Energetics & Thermochemistry

Ms. KielyIB Chemistry SLCoral Gables Senior High School

Bell RingerThe following equation shows the formation of magnesium oxide from magnesium metal.

2Mg(s) + O2(g) → 2MgO(s) ΔH = –1204kJ

Which statement is correct for this reaction?A. 1204 kJ of energy are released for every mol of magnesium reacted.B. 602 kJ of energy are absorbed for every mol of magnesium oxide formed.C. 602 kJ of energy are released for every mol of oxygen gas reacted.D. 1204 kJ of energy are released for every two mol of magnesium oxide formed.

Draft IA is due this upcoming MONDAY, 2/13! Must bring in a HARD COPY at the BEGINNING OF CLASS.

Draft IA is worth 10 grades.Final IA is worth 18 grades.

ANSWERThe following equation shows the formation of magnesium oxide from magnesium metal.

2Mg(s) + O2(g) → 2MgO(s) ΔH = –1204kJ

Which statement is correct for this reaction?D. 1204 kJ of energy are released for every two mol of magnesium oxide formed.

ANSWER COULD HAVE BEEN option B if it said: B. 602 kJ of energy are released for every mol of magnesium oxide formed.

Draft IA is due this upcoming MONDAY, 2/13! Must bring in a HARD COPY at the BEGINNING OF CLASS.

Draft IA is worth 10 grades.Final IA is worth 18 grades.

Today: IA Draft Review

-Self-review with Rubric: highlight areas that you think fulfill requirements of the IA rubric, and write acronyms for rubric requirements (Pe, Ex, An, Ev, Co)

-Peer Reviews with Rubric in another color

Must submit edited IA Draft via e-mail before end of TOMORROW: meaning your NEW draft is due via email to

eliza.kiely@gmail.com by 11:59 P.M. tonight.

Which statements about exothermic reactions are correct?I. They have negative H values.II. The products have a lower enthalpy than the reactants.III. The products are more energetically stable than the reactants.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

HOMEWORK: #25-32 on pgs. 184-187

-Topic 5 Test is NEXT WEEK Wednesday

-IA Final is due Tuesday 2/28 via Hard Copy and Email: eliza.kiely@gmail.com

Δ

Δ

In a calorimetry experiment, the temperature changes (ΔT) occurring to the water as well as to the can, their respective heat capacities (c), and their respective masses (g) can all be used to calculate the heat released by the combustion of a fuel; (given assumptions).

For example:In a reaction that occurs in 50 g of aqueous solution, the temperature of the reaction mixture increases by 20 °C. If 0.10 mol of the limiting reagent is consumed, what is the enthalpy change (in kJ mol⁻¹) for the reaction? Assume the specific heat capacity of the solution = 4.2 kJ⁻¹ K⁻¹.

A. −0.10×50×4.2×20

B. −0.10×0.050×4.2×20

C. −50×4.2×20/0.10

D. −0.050×4.2×20/0.10

Δ

AnswerIn a reaction that occurs in 50 g of aqueous solution, the temperature of the reaction mixture increases by 20 °C. If 0.10 mol of the limiting reagent is consumed, what is the enthalpy change (in kJ mol⁻¹) for the reaction? Assume the specific heat capacity of the solution = 4.2 kJ⁻¹ K⁻¹.

D. −0.050×4.2×20/0.10

For this problem, you calculated the heat change of the system using q=mcΔT, which gives you the heat change in joules (J). However, they are asking you for the change in enthalpy, ΔH, which asks for the heat change of a reaction in kJ mol⁻¹, meaning you must divide your final answer by 1,000 in order to convert appropriately from J to kJ. The only option that incorporates this is option D.

Δ

Δ

Δ

ΔΔ Δ

Δ Δ Δ

Δ → Δ

Δ → Δ→ Δ

→ Δ

S(s) + 1½ O2(g)

H1 is the first known rx: S(s) + 1½ O2(g) → SO3(g) ΔH1 = -395 kJH2 is the second known rx that we altered: SO3(g) → SO2(g) + ½ O2(g) ΔH2 = 98 kJ

SO3(g)

SO2(g) + ½ O2(g)

ΔH1

ΔH2ΔH3

According to Hess’ Law: ΔH1 + ΔH2 = ΔH3 -395 + 98 = -297 kJ/mol

You can double check this by putting together the reactants and products of H1 and H2, respectively, into one large equation, and then simplifying it by cancelling out like substances. If you end up with the equation you are originally solving for, then you did it correctly! S(s) + 1½ O2(g) + SO3(g) → SO3(g) + SO2(g) + ½ O2(g)

Simplified: S(s) + O2(g) → SO2(g) ΔH = -297 kJ/mol

Practice QuestionUsing the equations below:

C(s) + O2(g) → CO2(g) ∆H = –390 kJ

Mn(s) + O2(g) → MnO2(s) ∆H = –520 kJ

Determine what is ∆H (in kJ) for the following reaction?

MnO2(s) + C(s) → Mn(s) + CO2(g)

A. 910B. 130C. –130D. –910

Δ Δ Δ

→ Δ→ Δ

Δ →

Δ

Δ Δ Δ

Exercises 16-19

16. A

17. ΔH = –394 kJ + (+283) kJ = –111 kJ

18. ΔH = –180.5 kJ + (+66.4 kJ) = –114.1 kJ

19. ∆H = (2 × (–33.2 kJ mol⁻¹)) + (+9.16 kJ mol⁻¹) = –57.24 kJ mol⁻¹