Unit 1 How do we distinguish substances? The central goal of this unit is to help you understand and apply basic ideas that can be used to distinguish the different substances present in a system. M1. Searching for Differences Identifying differences that allow us to separate components. M2. Modeling Matter Using the particulate model of matter to explain differences. M3. Comparing Masses Characterizing differences in particle’s mass and number. M4. Determining Composition Characterizing differences in particle’s composition.

How do we distinguish substances? Module 2: Modeling Matter Unit 1 How do we distinguish substances? Module 2: Modeling Matter Central goal: To explain the diversity in properties and behaviors of the different substances in a system based on the particulate model of matter.

The Challenge Modeling How do I explain it? As we have seen, each substance has at least one differentiating characteristic that makes it unique. Carbon dioxide Nitrogen Water What causes the differences? How could we explain, predict, and even design, these differences?

Models of Matter The central task of differentiating substances has been greatly simplified by the development models about their internal structure. These models allow us to explain and predict the properties of matter, and to develop better techniques to detect and identify them.

Particulate Model of Matter One of the most powerful models we have to explain and predict the physical properties and behavior of substances is the particulate model of matter. Solid Liquid Gas Basic Assumptions: Any macroscopic sample of a substance is composed of a large number of very small particles; 1 mL of water = 33444444444444444444444 particles = 3.34 x 1022 particles

Most substances are made of particles of “nanometer” size. How Small? 0.0000001 m = 10-7 m 0.01 m = 10-2 m 0.000,000,001 m = 1 x 10-9 m = 1 nm (1 nanometer) Most substances are made of particles of “nanometer” size. 0.001 m = 10-3 m 0.1 m = 10-1 m 0.00001 m = 10-5 m 1 m

Being Cautious What do these “particles” represent? Gas What do these “particles” represent? What are the limitations of these representations? Liquid Solid

What determines the speed (v) of the particles? Dynamic Nature Basic Assumptions: 2. Particles are constantly moving; What determines the speed (v) of the particles?

What is the “average pressure” in this model? Dynamic Nature Basic Assumptions: 2. Particles are constantly moving; What is the “average pressure” in this model?

Interactions Basic Assumptions: 3. Particles interact with each other. The nature and strength of the interactions depend on the distance between particles; Distance

Go to: http://www.chem.arizona.edu/chemt/C21/sim (Ideal Gas) Let’s Explore Go to: http://www.chem.arizona.edu/chemt/C21/sim (Ideal Gas) Explore the properties of the particulate model of matter when interactions among particles are neglected: How does pressure depend on temperature, volume, and number of particles? Build graphs of the type P vs. T, P vs. V, and P vs. N as part of your analysis.

Molecular Dynamics Simulation Let’s Explore Molecular Dynamics Simulation F(r)  a(t)  v(t)  r(t)

The model predicts the following type of behavior: Predictions The model predicts the following type of behavior: P T P N P V This behavior is observed in all gases at high temperatures and low pressures. Why?

Go to: http://www.chem.arizona.edu/chemt/C21/sim (Real Gas) Let’s Explore In the absence of interactions among particles (intermolecular forces) the model does not predict the existence of phase transitions as we change T. Go to: http://www.chem.arizona.edu/chemt/C21/sim (Real Gas) Analyze the behavior of the model when intermolecular forces (IMF) among particles are introduced. Discuss: What is the effect of the on the behavior of the particles and the system?

Let’s Explore

Force strength does not change; Size of particles does not change; Phase Transitions To explain the existence of phase transitions we have to assume that there are intermolecular forces among particles. When temperature decreases, the average kinetic energy per particles decreases. Attractive forces between particles are then able to hold them together. Force strength does not change; Size of particles does not change; Let′s think! Why does the temperature remain constant during a phase transition?

Potential Energy Particles that attract each other are said to have negative potential energy compared to free particles. Ep r r1 r3 r4 More Negative r2

Potential vs. Kinetic Energy Ep r Need to add energy to separate Need to extract energy to get them closer r1 r4

Why do liquids get colder when they evaporate? Explain and Predict Let′s think! Why does the boiling temperature of water decrease with decreasing external pressure? Let′s think! Why do liquids get colder when they evaporate?

Explain and Predict T1 < T2 Let′s think! How do you explain that nitrogen condenses at a lower temperature than oxygen in terms of IMF?

Explain and Predict Substance A Substance B

Understanding Differences In the particulate model of matter, many differences between substances are attributed to the presence of different intermolecular forces among particles. Why are the intermolecular forces different? We assume the composition and the structure of the “particles” are different. What does this mean?

How many different substances are included in this representation? Modeling Substances This is a typical chemical representation at the particulate level of the main components of “pure” air. Let′s think! How many different substances are included in this representation? What similarities and differences do you observe between the different types of particles present in the system?

The representation conveys the idea that we model air as a “mixture”: Modeling Substances Free atom of argon The representation conveys the idea that we model air as a “mixture”: a system composed of two or more types of independent particles present in proportions that may vary from sample to sample. Atom or molecule? Molecule of carbon dioxide Molecule of oxygen Bonded atom of nitrogen Individual particles of different substances are modeled as made of free or bonded atoms of different types. Molecules are made of two or more bonded atoms

Classifying Substances As we have seen, the different components of a mixture can be separated by physical means (filtration, distillation): Nitrogen (liquid) Water S L G Argon (gas) Chemists classify substances as “elements” or “compounds” based on particle composition. Carbon dioxide (Solid, Dry ice) Let′s think! Which of these are elements/compounds? What makes the difference?

Chemical Elements Elements are the most simple substances in Nature. They are composed of identical particles made of free or bonded atoms of the same type. Macroscopic Symbolic Particulate Atomic Element Molecular Element

Chemical Elements Cl2 C Na P4 There are relatively few elements in Nature (fewer than 100), and a few more have been synthesized in lab. None of them can be decomposed in simpler substances by physical or chemical means. Cl2 Chlorine Phosphorus P4 Na Sodium Carbon C

Atoms vs. Elements We need to differentiate between the elements, as real substances, and the atoms they are made of. The Periodic Table summarizes the properties of the individual atoms, not of the actual elements.

Chemical Compounds CO2 H2O Most substances in nature are chemical compounds. They are composed of identical particles made of bonded atoms of two or more different types. Macroscopic Symbolic Particulate Carbon dioxide CO2 Water H2O C H O N Color Code Molecular Formula

There is a wide variety of molecular compounds in Nature. H2O and CO2 belong to a group of compounds called “molecular compounds”: They are made of molecules. Methane CH4 There is a wide variety of molecular compounds in Nature. This diversity is due to the possibility of having molecules with different compositions, sizes, and structures. Caffeine C8H10N4O2 Hemoglobin C2952H4664N812O832S8Fe4

Models and Formulas CH2O Space-filling Ball-and-stick Models Keep in mind that molecules are represented in a variety of ways: Space-filling Ball-and-stick Models Formaldehyde (Air Pollutant) Structural formula CH2O Molecular Formula Formulas

Let’s Explore Decide whether these particulate models correspond to an element, a compound or a mixture. C E M

Let′s apply! Assess what you know

Analyze What does this system represent? Let′s apply! T = 280 K P = 2 atm What does this system represent? How many phases are present in this system? How many substances are in each phase? How many elements? How many compounds? How would you separate the different components in the system?

Predict In which of the phases: Let′s apply! T = 280 K P = 2 atm In which of the phases: Are the intermolecular forces strongest? Is the average potential energy per particle the lowest (most negative)? Is the average particle speed the highest? Is the average kinetic energy per particle the lowest? Which of the two main substances, water or hexane, has the greater vapor pressure?

Write down one central idea that you learned in this module. Share your idea with the members of your group.

Useful to analyze, synthesize, and transform chemical substances. Modeling Matter Summary: The particulate model of matter allows us to explain and predict the properties of chemical substances. Useful to analyze, synthesize, and transform chemical substances. Differences in the intermolecular forces among the particles of different substances can be used to explain their different physical properties.

Modeling Matter Summary: Based on the composition of their particles, substances are classified as elements or compounds. N2 (Element) CO2 (Compound) The nature and strength of these interactions depend on the atomic composition and structure of a substance’s particles.

For next class, Investigate how we can determine how many times heavier is one type of atom than another. How can we use this knowledge to quantify, for example, how many molecules of O2 we breathe in a day?