1. Chemistry XXI So far, our focus has been on understanding the submicroscopic structure of chemical substances and its relationship with their macroscopic physical properties. Beyond the Physical However, an essential question in chemistry is how structure determines chemical reactivity. How, when, why do substances interact to produce new substances? Nylon Synthesis

2. Chemistry XXI The central goal of this unit is to help you understand and apply ways of thinking that can be used to model chemical change in a system. Unit 4 How do we model chemical change? Select the best fuel Produce new synthetic materials Control the production of air pollutants What do you need to know about chemical change to complete these tasks? Why do we care?

3. Chemistry XXI To what extent does it happen? Essential Questions How fast does it occur? How much substance do I need or do I get? How much energy do I need or do I get? Nylon Synthesis StoichiometryThermochemistry Thermodynamics Kinetics How do we explain, predict, and control it?

4. Chemistry XXI Unit 4 How do we model chemical change? M1. Understanding Proportions Determining the amount of substance formed or consumed. M2. Tracking Energy Predicting the amount of energy absorbed or released. THREE MAIN MODULES M3. Analyzing Rate and Extent Identifying the factors that affect chemical transformations.

5. Chemistry XXI Integration To illustrate the power of chemical ideas in modeling chemical change we will focus our attention on two important types of systems: Processes that generate energy. Processes that generate pollutants.

6. Chemistry XXI Unit 4 How do we model chemical change? Module 1: Understanding Proportions Central goal: To make quantitative predictions about the amount of substances consumed or produced during chemical reactions.

7. Chemistry XXI Chemical Change We say that a chemical change has occurred whenever we detect the appearance of a new substance in a given system. However, What “differentiating characteristics” could we use to distinguish chemical changes such as these? Why would be important to be able to identify different types of chemical changes? Let′s think!

8. Chemistry XXI All chemical changes involve the transformation of matter and energy into new forms. Thus, they can be differentiated based on: Signature Features

9. Chemistry XXI The Challenge We would like to generate models that allow us to answer questions such as this: What amounts of reactants and products are involved? How much energy will be needed or produced? How fast will the process go and how can I control it? To what extent will the reactants be changed into products? Modeling How do I explain it?

10. Chemistry XXI Characterizing Change Consider this relevant process: We are using natural gas to power vehicles. Natural gas, composed mainly by methane (CH 4 ), is one of the “cleaner” fossil fuels. Methane CH 4 CH 4 reacts with O 2 inside the combustion engine. The main outcomes are CO 2 and H 2 O.

11. Chemistry XXI Consider these facts: Without a spark, the reaction is too slow. Spark Let′s think! 1 mol CH 4 3 mol O 2 Isolated M = T fast 1 mol CO 2 2 mol H 2 O 1 mol O 2 C H O N Represent the chemical reaction that is taking place using: a)Particles b) Formulas(CH 4 ) Use your representations to explain why no more CO 2 and H 2 O were produced and why the total mass of the system did not change.

12. Chemistry XXI Limiting Reactant O 2 is in excess; CH 4 is the limiting reactant (determines the maximum amount of products). What about here?

13. Chemistry XXI Mass Conservation M = Why does the mass remain constant? CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O(g)

14. Chemistry XXI Methane CH 4 Let’s Think InitialInitial FinalFinal The analysis of the exhaust gas in a CH 4 -based car reveals the composition shown in the figure.  What is the balanced chemical equation for the combustion?  What was the composition of the initial mixture?  Which is the limiting reactant? ?

15. Chemistry XXI Reactant s O2O2 SO 2 Products Natural gas contains a small amount of sulfur compounds (~ 4 ppm). When burned, they produce the pollutant SO 2 that further reacts with oxygen in the atmosphere to produce another harmful substance. Let’s Think What is the balanced chemical equation? Which reactant is limiting this process?

16. Chemistry XXI Let’s Think The incomplete combustion of fossil fuels, such as coal (mainly made of carbon), and ethane (C 2 H 6 ) produce a toxic pollutant, carbon monoxide (CO). Write the balanced chemical equations and complete the particulate representation of each process. Which reactant is limiting?

17. Chemistry XXI The previous examples illustrate chemical processes in which the reactions go to completion (the limiting reactants is fully consumed). The process stops because there is no more reactant. What chemical equation represents this process? However, there are many cases in which the reaction does not fully go to products. Reaction Completion

18. Chemistry XXI Controlling the Outcome Recognizing that chemical substance react in definite proportions is of central importance to better control the outcome of a chemical reaction. For example, consider this problem More than 80% of the energy we use comes from burning fossils fuels, such as CH 4 and C 8 H 18 :  Non renewable;  Produce CO 2 and other pollutants. In order to replace them, or reduce pollution emission, we need to understand the chemical process in which they are involved.

19. Chemistry XXI Making Predictions Let’s take octane, C 8 H 18, one of the main components of gasoline. It reacts with oxygen (O 2 ) according to the following chemical equation: 2 C 8 H 18 + 25 O 2  16 CO 2 + 18 H 2 O This equation has a lot of implicit information: 228.4 g C 8 H 18 + 800.0 g O 2  704.2 g CO 2 + 324.2 g H 2 O 1028.2 g  1028.2 g  Particulate 2 mol C 8 H 18 + 25 mol O 2  16 mol CO 2 + 18 mol H 2 O Macroscopic 2251618

20. Chemistry XXI Mole to Mass How can we calculate the mass of CO 2 produced when burning 1 g of C 8 H 18 ? 2 C 8 H 18 (l) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O (g) Making Predictions 228.4 g C 8 H 18 + 800.0 g O 2  704.2 g CO 2 + 324.2 g H 2 O The same procedure can be used to calculate any amount of reactant needed or product produced. Mass to Mole Mole to Mole

21. Chemistry XXI General Strategy Mass reactant Moles product Chemical Equation Moles reactant Molar Mass Mass product Molar Mass The following diagram depicts the basic strategy that allows us to relate the mass of reactants and products in any chemical reaction. ReactantsProducts

22. Chemistry XXI Evaluating Substances Understanding the proportion in which substances react is of central importance to evaluate what substances to use in many practical processes. For example, Ethanol (C 2 H 6 O) is being used both as a gasoline additive and as an alternative renewable fuel. In which way does this substitution affects the amount of CO 2 released into the environment? How would you answer this question?

23. Chemistry XXI Let’s Think C 2 H 6 O(l) + 3 O 2 (g)  2 CO 2 (g) + 3 H 2 O(g) Calculate the amount of CO 2 produced when 1.0 g of ethanol (C 2 H 6 O) reacts with O 2. (the products are CO 2 and H 2 O). If 1.0 g of C 8 H 18 produces 3.08 g CO 2 and 1.0 g of C 2 H 6 O generates 1.91 g CO 2, what percent reduction in CO 2 emissions do we get by replacing octane with ethanol?

24. Chemistry XXI Not an Easy Decision Does the previous result implies that ethanol is “greener” than octane?

25. Chemistry XXI Not an Easy Decision

26. Chemistry XXI Reducing Pollution We may try to inject air and hydrocarbons into the combustion engine in the right proportions to try to ensure that all the hydrocarbon is consumed. For example: We have seen that the incomplete combustion of a hydrocarbon may lead to the formation of carbon monoxide (CO). How can we reduce the amount of CO formed? Given our dependence on fossil fuels, we should figure out ways to reduce the pollution they generate.

27. Chemistry XXI 2 C 8 H 18 (l) + 25 O 2 (g)  16 CO 2 (g) + 18 H 2 O (g) If we inject 1 g of C 8 H 18 in the engine, how many moles of O 2 are needed for full combustion? How many grams is this? Let’s Think Now, we do not inject O 2 in our combustion engines. We use air. How much air do we need?

28. Chemistry XXI Assess what you know Let′s apply!

29. Chemistry XXI Represent Let′s apply! Liquid Hydrogen Hydrogen (H 2 ) is now being used as a substitute of gasoline in some combustion engine cars.  What is the balanced chemical equation for the combustion?  What was the composition of the initial mixture?  Which is the limiting reactant?

30. Chemistry XXI Let′s apply! Determine the stoichiometric AF for this fuel (Grams of air needed per gram of H 2 ). Remember air is 21% O 2 and 79% N 2 by mol. Compare the AF st for gasoline (14.7:1) and hydrogen. Discuss possible implications of this difference. Calculate

31. Chemistry XXI Identify one type of question or problem presented in this module in which you need more practice.

32. Chemistry XXI Summary Understanding Proportions During a chemical reaction new substances are formed and other are consumed. If the reaction occurs in a closed container, the total mass of the system is conserved. Although the number of molecules of the different substances present in the system will vary, the total number of atoms of each type will remain the same.

33. Chemistry XXI Summary A balanced chemical equation, together with information about the molar masses of the reactants and products, can be used to predict the mass, amount of substance, and number of particles that will be formed or consumed during the process. Understanding Proportions Mass reactant Moles product Chemical Equation Moles reactant Molar Mass Mass product Molar Mass Reactants Products

34. Chemistry XXI For next class, Investigate why energy is always absorbed or released during a chemical reaction. In which ways the amount of energy released during chemical reactions may be related to your body weight?