Thermal Energy & Heat

Temperature Temperature  measure of the average KE of the particles in a sample of matter

Temperature Conversions o C to o F: o F = 9/5 o C + 32 o F to o C: o C = 5/9( o F – 32) o C to K: K = o C K to o C: o C = K – 273

Going from Celsius to Fahrenheit Convert 37 o C to o F. o F = 9/5 o C + 32 o F = 9/5(37 o C) + 32 = = 98.6 o F

Going from Fahrenheit to Celsius Convert 68 o F to o C o C = 5/9( o F – 32) o C = 5/9(68 – 32) = 5/9(36) = 20 o C

Going from Celsius to Kelvin Convert 100 o C to K K = o C K = = 373 K

Going from Kelvin to Celsius Convert 310 K to o C o C = K – 273 o C = 310 – 273 = 37 o C

Thermal Energy  the total energy of the particles in a material  KE - movement of particles  PE - forces within or between particles due to position  depends on temperature, mass, and type of substance

Thermal Energy Which beaker of water has more thermal energy?  B - same temperature, more mass 200 mL 80ºC A 400 mL 80ºC B

Heat Transfer Heat  thermal energy that flows from a warmer material to a cooler material Like work, heat is...  measured in joules (J)  a transfer of energy

Heat Transfer Why does A feel hot and B feel cold? 80ºC A 10ºC B  Heat flows from A to your hand = hot.  Heat flows from your hand to B = cold.

Heat Transfer Heat is always transferred from hot to cold. Insulators slow the transfer of heat due to air pockets. Conductors easily allow the transfer of heat, like metals. Heat is transferred by conduction, convection, and radiation.

Conduction Heat is transferred due to objects touching each other or through collisions. Occurs best in solids. Heat continues to be transferred until both objects reach the same temperature, called a thermal equilibrium.

Convection Transfer of heat through a liquid or gas through moving currents, called convection currents. The cause of wind and weather.

Radiation Transfer of heat through electromagnetic radiation (light from stars or light bulbs). Transferred in all directions. No contact required! Dark or dull objects absorb more than light or shiny objects do.

Heat Transfer Specific Heat (C p )  amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin  units: J/(kg·K) or J/(g·°C)

Heat Transfer Which sample will take longer to heat to 100°C? 50 g Al50 g Cu Al - It has a higher specific heat. Al will also take longer to cool down.

Heat Transfer Q = m   T  C p Q:heat (J) m:mass (kg)  T:change in temperature (K or °C) C p :specific heat (J/kg·K or J/g.o C)  T = T f - T i – Q = heat loss + Q = heat gain

Heat Transfer Calorimeter  device used to measure changes in thermal energy Coffee cup Calorimeter  in an insulated system, heat gained = heat lost

Heat Transfer A 32-g silver spoon cools from 60°C to 20°C. How much heat is lost by the spoon? GIVEN: V m = 32 g T i = 60°C T f = 20°C Q = ? C p = 235 J/kg·K WORK: F Q = m·  T·C p m = 32 g = kg  T = 20°C - 60°C = – 40°C  T = -40 o C = 233 K SQ = (0.032kg)(233K)(235J/kg·K) A Q = 1752 J

Heat Transfer How much heat is required to warm 230 g of water from 12°C to 90°C? GIVEN: V m = 230 g T i = 12°C T f = 90°C Q = ? C p = J/g· o C WORK: F Q = m·  T·C p m = 230 g  T = 90°C - 12°C = 78°C SQ = (230 g)(78 o C)(4.184 J/g· o C) AQ = 75,061 J

When energy is changed from one form to another, all of the energy can be accounted for. This is called the Law of Conservation of Energy.

Heat is energy. If heat is released by a chemical system, an equal amount of heat will be absorbed by the surroundings

When your body breaks down sugar completely, the body releases the same amount of heat as compared to burning the same amount of sugar in a flame.

Heat is transferred due to a temperature difference. Faster molecules speed up the slower molecules. This slows the faster molecules.

In an endothermic process, the system absorbs heat from the surroundings. A process that absorbs heat is a(n) endothermic process. In an exothermic process, the system gives off heat to the surroundings.

If you were to touch the flask in which an endothermic reaction was occurring, the flask would probably feel cooler than before the reaction started. In an exothermic reaction, the energy stored in the chemical bonds of the reactants is greater than the energy stored in the bonds of the products.

As perspiration evaporates from your skin, your body is cooled. With respect to your body, this process is said to be exothermic.

A calorie is the quantity of heat that raises the temperature of 1 gram of pure water 1  C. 1 Calorie = 4.18 kJ

How many kJ of energy can be released by a banana containing 150 Cal?

An object's heat capacity is the amount of heat required to change the temperature of an object by exactly 1°C.

Chem Notes 17B The specific heat capacity or specific heat is the heat divided by the mass and temperature change.

Q = m   T  C p C p or c = specific heat Q= heat m= mass  T=change of temperature

The temperature of an 10. g sample of metal changed from 25°C to 50°C when it absorbed 500. J of heat. What is the specific heat of this sample?

V c=? m = 10. g  T = 50°C-25°C = 25°C Q = 500. J F Q = m   T  C p S A

Determine the specific heat of a material if an 18 g sample absorbed 75 J as it was heated from 15°C to 40.°C.

V c = ? m = 18 g  T = 40.°C-15°C = 25°C Q = 75 J FQ = m·  T·Cp SA

What is the specific heat of a substance if cal are required to raise the temperature of a 300.-g sample by 20.C?

V c = ? Q = cal m = 300. g  T = 20.C F Q = m·  T·Cp S A

What is the amount of heat required to raise the temperature of g of aluminum by 10.C? (specific heat of aluminum = 0.21 cal/g  C)

V Q = ? m = 200. g c = 0.21 cal/g  C  T = 10.C F Q = m·  T·Cp S (200. g)(0.21 cal/g  C)( 10.C) = A 420 cal

Water has the highest specific heat of all common substances.

Chem Notes 17C The symbol  H stands for the heat of reaction for a chemical reaction. This is also called the change in enthalpy.  H = Q

The heat content of a system is equal to the enthalpy only for a system that is at constant pressure.

Calorimetry depends on the law of conservation of energy.

If 27.0 mL of water containing HCl is mixed with 28.0 mL of water containing NaOH in a calorimeter such that the initial temperature of each solution was 24.0°C and the final temperature of the mixture is 33.0°C, how much heat (in kJ) is released in the reaction? Assume that the densities of the solutions are 1.00 g/mL.

V  H = ? m = 27.0 g g = 55.0 g  T = 33.0°C °C = 9.0°C c = kJ/g°C F  H =m  Tc S  H = (55.0g)(9.0°C)(.00418kJ/g°C) A

A lead mass is heated and placed in a foam cup calorimeter containing 40.0 mL of water at 17.0°C. The water reaches a temperature of 20.0°C. How many joules of heat were released by the lead?

V  H = ? m = 40.0 g  T = 20.0°C °C = 3.0°C c = 4.18 J/g°C F  H =m  Tc SA

Chem Notes 17D Fusion solid  liquid Solidification liquid  solid

Molar heat of fusion – heat required to melt a mole =  H fus Molar heat of solidification – heat required to freeze a mole =  H solid  H fusion = -  H solid

Vaporization liquid  vapor Condensation vapor  liquid

Molar heat of vaporization – heat required to vaporize a mole =  H vap Molar heat of condensation – heat required to condense a mole =  H cond  H vap = -  H cond

Given the equation I2(s) kJ  I 2 (g)  H = kJ

How much heat, in kJ, is released when 108 g of water at O°C freezes to ice at O°C if H solid for water = kJ/mol?

Change grams to moles and then multiply by the kJ needed for 1 mole.

How much heat is released in the condensation of 27.0 g of steam at 100°C to water at 100°C if H cond for water = kJ/mol?

Change grams to moles and then multiply by the kJ per mole needed.

How many grams of ice at 0°C can be melted into water at 0°C by the addition of 75.0 kJ of heat? H fus for water = 6.01 kJ/mol

Chem Notes 17E The heat of solution is the amount of heat absorbed or released when a solid dissolves

If the molar heat of solution of NaOH is kJ/mol, how much heat (in kJ) will be released if g of NaOH are dissolved in water?

Hess's law makes it possible to calculate  H for complicated chemical reactions. Hess’s Law – If you add 2 or more thermochemical equations to give a final equation, then you also add the heats of reactions to give the final heat of reaction.

Given the equation C 2 H 4 (g) + 3O 2 (g) + 2H 2 O(l) kJ, how much heat is released when 8.00 g of O 2 react?

What is the heat of reaction (H) for the combustion (with O 2 ) of benzene, C 6 H 6 to form carbon dioxide and water? Write the balanced equation for the reaction.

Standard heats of formation: C 6 H 6 = kJ O 2 = 0.0 kJ CO 2 = kJ H 2 0 = kJ C 6 H 6 +(15/2) O 2  6CO H 2 0 H kJ + (15/2)(0.0 kJ) = 6( kJ) + 3( kJ) H = kJ