Unit 2: Energy Changes and Rates of Reaction

1. Unit 2: Energy Changes and Rates of Reaction Chapter 5 Thermochemistry Changes in Matter and Energy System vs. Surroundings Heat vs. Temperature q=mc∆T

2. What is THERMOCHEMISTRY? THERMOCHEMISTRY is the study of the energy changes that accompany physical or chemical changes in matter.

3. Physical and chemical changes occur with changes in ENERGY (heat) occurs in so many forms ENERGY Ep and Ek the ability to do work

4. CHEMISTRY & Energy • Ep (potential energy): energy of position • chemical bonds  stored energy • Ek (kinetic energy): energy of motion • Molecular motion • Thermal energy (heat energy) is form of Ekis the energy associated with the random motion of atoms and molecules • Chemical energy is one kind of stored energy in which the chemical bonds of molecules act as the “storage” medium.

5. TWO things energy is NOT: • some sort of invisible fluid • something which can be measured directly

6. Laws of Conservation of Energy • THE TOTAL ENERGY OF THE UNIVERSE IS CONSTANT (First Law of Thermodynamics) • ENERGY CAN BE NEITHER DESTROYED NOR CREATED • ENERGY CAN, HOWEVER, BE TRANSFERRED FROM ONE SUBSTANCE TO ANOTHER OR INTO VARIOUS FORMS

7. Surroundings Chemical System Chemical System: the set of reactants and products that are being studied (usually represented by a chemical equation) Thermal Energy: every molecule has thermal energy if the temp is above 0K. (energy of motion) Surroundings: all matter around the system being studied that is capable of absorbing or releasing energy. ice Thermal energy

8. SURROUNDINGS SYSTEM The system is the specific part of the universe that is of interest in the study. closed isolated open energy nothing Exchange: mass & energy 6.2

9. Second Law of Thermodynamics: Heat flows spontaneously from objects with a higher temperatureto objects with a cooler temperature until a thermal equilibrium is reached First Law of Thermodynamics (Law of Conservation of Energy) : Energy cannot be created or destroyed, merely converted from one form to another.

10. Chemical Reaction and ENERGY

11. Thermochemistry In most chemical reactions, energy is: absorbed Old bonds broken released New bonds form

12. q q Exothermic and Endothermic Processes Heat (q) is a form of energy transfer. Units: 1 calorie (cal) = 4.184 J q > 0 Heat is transferred from surrounding to system. Process is endothermic. q < 0 Heat is transferred from system to surroundings. Process is exothermic. System Surroundings

14. Energy and Enthalpy • Chemists give a special symbol, ΔH(delta H) to the heat change in a reaction • This heat change is called ENTHALPY • ΔH (enthalpy) = energy absorbed or released to the surroundings when a system changes from reactants to products. • ΔHsystem = ±|qsurrounding|

15. Representing enthalpy changes • Enthalpy changes of reaction = ΔHrxn • ΔHrxn is dependent on temperature and pressure • ΔH0rxn  standard enthalpy of reaction at SATP – 250C and 100kPa Enthalpy of reaction is also called heat of reaction

16. Molar Enthalpies Molar Enthalpy: the heat (enthalpy) change for a chemical, physical or nuclear change per mole of a substance. The symbol for Molar Enthalpy is Hx,, where x represents the type of reaction and it is measured in J/mole or kJ/mole. Hx = nHx

17. Representing enthalpy reactions • Thermochemical equation (within a chem. Eq) • Enthalpy of reaction at the end of the equation • Enthalpy diagram (more on this later) Enthalpy (amount of heat released or absorbed) of reaction is linearly dependent on the quantity of products.

18. 2H2(g) + O2(g) 2H2O (l) + energy H2O (g) H2O (l) energy energy + 2KClO3(s) 2KCl (l) + 3O2(g) energy + H2O (s) H2O (l) Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. demo Endothermic process is any process in which heat has to be supplied to the system from the surroundings. 6.2

19. 2H2O (s) 2H2O (l) H2O (s) H2O (l) H2O (l) H2O (s) DH = -6.01 kJ DH = 6.01 kJ DH = 2 x 6.01= 12.0 kJ Thermochemical Equations • The stoichiometric coefficients always refer to the number of moles of a substance • If you reverse a reaction, the sign of DH changes • If you multiply both sides of the equation by a factor n, then DH must change by the same factor n. 6.3

20. TEST YOUR UNDERSTANDING? • MgCO3(s)  MgO(s) + CO2(g) ΔH= 117.3 kJ • H2(g) + ½ O2(g) H2O(l) + 285.8 kJ • MgCO3 (s) + 117.3 KJ  MgO(s) + CO2(g) • H2(g) + ½ O2(g) H2O(l) ΔH= - 285.8 kJ

21. Clickers Test you UNDERSTANDING

22. How much heat is evolved when 266 g of white phosphorus (P4) burn in air? P4(s) + 5O2(g) P4O10(s)DH = -3013 kJ x H2O (l) H2O (g) H2O (s) H2O (l) 3013 kJ 1 mol P4 x DH = 6.01 kJ DH = 44.0 kJ 1 mol P4 123.9 g P4 Thermochemical Equations • The physical states of all reactants and products must be specified in thermochemical equations. = 6470 kJ 266 g P4 6.3

23. Enthalpies of Reaction DH is a conversion factor between heat trans-ferred and moles of substance. What mass of butane must react in order to produce 100 kJ of heat? 2 C4H10(g) + 13 O2(g) ® 8 CO2(g) + 10 H2O(g) DH = -5317 kJ = 2.19 g C4H10 100 kJ

24. Thermochemistry and stoichiometry • Enthalpy of reaction is linearly dependent on the quantity of products. Varies directly with Varies directly with Heat absorbed or released Amount of compound (A) Mol Amount of compound B (mol) Factor: ΔHrxn Factor; mole ratio WORKBOOK – stoichiometry and thermochemistry

25. Quantifying HEAT CALORIMETRY

26. Studying Energy Changes FIRST LAW OF THERMODYNAMICS Universe = System + Surroundings ∆Euniverse = Esystem + Esurroundings = 0 Esystem = - Esurroundings Energy lost by the system = energy gained by surroundings

27. Measuring Energy Changes Calorimetry is the process through which energy changes in a chemical system are measured. A Cheaper Calorimeter A Bomb Calorimeter

28. Heat vs. Temperature • Heat (q) is the amount of energy transferred between a system and its surroundings as a result of a temperature difference. (Units: Joules (J), kJ, kJ/mol) 1000 J = 1 kJ • TEMPERTURE _Intensive Property of a material • measure of the Ek of the particles in a sample of matter (Units: oC, K, oF) • is a quantitative measure of heat intensity on some defined scale

29. Demo – “Colorful HEAT” • This demonstration provides a collorful illustration of the relationship between heat and temperature – a key concept in thermochemistry… What is the relationship between the amount of heat added to a substance and the resulting temperature increase. Temperature is not the same thing as HEAT

30. Heat and Energy Changes • What are the products when methane combusts in air? CH4 + 2O2 CO2 + 2H2O + energy • Energy that is released from this CHEMICAL SYSTEM to the SURROUNDINGS is called thermal energy (heat).

31. Thermal energy focus of our UNIT • definition: energy due to chaotic molecular motions • three factors affecting thermal energy • temperature • higher temperature leads to higher thermal energy • sample size • a cup of hot coffee has more energy than a teaspoon of coffee, all other things being equal. • composition • E(solid) < E(liquid) < E(gas), all other things being equal • anything that changes temperature, sample size and/or composition of an object can change its thermal energy

32. Energy, as heat, q , passes from a warmer body to a colder body until the average molecular kinetic energies of two bodies become the same, until the temperatures become equal. • The factors that affect energy change in a chemical system are: • mass (m) • temperature change (T) • the type of substance (c).

33. THE END