Chemistry 30 – Unit 1 Thermochemical Changes To accompany Inquiry into Chemistry PowerPoint Presentation prepared by Robert Schultz

Preparation Info Systems: Open, closed, and isolated - definitions First Law of Thermodynamics – Total energy of the universe is constant (energy can’t be created or destroyed) Second Law of Thermodynamics – In the absence of energy input, a system becomes more disordered (its entropy increases)

Preparation Meaning? A system at lower temperature will be more ordered as the particles have less average kinetic energy Two systems in thermal contact will transfer energy such that the more ordered (cooler) one gains energy and becomes more disordered Consequence: heat always flows from hotter systems to cooler ones

Preparation Important Definitions: Thermal Energy: the total kinetic energy of all particles of a system Temperature: a measure of the average kinetic energy of the particles of a system Heat: a transfer of thermal energy between 2 systems

Chapter 9, Section 9.1 Questions: Which has more thermal energy, a hot cup of coffee or an iceberg? Which has a larger average thermal energy, a hot cup of coffee or an iceberg? If an iceberg and a hot cup of coffee come into contact, in which direction will heat flow?

Preparation Heat energy transferred will be related to the temperature change of the system It takes different amounts of heat energy to change the temperature of 1 g of a substance by 1°C This number is called the specific heat capacity, c, and is measured in units of:

Water has a c value of This means that it takes 4.19 J of heat to raise the temperature of 1 g of water by 1°C Water has a very large c compared to most other common substances Preparation

To determine the amount of heat transferred the formula used is Despite what your text says on page 337, I would always take ∆t as positive If heat is absorbed, temperature of surroundings will decrease; if heat is released temperature of surroundings will increase Examples: Practice Problems 1 and 4, page 337 Preparation

Practice Problem 1, page 337 Since 1 J is such a small amount of heat energy I start my questions in kJ as shown above If necessary I move into MJ or GJ Preparation

Practice Problem 4, page 337 Putting kilo top and bottom cancels out and c stays the same The substance is granite Worksheet: WS 43 (Nelson) then BLM (back only) Preparation

Chapter 9, Section 9.1 Energy changes in chemical reactions crucial to life Not just in photosynthesis, fuels, and batteries, but in the very way that your body metabolizes food and makes the energy available for life processes Thermodynamics: the study of energy and energy changes

Chapter 9, Section 9.1 Recall the first law of thermodynamics: ∆E universe = 0 If a system loses energy, the surroundings gain energy (get warmer) If a system gains energy, the surroundings lose energy (get cooler) ∆E system = - ∆E surroundings

Chapter 9, Section 9.1 Energy types: Kinetic energy, E k, energy of motion of particles of a system Temperature is a measure of the average E k of the particles of a system Potential energy, E p, stored energy, usually in chemical bonds

Chapter 9, Section 9.1 Transfer of E k : heat flows from hotter objects to cooler ones (Preparation section of notes) Breaking bonds always requires energy (endothermic); forming bonds always releases energy (exothermic) Chemical reaction: breaking bonds + energy 1 forming bonds + energy 2 If energy 1 > energy 2, reaction is endothermic If reverse is true, it is exothermic Worksheet BLM input output

Chapter 9, Section 9.1 New term: enthalpy (not entropy) Enthalpy (change), ∆H: the difference in potential energy between reactants and products, measured at constant pressure – measured in kJ (or MJ, etc) Molar Enthalpy (change), ∆ r H: the enthalpy change for 1 mole of a specified substance – measured in kJ/mol (or MJ/mol etc) In common usage the word change gets left out

Chapter 9, Section 9.1 Negative ∆H’s are exothermic (think lose heat) and temperature of surroundings increases Positive ∆H’s are endothermic (think gain heat) and temperature of the surroundings decreases Note: this increase → negative, and decrease → positive is a stumbling block for many students

Chapter 9, Section 9.1 Chemical reactions can be written using ∆H notation: C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O(l) ∆H= kJ 4 NO(g) + 6 H 2 O(g) 4 NH 3 (g) + 5 O 2 (g) ∆H=+906 kJ They can also be written with the heat as a term in the equation: C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O(l) kJ 4 NO(g) + 6 H 2 O(g) kJ 4 NH 3 (g) + 5 O 2 (g) Do ∆H Worksheet! value for the reaction as written

Chapter 9, Section 9.1 Potential energy diagrams for the same 2 reactions are shown below: ∆H = kJ H (kJ) C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O(l) reactants products H (kJ) 4 NO(g) + 6 H 2 O(g) 4 NH 3 (g) + 5 O 2 (g) reactants products ∆H = +906 kJ

Chapter 9, Section 9.2 Recalling that breaking bonds always endothermic and forming new bonds is always exothermic, more complete E p diagrams might be shown as follows: EndothermicExothermic reactants intermediate products ΔHΔH E p (kJ) reactants intermediate products ΔHΔH E p (kJ)

Chapter 9, Section 9.1 Alternate forms of potential energy diagram (from Chemistry 30 Diploma Exam Bulletin)

Chapter 9, Section 9.1 Example: Practice Problem 3, page 346 a)C(s) + 2 H 2 (g) CH 4 (g) kJ b)C(s) + 2 H 2 (g) CH 4 (g) ∆H = kJ c) H (kJ) C(s) + 2 H 2( g) CH 4 (g) products reactants ∆H = kJ Do E p diagrams for formation of Cr 2 O 3 (s), simple decomp* of AgI(s), and formation of SO 2(g)

Chapter 9, Section 9.2 Formation of Cr 2 O 3 (s) E p (kJ) reaction coordinate 2 Cr(s) + 3/2 O 2 (g) Cr 2 O 3 (s) ΔH=ˉ kJ E p (kJ) reaction coordinate ΔH= kJ simple decomposition of AgI(s) AgI(s) Ag(s) + ½ I 2 (s) E p (kJ) reaction coordinate ΔH=ˉ296.8 kJ 1/8 S 8 (s) + O 2 (g) SO 2 (g) formation of SO 2 (g)

Chapter 9, Section 9.1 Molar enthalpy of combustion: the enthalpy change for the complete combustion of 1 mol of a substance Complete combustions of fossil fuels always yields CO 2 (g) and H 2 O Open systems – constant pressure – gases escape – H 2 O(g) Isolated systems – H 2 O(l) Human body – cellular respiration - H 2 O(l)

Chapter 9, Section 9.1 Table of Molar Enthalpies of Combustions of alkanes, page 347 Practice Problem 5b, page 347 (open system) OR: note change in units! In thermodynamics it is acceptable to write equations with fractional coefficients – don’t do this elsewhere Try question 5a, page 347 C 4 H 10 (g) + 13/2 O 2 (g) 4 CO 2 (g) + 5 H 2 O(g) ∆H = kJ 2 C 4 H 10 (g) + 13 O 2 (g) 8 CO 2 (g) + 10 H 2 O(g) ∆H = kJ

Chapter 9, Section 9.1 Question 5a page 347 Note that the value of ∆H varies directly as the number of moles of reacting substances This formula gets used to calculate enthalpy changes for ∆E p like phase changes, chemical reactions, and nuclear reactions C 5 H 12 (l) + 8 O 2 (g) 5 CO 2 (g) + 6 H 2 O(g) ∆H = kJ

Chapter 9, Section 9.1 Example Practice Problem 3a, page 349 Note: from table, page comment mol of pentane

Chapter 9, Section 9.1 Example Practice Problem 6, page 349 molar enthalpy change for? a)ammonia b)oxygen c)nitrogen monoxide d)water 4 NH 3 (g) + 5 O 2 (g) 4 NO(g) + 6 H 2 O(g) ΔH = -906 kJ

Chapter 9, Section 9.1 Do Worksheet BLM 9.1.6

Chapter 9, Section 9.2 Finding the value of energy changes experimentally: calorimetry Device: calorimeter The following diagrams show the principle behind calorimetry – note arrow directions

Chapter 9, Section 9.2 A simple calorimeter like the one you will use 2 nested styrofoam cups containing a measured volume of water sitting in a beaker so that it doesn’t fall over 3 rd styrofoam cup inverted on top with hole for thermometer (stirrer)

Chapter 9, Section 9.2 Assumptions in styrofoam cup calorimetry: Amount of energy transferred to cups and thermometer is small and can be ignored The system is isolated The solution produced has the same density and specific heat capacity as water The process occurs at constant pressure

Chapter 9, Section 9.2 The enthalpy change of a chemical reaction = energy lost or gained, and is indicated by the symbol ΔH Energy gained or lost by the water causes a temperature change and is indicated by the symbol Q In an ideal calorimeter ΔH = Q But recall: Therefore and calorimetry equation system calorimeter “water”

Chapter 9, Section 9.2 I will redo the example on page 354 using this formula limiting reagent, if not stated, or substance question asks about remember m c Δt is for the “water” and n (c v) for the CuSO 4 (aq) Since the temperature has gone up the process is exothermic Correct answer:

Chapter 9, Section 9.2 Practice Problem 9, page 355 Note that question asks for molar enthalpy of reaction for sodium n will be moles of sodium (question asks) Since temperature increases, answer is correctly expressed as Do Practice Problems 7, 10, 12, page 355

Chapter 9, Section 9.2 Investigation 9.A page 356 (goes with the questions you’ve been doing) Molar enthalpy of combustion: Investigation 9.B, page 357

Chapter 9, Section 9.2 Bomb Calorimetry: a bomb calorimeter is used to make accurate and precise measurements Not on diploma exam

Chapter 9, Section 9.2 Reaction takes place inside an inner container called the “bomb” that contains pure oxygen Chemicals are electrically ignited and heat is released to or absorbed from calorimeter water Calorimeter materials: stirrer, thermometer, containers are not ignored With calorimeter filled to a set level with water, all of their heat capacities are combined as shown: Not on diploma exam

Chapter 9, Section 9.2 Note that C contains the mass and specific heat capacity of each component of the calorimeter How do you know when to use bomb calorimeter equation Heat capacity of calorimeter Not on diploma exam

Chapter 9, Section 9.2 Look for: - words “bomb calorimeter” - no mention of the mass or volume of water - words “heat capacity” rather than “specific heat capacity” - units J/°C rather than J/g°C Question 2, Worksheet 46 Since temperature increases, answer is -286 kJ/mol Do rest of Worksheet 46 Not on diploma exam

Chapter 9, Section 9.2 More practice with WS 9.1.5

Chapter 9, Section 9.2 Review: page good questions: 1, 3, 4 (no actual calculation needed), 5c (data page 347), 6a (data page 347), 8, 10, 13, 15, 16, 17, 18, 19, 21

Chapter 9, Section 9.2