Saturday Study Session 1Theme of the Class

EnthalpySession 3 – Delta H Four Different

Ways

Enthalpy. Anyone not in Chemistry thinks you’re

saying Empathy. I feel your pain.

EnergyEnergy

The capacity to do work or to produce heat.

The Two Types of EnergyThe Two Types of Energy

Potential: due to position or composition - can be converted to kinetic.

Kinetic: due to motion of the object

State FunctionState FunctionDepends only on the present state of the system - not how it arrived there.

It is independent of pathway.

Enthalpy is a state function

It’s like going to Vegas. On the way home you know you lost $400. It doesn’t matter what casino or how many bets you placed. You still lost money.

First LawFirst Law

First Law of Thermodynamics:

The energy of the universe is constant.

If you lose $$ in Vegas, someone else made $$.

EnthalpyEnthalpyH = energy flow as heat (at constant pressure and

volume)Thus the change of enthalpy is the change in the

amount of energy of a system.A block of wood burning has a negative change in

enthalpy.A tree taking energy from the sun and building a tree

has a positive change in enthalpy.If you lost money in Vegas, that is negative $.If you won money, that is positive $ .

4 ways to calculate change in enthalpy

1. Calorimetry – use q=mct to find heat gained or lost by water.

2. Hess’s Law –keep or flip equations. Add em up.

3. Hess’s law #2– standard heats of formation.4. Bond energies – more to come on that as

well.

Enthalpy Δ H

Calorimetry• Use water in a device to measure the heat given

off or absorbed by a reaction or object.• q= mct• q is heat absorbed or lost by the water usually in

Joules• m is the mass of the water in grams• c is the specific heat of water (4.18 J/gxC)• t is the change in temperature of the water K

or C doesn’t matter.

Applying mcat• Find q for the water, don’t worry about the sign

of anything. • If the water got warmer, the reaction was

exothermic and H will be negative.• If the water got cooler, the reaction was

endothermic and H will be positive.

Hess’s LawHess’s Law

Reactants Products

The change in enthalpy is the same whether the reaction takes place in one step or a series of steps.

You can lose one $10,000 bet or lose 10 $1,000 bets. Either way the end result is the same. You’re out 10 large!

Calculations via Hess’s LawCalculations via Hess’s Law

1. If a reaction is reversed, H is also reversed.

N2(g) + O2(g) 2NO(g) H = 180 kJ

2NO(g) N2(g) + O2(g) H = 180 kJ

2. If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer.

6NO(g) 3N2(g) + 3O2(g) H = 540 kJ

Hess law example

• Thermite is powdered aluminum plus iron III oxide creating iron and aluminum oxide.

• 2Al(s) + Fe2O3(s) Al2O3(s) + 2Fe(s)

• This is extremely exothermic.• 2 Al + 3/2 O2 Al2O3 ∆H=-1676 kJ/mol

• 2 Fe +3/2 O2 Fe2O3 ∆H=-826 kJ/mol

• Which one gets flipped?

H using heats of formationH using heats of formationStandard heat of enthalpy for any element is zero.Can be calculated from enthalpies of formation of

reactants and products.

Hrxn° = Hf(products) Hf(reactants)

H using Bond EnergiesH using Bond EnergiesCan be calculated from bond energies of reactants

and products.Hrxn = bond energies of reactants)

– bond energies of products)

Notice this is opposite of the standard heats of formation.

In other words breaking all the bonds takes energy (+ H) and building new molecules with new bonds releases

energy (-H).

A) -1235 KJB) -1279 kJC) -1323 kJD) -1411 kJ

1. C2H4(g) + 3 O2(g) 2 CO2(g) + 2 H2O(g)For the reaction of ethylene represented above, Hrxn is -1323 kJ. What is the value of H if the combustion produced liquid water H2O(l), rather than water vapor H2O(g)? (H for the phase change H2O(g)⇆H2O(l) is -44 kJ mol-1.)

Question 1 Answer

DClue: Multiply one of the

equations to make it work.

C2H4(g) + 3 O2(g) → 2 CO2(g) + 2 H2O(g) Hrxn = -1323 kJ. H2O(g) → H2O(l) Hrxn = -44 kJ mol-1 Don’t flip anything. It works out for liquid water to be on the right.

You do need to multiply the 2nd equation by 2 to get stuff to cancel.C2H4(g) + 3 O2(g) → 2 CO2(g) + 2 H2O(g) Hrxn = -1323 kJ. 2H2O(g) → 2H2O(l) Hrxn = -44 kJ mol-1 x 2

= -88 kJ mol-1

Then add the two numbers together.

A) -607 kJB) -147 kJC) -19 kJD) +19 kJ

2. What is the standard enthalpy change H°, for the reaction:3C2H2(g) C6H6(g)H°f of C2H2(g) is 230 kJmol-1

H°f of C6H6(g) is 83 kJmol-1

Question 2 Answer

AClue: Products minus

reactants

Summation = add them together

• 3C2H2(g) C6H6(g)

• H°f of C2H2(g) is 230 kJmol-1

• H°f of C6H6(g) is 83 kJmol-1

• C6H6 – (3 x C2H2) = answer

• 83 kJ – (3 x 230 kJ) =• 83 kJ – 690 kJ = -607 kJ

A) H>0B) H<0C) H=0D) H is temperature dependent

3. True for the evaporation of water at 1 atm and 25 WC.

Question 3 Answer

AClue: sweat

When water evaporate it absorbs the energy and uses it to speed up the molecules so they can break free into a gas. Absorbing energy is + H.

A) 400 kJB) 12 kJC) 8 kJD) 1.2 kJ

4. A 10 g sample of a metal was heated to 100°C and then quickly transferred to an insulated container holding 100 g of water at 20°C. The temperature of the water rose to reach a final temperature of 50°C. Calculate the heat absorbed by the water. Specific heat of water is 4 J/(gx°C)

Question 4 Answer

BClue: You don’t always

use every number given.

q = m c t for the water since that is what the question asks.q = 100 g x 4 J/(gx ⁰ C) x (50-20 ⁰C)q = 12000 J or 12 kJ

A) Step 1 and Step 3 are exothermic and Step 2 takes no energy so is neither endothermic nor exothermic.

B) Step 1 and 2 add together to be 2 X Step 3.C) Steps 2 and 3 add together to be Step 1D) Step 3 releases energy while Step 1 and 2 absorb energy

5. The dissolution of an ionic solute in a polar solvent can be imagined as occurring in three steps, as shown in the figure above. In step 1, the separation between ions in the solute is greatly increased, just as will occur when the solute dissolves in the polar solvent. In step 2, the polar solvent is expanded to make spaces that the ions will occupy. In the last step, the ions are inserted into the spaces in the polar solvent. Which of the following best describes the enthalpy change, H, for each step?

Question 5 Answer

DClue: Read carefully.

Anytime you dissolve anything it takes energy to pull apart the solvent molecules and pull apart the solute particles (step 1 and 2)

When you combine them together energy is released. Thus D is the only statement that makes sense.

The steps do not have to add up to each other.

If there is more energy released than absorbed the temperature of the solution would increase.

If more energy is absorbed than released the temperature of the solution would decrease.But the question says nothing about the temperature so we don’t care.

Process H (kJ/molrxn)Na(s) Na(g) mNa(g) Na+(g) + e- nBr2(g) 2 Br(g) pBr (g) + e- Br -(g) qNa+(g) + Br -(g) NaBr(s) r

A) 72.2 kJ is released C) 36.1 kJ is absorbedB) 36.1 kJ is released D) 72.2 kJ is absorbed

Na(s) + ½ Br2(l) NaBr(s) H =-361 kJ/molrxn

The elements Na and Br react directly to form the compound NaBr according to the equation above. Refer to the information above and the table below to answer the questions 6-8.

6. How much heat is released or absorbed when 0.050 mol of Br2(g) is formed from NaBr(s)?

Question 6 Answer

CClue: Stoichiometry

0.050 mol Br2 x 361 kJ =36.1 kJ 0.5 mole Br2

Realize the equation must be reversed to produce Br2 so the sign of H must be flipped.Since the reaction has a positive H, then heat is absorbed so 36.1 kJ is absorbed.

A) r only C) p, q, and r onlyB) q and r only D) n, p, q, and r

7. Which of the values of H° for a process in the table is (are) less than zero (i.e., indicate(s) an exothermic process) ?

Question 7 Answer

AClue: Which one of these

is NOT like the others.

Knocking off an electron from a neutral atom or squishing another electron on a neutral atom takes energy to make it happen. ENDOthermic

Turning a liquid into a gas or splitting apart a diatomic element take energy to make it happen. ENDOthermic.

A) p + q C) p + 2qB) p - q D) (p/2)- q

8. Br2(g) + 2e- 2Br -(g)Which of the following expressions is equivalent to H° for the reaction represented above?

Question 8 Answer

CClue: Hess’s law

Value p is all set to match the target equation.Value q must be multiplied x 2 to match.Nothing needs to be flipped. Just add.

A) -190 kJ/molrxn

B) -290 kJ/molrxn

C) -580 kJ/molrxn

D) -1,270 kJ/molrxn

9. 4 NH3(g) + 3O2(g) 2 N2(g) + 6 H2O(g)If the standard molar heats of formation of ammonia, NH3(g), and gaseous water, H2O(g), are -46 kJ/mol and -242 kJ/mol, respectively, what is the value of H°298 for the reaction represented above?

Question 9 Answer

DClue: What is the heat of

formation for an element?

Heats of formation products-Heats of formation of reactants(6 x -242 kJ) – (4x -46 kJ) =

-1268 kJ

The nitrogen and oxygen have heats of formation of zero.

A) 62 kJ/mol rxn

B) 21 kJ/mol rxn

C) 31 kJ/mol rxn

D) 42 kJ/mol rxn

10. ½ H2(g) + ½ I2(s) HI(g) H = 26 kJ/mol

rxn

½ H2(g) + ½ I2(g)-> HI(g) H = -5.0 kJ/mol rxn

Based on the information above, what is the enthalpy change for the sublimation of iodine, represented below?I2(s) I2(g)

Question 10 Answer

AClue: flip and multiply

½ H2(g) + ½ I2(s) HI(g) H = 26 kJ/mol rxn

½ H2(g) + ½ I2(g) HI(g) H = -5.0 kJ/mol rxn

Multiply each equation by 2 to get the coefficients to match.The 2nd equation must be flipped.H2(g) + I2(s) 2 HI(g) H = 52 kJ/mol rxn

2 HI(g) H2(g) + I2(g) H = +10. kJ/mol rxn

Short Free Response 1(3 points possible)

Free Response 2 (9 pts possible)