Unit 5 How do we predict chemical change?

1. Unit 5How do we predict chemical change? In order to make predictions about the likelihood of a chemical process, we need to explore four main features: THERMODYNAMICS DirectionalityExtent KINETICS RateMechanism The central goal of this unit is to help you identify and apply the different factors that help predict the likelihood of chemical reactions.

2. Comparing the relative stability of different substances M1. Analyzing Structure Determining the directionality and extent of chemical reactions. M2. Comparing Free Energies Analyzing the factors that affect reaction rate. M3. Measuring Rates Identifying the steps that determine reaction rates. M4. Understanding Mechanism Unit 5How do we predict chemical change? FOUR MAIN MODULES

3. Why would the analysis of directionality, extent,rate, and mechanism of chemical reactions be important to understand the EMERGENCE OF LIFE in our planet? Why do we care? Context To illustrate the power of chemical ideas and models in predicting whether a chemical change will occur or not, we will focus our attention in some of the processes that seem to lie behind the origin of life.

4. The answer to that question depends on our understanding of how we went from simple molecules, such as N2 , H2O, and CO2, to complex, such as proteins and DNA. The Problem One of the central questions of modern science is how life started in our planet.

5. Unit 5 How do we predict chemical change? Module 1: Analyzing Structure Central goal: To evaluate the relative stability of substances based on relevant structural features.

6. TransformationHow do I change it? The Challenge Imagine that you have information about the chemical composition of the early Earth. How could you decide what reactions were likely to occur among these “primordial” components? What structural features of the reactants and products could help you make the prediction? How could you qualitatively predict the directionality of a chemical reaction?

7. The extent of the reaction is determined by the relative stability of the reactants and products. Relative Stability Properly evaluating the relative stability of two or more substances is a crucial skill in making predictions about reaction directionality. Potential Energy Ea

8. ENERGETIC FACTORS ENTROPIC FACTORS Two Crucial Factors In making judgments about relative stability, we need to consider two types of factors:

9. Which features of a substance, or its structural units (ions, molecules), may affect its potential energy? Let′s think! Energetic Factors In general, energeticstability increases with decreasing potential energy of a substance. These are some of the relevant features we need to consider: Bond strengthChemical composition Charge distributionState of matter Intermolecular Forces

10. Let′s think! Energetic Factors Let’s analyze more carefully the effect of bond strength on chemical stability. Compounds with strong bonds tend to be more energetically stable. Their decomposition requires more energy input. What seems to be the effect of both the types of atoms (e.g. O-O vs. C-O) involved and bond length on energetic stability?

11. Bond Strength In general, A-A bonds are weaker than A-B bonds. In general, longer bonds are weaker than shorter bonds. Thus, energy considerations favorshorterA-B bonds.

12. Electronegativityc Bond polarity (~Dc) Strength Let’s Think Consider these bond energies. How may you explain these values? What other atomic properties may be used to predict bond strength?

13. Let’s Think The amount of O2 in the primitive atmosphere was likely pretty small. However, in order for aerobic organism to develop, there must have been a source of O2 for them. Water maybe? 2H-O-H 2H-H+ O=O What is more energetically stable, the mixture of reactants or the product? Predict first, then calculate using bond energies

14. 2H2+ O2 -1362 kJ -2 x 432 - 498 2H2O -1856 kJ -4 x 464 Let’s Think 2H-O-H 2H-H+ O=O Ep DHrxn = 494 kJ 0 This process is not favored from the energetic point of view, but may occur if we supply the energy (e.g., sun radiation).

15. Let’s Think Consider the formation these three substances: HF, HCl, and HBr (HX) H-H+ X-X 2H-X Is the formation of these compounds energetically favored? What do you predict for the compound HI? Why? Based on these results, what experimental data about a chemical process could be used to evaluate how “energetically favored” a reaction is?

16. Thus, the heat of reaction (or Enthalpy change DHrxn) is an important piece of information in deciding whether a chemical process will be favored or not. Heat of Reaction The more energy is released during a chemical reaction (exothermic processes), the more energetically stable the products are relative to the reactants. The more stable the products, the more likely the reaction to proceed to completion (extent).

17. Calorimetry: Heat transfer is indirectly measured by quantifying changes in temperature. Standard Enthalpy Changes Measurements of the energy absorbed or released in the form of heat during a chemical reaction are of central importance in making predictions about the extent of a chemical process. These measurements are commonly done using substances in their standard state at 1 atm and 25oC. This heat of reaction is identified as the standard enthalpy changeDHorxn.

18. ½H2(g) +½F2(g) HF(g) DHof= -273.3 kJ/mol ½H2(g) + ½ Cl2(g) HCl(g) DHof= -92.3 kJ/mol ½H2(g) + ½ I2(s) HI(g) DHof= +26.5 kJ/mol Standard Enthalpy of Formation One particular useful quantity is the change in enthalpy when 1 mole of substance is formed from its constituent elements in their standard state. ? ½H2(g) + ½ Br2(l) HBr(g) DHof= -36.3 kJ/mol The more negative the standard enthalpy of formation (DHof), the more energetically stable the compound is with respect to the simple elements.

19. C2H6O(l) C2H6O(l) -271 kJ/mol -277 kJ/mol Let’s Think How would you justify the differences in DHofin these cases? H2O(l) H2O(g) -241.8 kJ/mol -286.0 kJ/mol

20. The effect of interparticle interactions is particularly important in ionic compounds, where the substance is comprised of a network of interacting ions. Intermolecular Forces The strength of the intermolecular forces between particles, determined by proximity between molecules and types of interactions, affects the potential energy of the bulk material. More negative DHfo Stronger IMFS

21. They may have served as catalysts or as replicators of “information.” Ionic Compounds Some scientist have proposed that some ionic compounds, either in crystal form or dissolved in water, played a central role in the origin of life. How may we make predictions about the energetic stability of these types of compounds?

22. r However, in this case we need to remember that we are not dealing with molecules but ionic networks. + The properties of an ionic compound are determined by the electrostatic forces among its ions, and between these ions any surrounding particle (atom, ion, or molecule). Coulomb’s Law Ionic Compounds Similar ideas about interparticle strength and enthalpy of formation can be used to make judgments of stability in the case of ionic compounds.

23. The stronger the forces, the lower the potential energy of the lattice. The lower the potential energy, the more energetically stable the ionic compound. 0 + + 2+ Ep Forces and Energy • Based on Coulomb’s Law, one may expect forces between ions to be stronger: • the larger the charge of the ions (larger q1, q2); • the smaller the size of the ions (smaller r);

24. Rank these sets of compounds from lower (less negative) to higher (more negative) lattice energy. Discuss which compounds are more energetically stable. Let′s think! NaF, NaBr, NaCl KF, LiF, NaF NaF, MgO, SrO Lattice Energy The energy released during the formation of an ionic compound starting from its ions in the gas phase is a good measure of the potential energy of the compound. A+(g) + B-(g)  AB(s) DHrxn= Lattice Energy

25. Lattice Energy Rb+152pm -600 K+138pm -700 Na+102pm -800 Li+76 pm Lattice Energy (kJ/mol) -900 MgO (-3795 kJ/mol)SrO (-3217 kJ/mol) -1000 -1100 F-133pm Cl-181pm Br-196 pm I-220pm

26. Let’s Think Based on our discussion: Rank the following synthesis reactions in order of more to less “energetically favored” (from more to less negative DHof). 4 Cs(s) +1/2Cl2(g) CsCl(s) DHof= -438 kJ/mol 1 2 Al(s) +3/2 O2(g) Al2O3(s) DHof= -1676 kJ/mol 3 Ba(s) +1/2O2(g) BaO(s) DHof= -548 kJ/mol 2 Ca(s) +1/2O2(g) CaO(s) DHof= -635 kJ/mol

27. ENTROPIC FACTORS Summary In making judgments about relative stability, we need to consider : ENERGETIC FACTORS Bond:strength, length, heterogeneity, polarityIMFs:StrengthIon:Size, charge

28. Let′s think! Arrange these systems from fewer to larger possible distinguishable “configurations” (different ways to distribute matter and energy). Entropic Factors In general, the stability of a system increases the larger the number of ways the system has to distribute both its matter and energy. Consider these systems with the same total kinetic energy:

29. The larger the number of different types of particles which can move and interact in different ways, the more distinguishable “configurations.” Entropic Factors

30. Which features of a substance, or its structural units (ions, molecules), may affect its entropy? Let′s think! Entropy The Entropy (S) of a system is an indirect measure of the number of different configurations that its matter and energy can take. The more possible configurations a system has (distributions of matter and energy), the larger its entropy (larger entropic stability). Molecular size Complexity State of Matter

31. Standard Entropy of Formation Fortunately, it is possible to measure the standard entropy of formation DSof of chemical substances. Basic Assumption: So (perfect crystal) = 0 at 0 K. Sof for any substance is a measure of the different configurations that matter and energy can take in 1 mole of the substance at 25oC and 1 atm (measure of entropic stability).

32. Let′s think! Consider this data: What patterns do you observe? How do you explain them?

33. S increases slightly with T S increases a large amount with phase changes Entropic Trends

34. Entropic Trends What trends do you observe? How would you explain these results? Let′s think! In general, the less constrained the atoms, molecules, or ions in a system, the larger the entropy.

35. Let′s think! What two major patterns do you observe? How do you explain them? Entropic Trends Consider this data:

36. Entropic Trends For a given state of matter, entropy generally increases with Molar mass and Molecular complexity.

37. Let′s think! How would you explain these results?

38. Entropy and Reaction Extent Chemical reactions in which the total entropy of the products is higher than the total entropy of the reactants are “entropically” favored. Let’s consider the reaction: A + B C + D DSorxn= Soproducts – Soreactants = (SoC + SoD) – (SoA+SoB) >0Entropically favored DSorxn>0 1) Fewer constraints (g, aq); 2) Larger molar mass; 3) More complexity (Number, Types).

39. Let′s apply! Assess what you know

40. Fuel Synthesis The following molecules could have been used as fuels to generate energy by primitive organisms in our planet. C6H12O6 C8H18 C4H10 H2 Glucose Butane Octane Hydrogen

41. Let′s apply! - - - - + - + + Predict Compare the energetic stability of reactants and products and predict the signs of DHorxn and DSorxn Chemical Reaction DHorxn DSorxn 4C(s) + 5H2(g) C4H10(g) 8C(s) + 9H2(g) C8H18(l) 6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g) CH4(g) + H2O(g) CO(g) + 3H2(g) Which of these reaction can we expect to be favored?

42. Summarize the main energetic and entropic factors that can help predict relative stability.

43. Analyzing Structure Summary Relative stability depends on: ENERGETIC FACTORS ENTROPIC FACTORS Bond:strength, length, heterogeneity, polarityIMFs:StrengthIon:Size, charge Constraints:structural, dynamicMolar MassComplexity:structural, dynamic(number and types of atoms)

44. For next class, Investigate what the Second Law of Thermodynamics is about. How can this law be used to decide on the directionality of a chemical process?