Thermochemistry. The study of energy transfers and chemical reactions

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1 Thermochemistry The study of energy transfers and chemical reactions

2 Energy Energy is the ability to do work Work = Force x distance SI unit is the Joule (J) 1000 J = 1 kj other unit: calorie (cal) 1000 cal = 1 kcal = 1 Cal (food) 1 cal = J

3 A few terms... System: what we are observing beaker, battery, cell, atmosphere, etc. Surroundings: everything outside of the system Boundary: a separation between system and surroundings (real or imaginary)

4 A few terms... If a system is prevented or hindered from transferring heat past the boundary, it is insulated

5 A few terms... the state of the system is its temperature (T), pressure (P), volume (V), concentration, phase (s,l,g,aq) a change in any of these ( T, P, V, etc.) is a change in the state of the system

6 There are many forms of energy Electrical electromagnetic nuclear heat chemical mechanical All are interconvertible chemical reactions usually involve at least heat and chemical

7 There are two types of energy Kinetic KE= 1 / 2 mv 2 energy due to motion HEAT Potential depends on position or composition ATTRACTIVE FORCES

8 Potential Energy The attraction between two objects may be gravitational, electrostatic, magnetic, or in the nucleus, the strong force PE is the energy added to the system whenever work must be done to change the distance between two objects

9 Potential Energy Whenever attractions are allowed to form, energy is RELEASED A decrease in PE Whenever attractions are broken, energy must be ABSORBED An increase in PE

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11 There are three forms of Potential gravitational Energy Depends on your position elastic Based on the degree of compression chemical Based on the arrangement of atoms within a compound

12 Kinetic Energy Heat energy is a form of kinetic energy hotter faster

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14 Heat flow Heat: the Energy that flows between any two objects at different temperatures that are in contact with each other. From the higher T object to the lower T object Two objects at the same T are said to be at thermal equilibrium.

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16 What is heat, anyway? The amount of heat is equal to the total KE of all the molecules of a system the degree of heat, or the temperature, is related to the average KE of the molecules of a system

17 All forms of energy are interconvertible because heat is easy to measure, E is usually considered to be heat lost or gained by a system the symbol for heat is q

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19 Exothermic reactions Reactions that involve the release of energy are called exothermic energy (heat) flows from the system to the surroundings ex. combustion, luminescence may feel hot, because your hand is part of the surroundings

20 Endothermic reactions Reactions that absorb energy are called endothermic energy (heat) flows from the surroundings to the system cold packs, photosynthesis may feel cold, because your hand, part of the surroundings, is losing heat to the system

21 Heat Flow Molecules that compose matter are in constant motion Translational Rotational vibrational Energy may be transferred from one object to another during collisions between the molecules

22 Heat flow There are three possible methods for heat transfer: 1. Conduction 2. Convection 3. Radiation

23 Conduction The transfer of heat by collisions between the particles in a substance (especially a solid) Solids made of particles with loosely held electrons are good conductors ex: metals

24 Convection The transfer of heat in a fluid (gas or liquid) by means of currents in the heated fluid. As the more energetic ( hotter ) molecules move throughout the fluid, they transfer heat to surrounding molecules via collisions Ex: water in a pot, warm air in a room

25 Radiation The transfer of heat by way of electromagnetic radiation Also known as radiant energy Ex: energy from the sun or a heat lamp Often in the infra-red part of the spectrum NOT nuclear radiation

26 Heat flow A + or - sign is used to show the direction of heat flow exothermic changes heat leaves the system -q endothermic changes heat is gained by the system +q

27 Thermal Properties Properties of a substance that relate to its ability to absorb or release heat without chemically changing heat capacity specific heat

28 Heat capacity (C) The amount of heat that must move into an entire object to raise its temperature by 1 o C units are J/ o C or cal/ o C specific for that object q=c T

29 Specific heat (s) The amount of heat that must move into 1.0g of a substance to raise its temperature 1 o C units are J/g o C or cal/g o C specific for that substance, regardless of the amount q=ms T

30 q=ms T m (mass) and s (specific heat) are ALWAYS POSITIVE numbers! The sign on T determines the sign on q T, T is (+), q is (+) heat flows into the system (endothermic) T, T is (-), q is (-) heat flows out of the system (exothermic)

31 The unit for heat called the calorie comes from the specific heat of liquid water it takes 1.00 cal of heat to raise the T of 1.00 g of H 2 O (l) by 1 o C remember, 1 cal = 4.184J s H2O = 1.00 cal/g o C = J/g o C

32 The larger the specific heat,... The harder it is to heat or cool a substance The more heat that must flow to cause a T the specific heat of water is relatively large it takes a lot of energy to heat up H 2 O ( the T) the H 2 O retains the heat well biological, climactic effects

33 q=ms T Another perspective Heat flow = energy = q instead of energy, think money The amount of a T you can buy depends on how much q you can spend The specific heat (s) is like the cost/item The larger the s, the more expensive the T will be The more q it will cost you to buy the same size T

34 Example problem Cu has a specific heat of s = J/g o C. How much heat is needed to be absorbed by 27g of copper to raise the T from 50 o C to 70 o C? q = ms T q = (27g)(0.387 J/g o C)(+20 o C) q = +209 J

35 Example problem How much heat is released if 250mL of water cools from 80 o C to 25 o C? (1.0 g = 1.0 ml) q = ms T q=(250g)(4.184 J/g o C)(-55 o C) q = -57,530 J = kj

36 Example problem What is the final T of 400mL of H 2 O at 30 o C if it absorbs 5 kj of heat? q = ms T +5000J=(400g)(4.184 J/g o C)( T) T = +3 o C T f = 33 o C

37 calorimetry A calorimeter is a device used to measure the T for a reacting system Often, filled with water to absorb or release heat The apparatus (and any water within in) are part of the SURROUNDINGS

38 Because the heat is absorbed by or released mostly from the water, and a bit from the calorimeter, measuring T of the water allows one to measure q for the reaction Heat out of system = Heat into surroundings q rxn = -(q H2O + q cal ) q rxn = -(ms T + C T )