Chemistry XXI Unit 1 How do we distinguish substances? 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. 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.

Chemistry XXI 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.

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

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

Chemistry XXI 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. 1 mL of water = particles = 3.34 x particles SolidLiquidGas Basic Assumptions: 1.Any macroscopic sample of a substance is composed of a large number of very small particles;

Chemistry XXI m = m m = m m = m 0.01 m = m0.1 m = m How Small? 1 m 0.000,000,001 m = 1 x m = 1 nm (1 nanometer) Most substances are made of particles of “nanometer” size.

Chemistry XXI Solid Liquid Gas What do these “particles” represent? What are the limitations of these representations? Particles could represent anything we want: atoms, molecules, ions. These representations are static, have unrealistic proportions, represent particles as solid objects, mix levels of representation. Being Cautious

Chemistry XXI 2. Particles are constantly moving; Dynamic Nature Basic Assumptions: In this model, Temperature is a measure of average kinetic energy (K = ½ mv 2 ) per particle. What determines the speed (v) of the particles? Same T  Same K v depends on T and m

Chemistry XXI 2. Particles are constantly moving; Dynamic Nature Basic Assumptions: Pressure is determined by the force of the particle collisions on the walls: Pressure = Force/Area What is the “average pressure” in this model?

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

Chemistry XXI Let’s Explore Go to: (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.

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

Chemistry XXI Predictions P T P N P  NP  N P  1/V P  TP  T  is used to indicate “direct proportionality” P V The model predicts the following type of behavior: This behavior is observed in all gases at high temperatures and low pressures. Why?

Chemistry XXI 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: (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?

Chemistry XXI Let’s Explore

Chemistry XXI 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?

Chemistry XXI Particles that attract each other are said to have negative potential energy compared to free particles. Potential Energy 0 EpEp r r1r1 r2r2 r3r3 r4r4 More Negative

Chemistry XXI Potential vs. Kinetic Energy During a phase transition all the energy is invested (or lost) in the form of POTENTIAL ENERGY. 0 EpEp r r1r1 r4r4 Need to add energy to separate Need to extract energy to get them closer The average KINETIC ENERGY per particle does not change during a phase change.

Chemistry XXI Explain and Predict Let′s think! Why does the boiling temperature of water decrease with decreasing external pressure? Water particles in the liquid phase have to push the surrounding gas (air) to escape; the higher the external pressure, the higher the kinetic energy needed for particles to push the surrounding gas. That is why boiling occurs when the pressure of the evaporating liquid (its vapor pressure) is equal to the external pressure. Let′s think! Why do liquids get colder when they evaporate?

Chemistry XXI Explain and Predict Not all of the particles in a substance move at the same speed at a given temperature. T 1 < T 2 The particles that escape to the vapor phase are the ones with higher kinetic energy. When they leave, the average kinetic energy in the system goes down and thus temperature goes down. Let′s think! How do you explain that nitrogen condenses at a lower temperature than oxygen in terms of IMF?

Chemistry XXI Substance A Substance B Explain and Predict The difference in boiling points, and many other physical properties, can be explained if we assume that the strength of the intermolecular forces among particles vary from substance to substance. A new question for us, Why?

Chemistry XXI 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?

Chemistry XXI 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? 5 types of particles  5 different substances

Chemistry XXI 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. Individual particles of different substances are modeled as made of free or bonded atoms of different types. Free atom of argon Molecule of oxygen Bonded atom of nitrogen Molecule of carbon dioxide Modeling Substances Molecules are made of two or more bonded atoms Atom or molecule?

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

Chemistry XXI 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. Atomic Element MacroscopicSymbolic Particulate Ar Argon Molecular Element Nitrogen N2N2

Chemistry XXI Chemical Elements 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. Cl 2 Chlorine Phosphorus P4P4 Na Sodium Carbon C

Chemistry XXI 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.

Chemistry XXI Chemical Compounds Most substances in nature are chemical compounds. They are composed of identical particles made of bonded atoms of two or more different types. MacroscopicSymbolic Particulate Carbon dioxide CO 2 Water H2OH2O CHONCHON Color Code Molecular Formula

Chemistry XXI Molecular Compounds H 2 O and CO 2 belong to a group of compounds called “molecular compounds”: They are made of molecules. 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. Methane CH 4 Caffeine C 8 H 10 N 4 O 2 Hemoglobin C 2952 H 4664 N 812 O 832 S 8 Fe 4

Chemistry XXI Models and Formulas Formaldehyde (Air Pollutant) Keep in mind that molecules are represented in a variety of ways: Models Formulas CH2OCH2OCH2OCH2O Molecular Formula Space-filling Ball-and-stick Structural formula

Chemistry XXI Let’s Explore Decide whether these particulate models correspond to an element, a compound or a mixture. C EM EMC

Chemistry XXI Assess what you know Let′s apply!

Chemistry XXI T = 280 K P = 2 atm Analyze Let′s apply! 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?

Chemistry XXI T = 280 K P = 2 atm Analyze Let′s apply! What does this system represent? SODA WATER How many phases are there? TWO: LIQUID AND GAS How many substances are in them? 4 G (N 2, O 2, H 2 O, CO 2 ); 3 L How many elements? 2 (N 2, O 2 ) How many compounds? 2 (CO 2, H 2 O) How would you separate the different components in the system? DECREASING T, SEPARATING EACH AS THEY CONDENSE

Chemistry XXI Predict 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? T = 280 K P = 2 atm Let′s apply!

Chemistry XXI Predict In which of the phases: Are the intermolecular forces strongest? Liquid Phase Is the average potential energy per particle the highest (least negative)? Gas Phase Is the average particle speed the highest? Liquid Phase Is the average kinetic energy per particle the lowest? The same in all phases (same T) Which of the two main substances, water or carbon dioxide, has the lowest vapor pressure? Water T = 280 K P = 2 atm Let′s apply!

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

Chemistry XXI 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.

Chemistry XXI Modeling Matter Summary: The nature and strength of these interactions depend on the atomic composition and structure of a substance’s particles. Based on the composition of their particles, substances are classified as elements or compounds. N 2 (Element) CO 2 (Compound)

Chemistry XXI 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 O 2 we breathe in a day?