Don't forget to. Mastering Chemistry everyday! October 9 th, Next week in Lab: Next week in disc: 10/9/2014.

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1 Solutions of nitric acid, sodium bicarbonate, silver nitrate and iron (III) acetate are mixed. Write all NIE s. There are 4! H + (aq) + HCO 2 3 (aq) H 2 O(l) + CO 2 () (g) H + (aq) + C 2 H 3 O 2 (aq) HC 2 H 3 O 2 (aq) 2Ag 2+ (aq) + CO 3 2 (aq) Ag 2 CO 3 (s) 2Fe 3+ (aq) + 3CO 3 2 (aq) Fe 2 (CO 3 ) 3 (s) October 9 th, Chem 1A F Chem 1A F Don't forget to check with Mastering Chemistry everyday! Next week in disc: Monday / Tuesday Thermochemistry Part 1 (Chapter 6 stuff) Next week in Lab: Wednesday / Thursday Experiment 6: Analysis of Mg (read the lab carefully 1 st ) Quiz 6: gas/stoichiometry calcs, KMT, heat, work energy & MCAT Chem 1A F Chem 1A F

2 Last time... Chapter 5 Vapor Pressure problem Kinetic Molecular Theory Real Gasses Heat, energy and work Today: Chapter 6 The 1 st law Calorimetry Chem 1A F When it comes to your Homework: You know that... The due date is not the start date! Chem 1A F If you are at or below 65% of the total class points, you are in serious trouble at this time! Chem 1A F Chem 1A F

3 Relating ΔE to Heat and Work Δ E q+ w change in system energy heat lost or gained + by thesystem work done by or on the system When heat (q) enters the system from the surroundings, it is given a positive sign (+). When heat (q) leaves the system into the surroundings, it is given a negative sign ( ). When the surroundings does work on the system (w), it is given a positive sign (+). rgy the system s ener E Final E initial work in (w > 0) Energy in work and heat can Energy out E f > E i balance! E f < E i E initial Work work out (w < 0) q in Heat q out (q > 0) (q < 0) E Final When the system does work (w) on the surroundings, it is given a negative sign ( ). ΔE system > 0 (+) ΔE system 0 ΔE system < 0 ( ) Chem 1A F Chem 1A F Heat and the Specific Heat Capacity: When heat is absorbed or lost by a body, the temperature must change as long as the phase (s, g or l) remains constant. The amount of heat (q) transfer is related to the mass and temperature by: q heat lost or gained () m mass of substance (g) q m C ΔT Does it matter if we calculate a temperature change in Kelvin or degrees C? Recall that ΔT T f T in T f T 25.0 o in C let T in 25.0 o C and T f 50.0 o C 50.0 o C 50.0 o C K (25.0 o C ) K C the Specific Heat Capacity of a compound o g C or K ΔT is the temperature change in degrees Celsius or Kelvins ΔT 25.0 o C ΔT 25.0 K The are the same! Chem 1A F Chem 1A F

4 Determine the final temperature of a 25.0g block of metal that absorbs 255 cal of energy. The initial temperature of The specific heat 2.72 the block was 17.0 o C capacity of the metal is: g K rearranging: T f f Δ m C ( T T ) q m C ΔT +255 cal cal 25.0g 2.72 g o Remember, it does not matter if it is o C or K!!! T f C q +T m C in o C f in 32.7 o C T f > T in as expected q is positive (+) since the block absorbs heat Work: Energy is the capacity to do work. Work equals a force applied through a distance. When you do work, you expend energy. When you push down on a bike pump, you do work. P force area force 2 x change in volume ΔV x 3 F x 3 P ΔV x F x 2 P ΔV work Therefore work is equal to a change of volume at constant pressure Chem 1A F Chem 1A F Pressure/Volume work and Units: Recall that the gas law constant an be written in two forms: Latm R R mol K mol K Setting the two equal: Latm molk molk 1 L atm Chem 1A F A balloon expands from an initial volume of 1.75 L to 3.45 L at a constant pressure of 755 torr at constant temperature. Calculate the work done by the system (by the balloon in.) Since the balloon expands against the surroundings (the atmosphere) the system does work on the surroundings so the sign of work is negative ( ). Final volume w (P ΔV) initial volume Chem 1A F

5 A balloon expands from an initial volume of 1.75 L to 3.45 L at a constant pressure of 755 torr at constant temperature. Calculate the work done by the system (by the balloon in ) w P ΔV 1 atm w P Δ V 755 torr ( 3.45L 1.75) 1.69 Latm 760 torr L atm L atm A balloon expands from an initial volume of 1.75 L to 3.45 L at a constant pressure of 755 torr at constant temperature. Calculate the work done by the system (by the balloon in ) w P ΔV 171 What would the change in energy for the system (balloon) be if 0.45 k of energy were added to the balloon as it expanded? (+) because q was added 3 10 Δ E q + w k k Chem 1A F Chem 1A F Coffee Cup Calorimetry Thermometer immersed in the water. water in cup anything in the q water is the system Coffee Cup Calorimetry Thermometer immersed in the water. water in cup anything in the water is the system glassstir stir rod (to mix things) surroundings glassstir stir rod (to mix things) surroundings Water inside cork lid q water m water C water ΔT water insulation creates an Isolated system Any temperature change in the water is a result of action by the system. If q leaves the system, ΔT water > 0 If q enters the system, ΔT water < Chem 1A F Water inside cork lid insulation creates an Isolated system q system + q water 0 q system q water Therefore, ΔT water yields information about the energy change for the system! Chem 1A F

6 Because the coffee cup is insulated (isolated system) What happens in the coffee cup Stays in the coffee cup! No heat loss to the surroundings. No mass loss to the surroundings Chem 1A F A g cube of steel initially at 90.0 o C is placed into a Coffee Cup calorimeter containing ml of water. The water is initially at 22.0 o C. At thermal equilibrium the temp. is 26.5 o C? What was the specific heat capacity of the cube? Since the steel at a higher temperature than the water, heat flows from the block to the water. We define the water is the surroundings, that leaves the block as the system. That means: q steel + q water 0 q steel q water Chem 1A F A g cube of steel initially at 90.0 o C is placed into a Coffee Cup calorimeter containing ml of water. The water is initially at 22.0 o C. At thermal equilibrium the temp. is 26.5 o C? What was the specific heat capacity of the cube? To solve the problem, we start with: q m C ΔT q steel + q water 0 Given information: T f 26.5 o C (for both) T initial (Steel) 90.0 o C T initial (H 2 O) 22.0 o C m water 250.0g m steel 125.0g C HO 2 Look up: gc o What do we need? C steel? Expanding both q steel and q water : q steel q water msteel Csteel Δ Tsteel mwater Cwater ΔTwater Δ T T T final initial T final Thermal equilibrium Temperature Chem 1A F Chem 1A F

7 A cube gram of steel initially at 90.0 o C is placed into a Coffee Cup calorimeter containing ml of water. The water is initially at 22.0 o C. At thermal equilibrium the temp. is 26.5 o C? What was the specific heat capacity of the cube? msteel Csteel Δ Tsteel mwater Cwater ΔTwater Entering the values: o o o o ( ) ( ) 125.0g Csteel 26.5 C 90.0 C 250.0g 26.5 C 22.0 C gc Solving: o o ( ) o o ( ) g 26.5 C 22.0 C gc 125.0g 26.5 C 90.0 C 2 sig figs! Csteel 0.59 g o C Chem 1A F I have placed an example that covers bomb calorimetry in the notes. Please read this in addition to your text and HW reading. You will see additional problems in discussion next week Chem 1A F Measuring the energy associated with a chemical reaction: Bomb or constant volume Calorimetry. Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived dfrom the Latin Lti word calor, meaning heat. Bomb Calorimetry (Constant Volume Calorimetry) The energy changes for reactions (ΔE Reaction ) are measured using a device called a Bomb Calorimeter. The bomb loaded with a sample is immersed in water enclosed in thermally isolated container. The combustible sample is ignited with excess O Chem 1A F Chem 1A F

8 Bomb Calorimetry (Constant Volume Calorimetry) When the sample burns inside of the bomb, the heat from the reaction passes through the bomb, then through the water where the temperature change is measured by a thermometer. The temperature change is measured: Since the bomb s mass is fixed, the heat absorbed by the bomb is given by: q bomb C bomb ΔT C bomb has units of C o ΔT T final T initial q The mass of the bomb (m) is within the heat capacity of the bomb Chem 1A F Chem 1A F A 1.00 g sample of octane is burned in a bomb calorimeter unit that contained the reaction. Octane, the primary component of gasoline combusts by the reaction: 2C 8 H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18H 2 O(l) The temperature of the water rises to o C from o C when the octane is reacted. If the heat capacity of the bomb is 5858 / o C, calculate the ΔE of reaction per mole of octane. The reaction in the bomb, the bomb itself and the water surrounding the bomb represent an closed system. Therefore one can write: q RXN + q bomb 0 A 1.00 g sample of octane is burned in a bomb calorimeter unit that contained the reaction. Octane, the primary component of gasoline combusts by the reaction: 2C 8 H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18H 2 O(l) The temperature of the water rises to o C from o C when the octane is reacted. If the heat capacity of the bomb is 5858 / o C, calculate the ΔE of reaction per mole of octane. The reaction in the bomb, the bomb itself and the water surrounding the bomb represent an closed system. Therefore one can write: q RXN + q bomb Chem 1A F Chem 1A F

9 Bomb Calorimetry (Constant Volume Calorimetry) Since the bomb is a closed system, energy must be conserved: q rxn + q bomb b 0 A 1.00 g sample of octane is burned in a bomb calorimeter unit that contained the reaction. The temperature of the water rises to o C from o C when the octane is reacted. If the heat capacity of the bomb is 5858 / o C, calculate the heat of reaction per mole of octane. q RXN C bomb ΔT water q rxn q bomb q RXN o C C ( ) o q bomb C bomb ΔT q RXN or 48.1 k Therefore: q C ΔT rxn bomb Chem 1A F Chem 1A F A 1.00 g sample of octane is burned in a bomb calorimeter that contains 1.20 kg of water surrounding the bomb. The temperature of the water rises to o C from o C when the octane is reacted. If the heat capacity of the bomb is / o C, calculate the ΔE of reaction per mole of octane. 2C 8 H 18 (l) + 25 O 2 (g) 16 CO 2 (g) + 18H 2 O(l) Heat transferred per mole q V : q RXN or 48.1 k 48.1 k 3 k ΔE 1 mol C8H g C mol 8H g rxn Enthalpy: Enthalpy, H is the heat transferred between the system and surroundings under conditions of constant pressure. H q p the subscript p indicates constant pressure The change in enthalpy is the change in internal energy at constant pressure, so we can write: ΔH ΔE p Chem 1A F Chem 1A F

10 Enthalpy: Enthalpy, H is the heat transferred between the system and surroundings under conditions of constant pressure. if no PV work is done by the system, ΔV V 0 H q p H E+ W H E+ PV Δ HΔ ( E+ PV) Δ E+ PΔV 0 ΔH ΔE p the subscript p indicates constant pressure the change in enthalpy is the change in internal energy at constant pressure Chem 1A F When the system gains heat from the surroundings Δ H, is positive (+) and the process is said to be Endothermic When the system gives heat to the surroundings Δ H, is negative ( ) and the process is said to be Exothermic Chem 1A F Enthalpy is a State Function The results, Final initial are the same! Since individual Enthalpies cannot be directly measured, we only deal with enthalpy changes No matter which path is taken: (ΔH H final H in ) For a system the overall change in Enthalpy is path independent Chem 1A F Enthalpy Conditions: Since Enthalpies are state functions, one must specify the conditions at which they are measured. H(T,P): Enthalpy is a function of temperature and pressure. H o indicates that the Enthalpy is taken at Standard State conditions. Standard State Conditions are defined as: 1 atm 760 mm Hg or 760 torr & K or 25 o C Chem 1A F

11 Enthalpies and Chemical Reactions All chemical processes are accompanied by energy changes: If a reaction gives off energy (heat), it is an exothermic reaction. Reactants Products + Energy Enthalpies and Chemical Reactions: ΔH rxn Reactants (initial) Products (final) ΔH H final H initial Enthalpy of reaction ΔH rxn H products H reactants If a reaction absorbs energy (heat), it is an endothermic reaction. H (Products) H (Reactants) Energy + Reactants Products Energy can be treated as part of the reactants or products! Energy ΔH rxn > 0 ΔH rxn < 0 H (Reactants) Endothermic H (Products) Exothermic Chem 1A F Chem 1A F How many k of energy are released when g of methane, CH 4 (g) is combusted? How many hours would k power a 100 W light bulb if one could use all of this energy? CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) g mols molar Reaction mass enthalpy mol CH g CH g +802 k 1mol CH 4 ΔH r 802 k exothermic reaction, positive energy out! k k 10 3 k 100 W 1 s s min hr 60s 60min 17.8 hrs Chem 1A F Chem 1A F