Thermochemistry

Specific Heat

100

A chunk of 50.0 g iron metal at 150.C is placed in 50.0 mL water at 20.C. The water and metal are allowed to reach equilibrium. The final temperature of the water will be...

a) 85C

b) more than 85C

c) less than 85C

d) impossible to tell

less than 85C

The water gains the same amount of energy as the metal loses, BUT the high specific heat of water allows for less of a change in temperature.

200

What is the specific heat of a metal which has a mass of 25.0g, a temperature change from 25.0 degrees Celsius to 75.0 degrees Celsius, and absorbed 162.4J of heat?

What is 0.130 J/g°C

300

A 250.0 g sample of mercury is at 50.0°C. What is the final temperature of the mercury when it absorbs 5.50 kJ of energy? (the specific heat for mercury is 0.140 J/g°C)

207°C

400

In a coffee-cup calorimeter, 50.0 g of H2O and 12.5 g NaCl are mixed. The water was originally at 24.6 C. After the reaction, the temperature of both substances is 35.3 C.

What is the enthalpy of solution for the salt in kJ/mol

10.5 kJ/mol

500

_{A piece of metal weighing 59.047 g was heated to 100.0 °C and then put it into 100.0 mL of water (initially at 23.7 °C). The metal and water were allowed to come to an equilibrium temperature, determined to be 27.8 °C. Assuming no heat lost to the environment, calculate the specific heat of the metal.} 

0.402 J/gC

Enthalpy Calculations

100

Use BOND ENERGIES to calculate the enthalpy of the following equation

__H₂ + __Cl₂ --> __HCl

-183 kJ/mol

200

Use BOND ENTHALPIES OF FORMATION to determine the enthalpy of reaction

3SO₂(g) + 2HNO₃(aq) + 2H₂O(l) → 3H₂SO₄(aq) + 2NO(g)

671.5 kJ/mol

300

Is breaking bonds endothermic or exothermic?

endothermic

400

Use BOND ENTHALPIES to determine the enthalpy of reaction

2C₂H₆ (g) + 7O₂ (g) --> 4CO₂ (g) + 6H₂O (g)

-2857.2 kJ

500

Given that ∆H^°_rxn=47.2kJ/mol for the following, calculate the ∆H^°_f for Fe₃O₄(s).

2 Fe₃O₄ (s) + CO₂ (g) → 3 Fe₂O₃ (s) + CO (g)

-1118.4 kJ

Thermo Meets Stoich

100

A chemical reaction is found to be endothermic. This means energy went from...

a) surroundings to system

b) system to surroundings

c) universe to surroundings

d) surroundings to universe

a) surroundings to system - by definition, endothermic reactions take energy in.

200

How much heat will be released when 25.0 g of carbon reacts with excess O₂ according to the following equation?

C + O₂ → CO₂       ∆H = -393.5 kJ

819 kJ

300

What mass of propane, C₃H₈ must be burned in order to produce 88,000 kJ of energy?

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O      ∆H = -2200 kJ

1800 g

400

What mass of propane, water is produced when 88,000 kJ of energy is released?

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O      ∆H = -2200 kJ

2900 g

500

How much heat will be released when 20.0 g of H₂ reacts with 106.0 g of O₂ according to the following equation?

2H₂ + O₂ → 2H₂O ∆H = -571.6 kJ

1893 kJ

Hess's Law

100

Determine enthalpy in kJ for the following reaction using the listed enthalpies of reaction:

N2H4(l) + 2 H2O2(g)→ N2(g) + 4 H2O(l)

N2H4(l) + O2(g) →N2(g) + 2 H2O(l)    ΔH = -622.3 kJ
H2(g) + 1/2 O2(g) → H2O(l)            ΔH = -285.8 kJ
H2(g) + O2(g) → H2O2(g)            ΔH = -187.8 kJ

-818.3 kJ

200

Given the following equations and Ho values, determine the heat of reaction (kJ) at 298 K for the reaction:

2 OF2(g) + 2 S(s)→ SO2(g) + SF4(g)

OF2(g) + H2O(l)  →O2(g) + 2 HF(g)        ΔH = -276.6
SF4(g) + 2 H2O(l) → 4 HF(g) + SO2(g)    ΔH = -827.5
S(s) + O2(g) → SO2(g)                ΔH = -296.9

-319.5 kJ/mol

300

Given the following equations and Ho values, determine the heat of reaction at 298 K for the reaction:

C(s) + 2 H2(g)→ CH4(g)

C(s) + O2(g) → CO2(g)            ΔH = -393.5
H2(g) + 1/2 O2(g) → H2O(l)     ΔH = -285.8
CO2(g) + 2 H2O(l) → CH4(g) + 2 O2(g)    ΔH = +890.3

-74.8 kJ

400

What mass of oxygen are needed to give off 8500 kJ of energy according to the following combustion reaction? 

C2H2(g) +  5/2O2(g)  →  2CO2(g)  +  H2O(g) 

C2H6(g)  →  C2H2(g) + 2H2(g)      ΔH = 283.5 kJ

H2(g) + 1/2O2(g)  →  H2O(g)         ΔH = -213.7 kJ

2CO2(g)  +  3H2O(g)  →  C2H6(g) +  7/2O2(g)      ΔH = 849.0 kJ

-705.1 kJ

500

15.0 L of O2 gas and excess N2 at STP are used in the following reaction. How much energy is absorbed/released from this reaction?        

N2(g) +  2O2(g)  →  2NO2(g) 

N2(g)  +  3H2(g)  →  2NH3(g)     ΔH = -115.0 kJ

2NH3(g)+4H2O(l)→2NO2(g)+7H2(g)  ΔH = -142.5 kJ

H2O(l)  →  H2(g)  +  1/2O 2(g)  ΔH = -43.7 kJ 

ΔH=-82.7 kJ

BUT the final answer is...27.7 kJ (use stoichiometry)

Entropy and Gibbs

100

Which of the following has the greater entropy in each pairing?

1. CO2 (g) or CO2(s)

2. C2H4 (g) or C6H14 (g)

3. Copper at 100 ⁰C or copper at 1000 ⁰C

1. gas

2. C6H14

3. 1000 C

200

For a certain process at 300.0 K, ΔG = -77.0 kJ/mol and ΔH = -56.9 kJ/mol.  Find the entropy change for this process in J/(molK).

67.0 J/(molK)

300

A student collected the following data during a chemical reaction

ΔH =+12 kJ/mol        ΔS =+10. J/(K mol)

What temperature is required to make this reaction spontaneous?

Temperatures higher than 1200 K

400

The vaporization of bromine requires +31.0 kJ/mol and has an increase in disorder (ΔS˚ = 93.0 J/(mol•K)).  At what minimum temperature will this process be spontaneous?

                Br2 (l) → Br2 (g)

333 K

500

Methanol (CH3OH) is used as rocket fuel. Look at the reactants and products to analyze.

CH3OH (l) + 3/2 O2 (g) → CO2 (g) + 2 H2O (g)

a.Predict the sign of ΔS

b.Predict the sign of ΔH  

c. Is this reaction favorable at high, low, all temperatures, or never favorable?

a) +

b) -(think about the fact it is ROCKET FUEL and combusts!)

c) All temps(don't get this confused with activation energy which this will need a "spark" to get to begin to ignite)