HeatHeat SolidLiquidGas Heat = AMOUNT of internal energy Temperature = a MEASURE of the average molecular kinetic energy

10 g Pb T = 40 °C 100 g Pb T = 40 °C Both blocks are at the same temperature. Do they both contain the same amount of heat?

Which substance requires more heat to increase the temperature by 5 °C? Specific heat capacity (C p ): amount of heat(q) required to raise 1 g of substance by 1 °C C p (Pb) = J/g°C C p (paraffin) = 2.1 J/g°C Pb 100 g

How much heat is required by the 100 g candle to increase the temperature by 5 °C? C p (paraffin) = 2.1 J/g°C q = C p (mass)(  T) q = (2.1 J/g°C)(100 g)(5 °C) q = 1050 J q = C p (mass)(  T) 1050 J = (4.184 J/g°C)(100g)(  T)  T = 2.5 °C If the same amount of heat was used to heat 100 g of water [C p (liquid water) = J/g°C], what would be the  T of the water? decreases increases For the same amount of heat and mass,  T decreases as the specific heat of the substance increases

Baltimore Shot Tower ft 100 kg Pb If the temperature of the lead is 327°C before it hits the water, what is the final temperature of the lead after hitting the water? C p (Pb) = 0.13 J/g°C C p (H 2 O) = 4.18 J/g°C q = mC p  T  T = T f - T i T f = 93 °C -q Pb = q H2O -(1x10 5 g)(0.13 J/g°C)(T f – 327°C) = (1x10 4 g)(4.18 J/g°C)(T f – 20°C) 10 kg H 2 O T i = 20 °C

Melting one 14-gram Al soda can requires 5.55 kJ of energy. What is its molar heat of fusion? 105,000 cans are recycled in the US every minute. How many kJ/s are being used in recycling Al cans? 105,000 cans are recycled in the US every minute. How many kJ/s are being used in recycling Al cans? That’s equivalent to burning 2300 food Calories/s!

18 g H 2 O = 1 mole H 2 O - 10 °C 90 °C Experiment: Heat two beakers containing 18 g of water at the same rate, and monitor their temperatures. Question: Will their temperatures increase at the same rate? 0 °C 100 °C

Experiment: Heat two beakers containing 18 g of ice and water at the same rate, and monitor their temperatures. Question: Will their temperatures increase at the same rate? Answer: It takes twice as long to increase the temperature of the liquid water by 10 °C than it does to increase the temperature of the ice by the same amount.

Temperature (°C) Heating curve of water solid warming solid + liquid present liquid warming liquid + gas present Gas warming Heat (kJ/s)

Temperature (°C) Heating curve of water melting/freezing point boiling/condensation point Temperature is constant during phase transitions!! All heat energy goes to changing the state of matter. Heat (kJ/s)

Temperature (°C) Heating curve of water  H fus = the amount of heat needed to covert a solid into its liquid phase  H fus  H vap (heat of fusion) (heat of vaporization)  H vap = the amount of heat needed to convert a liquid into its gaseous phase

Temperature (°C) Heating curve of water Heat (kJ/s) H 2 O:  H fus = 6.01 kJ/mol  H vap = 40.7 kJ/mol  H fus = 20.2 kJ/mol  H vap = 10.3 kJ/mol H 2 PEw: A greater  H fus = more time to melt And vice versa

Heating Curve Wrap Up: The specific heat capacity (C p )of a substance determines the temperature change observed when heat is added or withdrawn from the substance. Temperature is INVARIANT during phase transitions. The amount of heat required to convert one mole of the substance from one phase to another is its molar enthalpy of transition (  H fus,  H vap,  H sub ). The amount of heat given off for one mole of a substance during a phase transition while cooling is its molar enthalpy of transition (  H cond,  H sol,  H dep ). The shape of a heating curve depends upon the heating rate, specific heat capacities of the phases involved, and the enthalpies of transition. What is the sign for all three?+H+H -H-H

2 AlBr Cl 2 2 AlCl Br 2 Energy 2 AlBr Cl 2 2 AlCl Br 2  H rxn = Heat content of products – heat content reactants  H rxn < 0 Reaction is exothermic But how do we determine the heat content in the first place?

Heat of formation,  H f The  H f of all elements in their standard state equals zero. The  H f of all compounds is the molar heat of reaction for synthesis of the compound from its elements  H f (AlBr 3 ): 2 Al + 3 Br 2 2 AlBr 3  H rxn = 2  H f (AlBr 3 )  H rxn 2  H f (AlBr 3 ) = Since the  H rxn can be used to find  H f, this means that  H f can be used to find  H rxn WITHOUT having to do all of the calorimetric measurements ourselves!! The Law of Conservation of Energy strikes again!!

Hess’s Law:  H rxn =  H f (products) –  H f (reactants) 6 CO 2 (g) + 6 H 2 O (l)C 6 H 12 O 6 (s) + 6 O 2 (g)  H rxn = [  H f (C 6 H 12 O 6 ) + 6  H f (O 2 )] – [6  H f (CO 2 ) + 6  H f (H 2 O)] From  H f tables:  H f (C 6 H 12 O 6 ) = kJ/mol  H f (CO 2 ) = kJ/mol  H f (H 2 O) = kJ/mol  H rxn = [-1250 kJ/mol] – [6( kJ/mol) + 6( kJ/mol)]  H rxn = kJ/mol

H 2 O (l)H 2 O (g) Energy H 2 O (l) H 2 O (g)  H vap = kJ/mol Water will spontaneously evaporate at room temperature even though this process is endothermic. What is providing the uphill driving force?

a measure of the disorder or randomness of the particles that make up a system Water will spontaneously evaporate at room temperature because it allows the disorder of the water molecules to increase. The entropy, S, of gases is >> than liquids or solids. If S products > S reactants,  S is > 0 Predict the sign of  S: ClF (g) + F 2 (g)ClF 3 (g)  S < 0 CH 3 OH (l)CH 3 OH (aq)  S > 0

Are all +  S reactions spontaneous? 2 H 2 O (l) 2 H 2 (g) + O 2 (g)  S is large and positive… …but  H is large and positive as well. Gibb’s Free Energy,  G, allows us to predict the spontaneity of a reaction using  H AND  S.

2 H 2 O (l) 2 H 2 (g) + O 2 (g) What is  G for this reaction at 25  C?  H rxn =  H f (products) –  H f (reactants)  H rxn = [2(0) + 0] - 2( kJ/mol ) = kJ/mol  S rxn = [2( J/molK ) J/molK ] - 2(69.91 J/molK )  S rxn =  S f (products) –  S f (reactants)  S rxn = J/molK = kJ/molK  G rxn =  H rxn – T  S rxn = kJ/mol - 298K( kJ/molK )  G rxn = kJ/mol

2 H 2 O (l) 2 H 2 (g) + O 2 (g)  G rxn =  H rxn – T  S rxn = kJ/mol - T( kJ/molK ) What is the minimum temperature needed to make this reaction spontaneous? Set  G rxn = 0 to find minimum temperature 0 = kJ/mol - T( kJ/molK ) T = ( kJ/mol)/( kJ/molK) = 1751 K T > 1479  C