data Analysis Lab.

Why does the ozone concentration drop in the spring (August)? Answer: That is when temperature is high enough for atomic chlorine to react with

ozone. (The sunlight catalyzed reaction to form atomic chlorine occurs even during the cold winter months and levels build up until the temperature is high enough for the ozone destroying reaction to occur.)

Here is the Think Critically exercise on page 21 Describe the trend in the data for the total ozone and temperature at the 20-24 km layer.

Answer: Temperature drops from January to July then rises from July to December. Ozone concentration is fairly constant from January to the end of July then drops sharply through the month of August. There is a spiky up and down but general increase in ozone from September through December.

Evaluate how the 2004 data compare with the 2005 data. Answer: The trends are nearly identical for both years.

Identify the month during which the ozone levels were the lowest. Answer: September.

Assess: Do these data points back up what you learned in this chapter about ozone depletion? Explain your answer.

Answer: Yes. The sunlight reaction that forms atomic chorine (and bromine) occur even when the temperature is cold allowing the levels of chlorine and bromine to build up through the winter. In August, the temperature warms and the atomic chlorine and bromine levels deplete quickly allowing ozone levels to increase from October through December.

When do scientists predict the ozone to recover? Answer: Early models predicted the ozone might recover in about 2050 but newer

models push the date back until about 2068 but scientists are not really certain when ozone levels will recover but they are fairly certain that, given enough time, they will recover.