Thermochemistry - Penn State Universitycourses.chem.psu.edu/chem110/summer/Lectures/Web pdf/CHAPT5.pdf · Mary J. Bojan Page 1 Chapter 5 Thermochemistry Energy • kinetic • potential

Mary J. Bojan Page 1 Chapter 5

Thermochemistry Energy

kinetic potential

First Law of Thermodynamics E = q + w

Enthalpy H Thermochemical Equations H for chemical reactions Calorimetry, Heat Capacity Hesss Law Heat of formation Hf Standard State H f Foods and Fuels Thermochemistry:

Study of energy changes in chemical processes

2H2 + O2 2H2O + energy


ENERGY Energy can be converted from one form to

another

Kinetic Energy energy of motion Mechanical moving mass (1/2mv2) Electrical moving charge Light photons Sound molecules moving

uniformly Heat molecules moving

randomly Potential Energy stored energy Mechanical mass in a place where

a force can act Chemical bonds Nuclear binding energy

Chemical energy the Potential Energy associated with bonding

HH + HH + O=O O

HH +

OH

H 3 bonds 4 bonds


Energy Units

SI Unit: ??? What is the SI Unit of energy? a) Cal b) kcal c) ergs d) joules e) BTUs


ENERGY but total energy remains constant.

First Law of thermodynamics: Law of conservation of energy Energy lost by the SYSTEM is gained by the SURROUNDINGS. The system: what you are interested in

(a chemical reaction) Surroundings: everything +w else

E +q E


Changes in Energy E = internal energy:

the capacity to do work or transfer heat. E =Efinal Einitial = q + w

w= work= action of force through a distance

(often PV) work done to the system (+)

q = heat (thermal energy) = energy transferred due to

a difference in temperature. heat added to the system (+)

energy

System Surroundings E = exothermic energy

Surroundings System E = + endothermic


Energy Changes - Heat and Work A B Dry ice (CO2) is heated to room temperature at constant P or at constant V What are the signs of q and w for each? Does Esystem increase or decrease? How do you know? Which system has higher E at the end? Where does the energy come from?

w = P!V

+q +q

!V = 0


90C H2O 100ml cool 25C A H2O B H2O heat Which one has more energy? 0C H2O


State function: a function whose value does not depend on the pathway used to get to the present state:

State Function: only depends on the current state (composition, T,P): does not depend on past history

Chemical Reaction

A

A A

B B

C

D

E

B

elevation


State Functions

State functions are written as uppercase letters (E, H, P, V, T, S) q and w are not state functions but E (= q + w) is a state function Changes in state functions are path-independent:

reactants

products

1

2 E


Chemical Reactions

When natural gas burns: CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

Reaction produces heat and light

Is energy conserved? What are the signs of q and w for the overall reaction? Where does the energy come from?

Is energy stored or released when bonds are broken?


Relationship between Energy (E) and Enthalpy (H)

Energy transferred when V is constant

= E Usually run chemical reactions at constant

pressure (atmospheric).

E = q + w PV = work done on system at constant P

arises from expansion or contraction of the system: V = volume change

of system

E = qp PV P = constant qp = E + PV H

H = enthalpy

Energy transferred when P is constant

(=qp) = H

For many chemical processes, PV is small and

E H Like E, H is a state function


Thermochemical Equations

A balanced chemical equation that also includes the energy change

H2(g) + 1/2 O2(g) H2O(g) H = 241.8kJ H = enthalpy: heat given off or absorbed in the reaction

ENTHALPY 1) Enthalpy is an extensive property. 2) H for a reaction is equal in magnitude

and opposite in sign to H for the reverse reaction.

3) H for a reaction depends on the states

of reactants and products (gas, liquid, solid).


ENTHALPY OF REACTION

H = H(products) H(reactants) If H0 (+) endothermic

(heat absorbed) Ba(SCN)2(aq) + 2NH3(aq)+10H2O(l) H H = + Ba(OH)28H2O(s) + 2NH4SCN(s)


H2(g) + 1/2 O2(g) Hrxn = 241.8kJ H2O(g) H2O(l) 5.1 Is this reaction

1 exothermic 2 endothermic

5.2 How much heat is given off per mole of H2?

1 241.8 kJ 2 120.9 kJ 3 not enough information

5.3 How much heat is given off per mole of O2?

1 483.6 kJ 2 241.8 kJ 3 120.9 kJ 4 not enough information


H2(g) + 1/2 O2(g)

Hrxn = 241.8kJ H2O(g) H2O(l) 5.4 If I convert water to H2 + O2, the reaction

will be ___________

1 exothermic 2 endothermic

5.5 How much heat will be needed to convert

9g of water into H2 + O2? 1 483.6kJ 2 241.8 kJ 3 120.9 kJ


H2(g) + 1/2 O2(g) Hrxn = 241.8kJ H2O(g) H2O(l) 5.6 How much heat will be given off if liquid

water is formed instead of gaseous water. Hvap of water = 44kJ/mol

1 241.8kJ 2 +241.8kJ 3 285.8kJ 4 44kJ

5.7 How much heat will be given off if 10g of H2 is consumed?

1 24.18kJ 2 241.8kJ 3 1209 kJ 4 2418 kJ


CALORIMETRY Experimental measure of heat flow (used to determine Hrxn)

q = C m T q = heat flow C = specific heat (heat capacity per gram) m = mass T= Tfinal - Tinitial For H2O: C = 4.184 J/g C

Molar heat capacity = 75.2 J/mole C

Note: H2O is usually part of the surroundings

qsurr = Csurr m T


Mix 50ml of 1M NaOH+50ml of 1M HCl What is H rxn ? =

1. Write the balanced reaction. 2. Ti = Tf = T = Tf Ti = 3. Is qp (H rxn) positive or negative? 4. qp =H rxn= CmT

V=100ml, assume d = 1g/ml Then: m = (100ml)(1g/ml) = 100g

qp =(4.184J/goC)(100g) T


HESSS LAW

H for a sum of steps is the same as H for the overall process.

True because H is a state function. C H2 Hrxn B

H1 A A B H1 B C H2 A+B B + C H1 + H2= Hrxn


Example: Given the following information A H2(g) + F2(g) 2HF(g) HA = 537kJ B 2H2(g) + O2(g) 2 H2O(g) HB = 572kJ Determine H for the reaction: C 2F2(g) + 2H2O(g) 4HF(g) + O2(g) HC =? IDEA: find combinations of reactions

such that

n A + m B = C then

n HA + m HB = HC Here: 2xA 2H2(g) + 2F2(g) 4HF(g) H = 2(537kJ) 1xB 2 H2O(g) 2H2(g) + O2(g) H = (572kJ) _________________________________________________________________________________________________

2F2(g) + 2H2O(g) 4HF(g) + O2(g) HC = 2(537kJ) 1(572kJ) = 502kJ


Given the following information: H 2SO2(g)+ O2(g) 2SO3(g) 196kJ 2S(s) + 3 O2(g) 2SO3(g) 790kJ What is Hrxn for

S(s) + O2(g) SO2(g) 1. + 986 kJ 2. 986 kJ 3. 594 kJ 4. + 594 kJ 5. 297 kJ


Heat of Formation

Hf enthalpy of formation heat given off (or absorbed) when elements combine to give a compound

combine Elements Compounds Hf H f standard enthalpy of formation

heat given off (or absorbed) when all substances are in their Standard State

STANDARD STATE

P = 1 atm T = 25C (298K) most stable state (gas/liquid/solid)

For an element in its standard state: H f = 0 (by definition)


For which of the following reactions (at 25 C and 1 atm) is Hrxn = H f ? a) H2(g) + F2(g) 2HF(g) b) NO(g)+ 1/2 O2(g) NO2(g) c) 2C(graphite)+3H2(g)+1/2O2(g)C2H5OH(l) d) Pb(s) + Cl2(g) PbCl2(s) e) S(s) + O3(g) SO3(g) f) Br2(l) Br2(g)


Standard Enthalpy of a reaction: H rxn

Heat of reaction (Hrxn) when all reactants and products are in the standard state.

H rxn from H f H rxn = n H f (prod) m H f (react)

n, m are stoichiometric coefficients of products and reactants. (This is an application of Hesss Law.)


Using the information in Table 5.3 of your text book, what is H rxn for the reaction:

C2H5OH(l) + 3O2(g)2CO2(g) + 3H2O(l)

a) 115.8kJ b) 115.8kJ c) 1366.7kJ d) 13366.7kJ e) There is not enough information in

Table 5.3 to answer this. If a piece of fruit contains 16.0 g of fructose, how many food calories does it contribute? C6H12O6(s) +6O2 6CO2+ 6H2O Hrxn = 2803kJ 1 cal = 4.184J 1kcal = 1 Cal (= 1 food calorie)

a) 2803Cal b) 669.9Cal c) 10.72Cal d) 59.5Cal


Bond properties Review

COVALENT BOND LENGTHS and ENERGIES

Bond length: distance between nuclei bond Bond energy

kJ/mol Bond length pm

CC 348 154 C=C 614 134 CC 839 121 more electrons shared, shorter bond length

bond Bond energy

kJ/mol Bond length

pm CH 413 110 CCl 328 176 CBr 276 196 Shorter bond length, stronger the bond Table 8.4 and 8.5


BOND ENERGY bond (dissociation) energy: D

enthalpy of bond breaking reaction in the gas phase.

D > 0 (H > 0) for diatomics, D is H of one reaction: HH(g) 2H(g) DH-H= Hrxn = 436kJ/mol for polyatomics, D is an averaged quantity

HOH(g) HO(g) + H(g) +494kJ/mol HO(g) H(g) + O(g) +424kJ/mol

DOH = 463 kJ/mol *

* value obtained from averaging over many molecules

not exact for any one case (like H f)


Estimating Hrxn

Bond energies provide estimates of reaction enthalpies:

Hrxn nDbroken mDformed

n, m = # bonds Energy is given off () when bonds form. Helps understand origins of Hrxn Example:

2 H2O2 2 H2O + O2 Hrxn = ? Draw Lewis structures of reactants and products:

Reactants(broken) Products (formed)

bond # D bond # D

Documents

Thermochemistry - Penn State Universitycourses.chem.psu.edu/chem110/summer/Lectures/Web pdf/CHAPT5.pdf · Mary J. Bojan Page 1 Chapter 5 Thermochemistry Energy • kinetic • potential