Chemistry: 13 T

What is the amount of heat (in joules) required to raise the temperature of 200.0 g of water from 20.0 °C to 50.0 °C? The specific heat of water is 4.18 J/g • °C.

25,100 J Q=mc_spDt (200.0g)(4.18J)(30°C) = 25,080

Thermochemistry concentrates primarily on transformations of electrical energy.

False

The lower the specific heat of a substance is, the more heat will be required to cause a temperature change in the substance.

False. Specific heat is the amount of heat required to raise the temperature of 1 g of the substance by 1 °C, and thus a greater specific heat means more heat would be required to raise the substance's temperature.

An exothermic reaction involves

breaking weaker bonds to form stronger bonds.

Changes in enthalpy can be measured in a reaction vessel called a __________.

calorimeter

the amount of heat released by the complete burning of 1 mole of a substance

molar enthalpy of combustion

the change in enthalpy that occurs when 1 mole of a compound is produced from its elements

molar enthalpy of formation

the quantity of heat required to change 1 mole of a solid to a liquid with no temperature change

molar enthalpy of fusion

the amount of heat required to convert 1 mole of liquid at its boiling point to its vapor at the same temperature

molar enthalpy of vaporization

What value sign does ΔS° have when the entropy decreases?

negative

A certain reaction has a ΔH of -327 kJ. What is the ΔH for the reverse reaction?

+327 kJ

What is the ΔH° for P₄ + 6Cl₂ → 4PCl₃, given that the ΔH°f(PC₃3) = -319.7 kJ/mol? Observe significant digits.

-1279 kJ ΔH° = (4 mol)(-319.7 kJ/mol) = -1279 kJ

What is the ΔS° for the following reaction at 25 °C and 1 atm? C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (g) ΔS° of C₃H₈ = 270.3 J/(mol • K) ΔS° of O₂ = 205.2 J/(mol • K) ΔS° of CO₂ = 213.8 J/(mol • K) ΔS° of H₂O = 188.8 J/(mol • K)

100 J/(mol • K) ΔS° = ΣS°products - ΣS°reactants ΔS° = [(3 mol)(213.8 J/mol • K) + (4 mol)(188.8 J/mol • K)] − [(1 mol)(270.3 J/mol • K) + (5 mol)(205.2 J/mol • K)] = 100.3 J/(mol • K)

What is the ΔH° for the decomposition of sodium bicarbonate (baking soda)? 2NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g) ΔH°f of NaHCO₃ = -947.7 kJ/mol ΔH°f of Na2CO₃ = -1130.9 kJ/mol ΔH°f of H₂O = -241.8 kJ/mol ΔH°f of CO₂ = -393.5 kJ/mol

129 kJ ΔH° = ΣH°f(products) - ΣH°f(reactants) ΔH° = [(1 mol)(−1130.9 kJ/mol) + (1mol)(−241.8 kJ/mol) + (1 mol)(−393.5 kJ/mol)] - [(2 mol)(−947.7 kJ/mol)] = 129.2 kJ

How much heat is required to melt 200.0 g of ice? The heat of fusion of ice is 6.01 kJ/mol.

66.7 kJ molar mass ↓ (200g ice) / (18.02) x (6.01)

The stronger the bond is, the less energy will be released when it forms.

False. The amount of energy that must be used to break the weak bonds is small compared to all the energy that is given off when strong bonds form, so forming stronger bonds means more energy will be released.

When ice melts, its entropy increases.

Hell yeah

According to ______________, the enthalpy change of a reaction equals the sum of the enthalpy changes for each step of the reaction.

Hess's law

In an endothermic reaction, what is the relationship of the enthalpy of the reactants to the enthalpy of the products?

The reactants have less enthalpy.

A reaction is favorable if the free-energy change (ΔG) is negative.

True

Energy is always required to break bonds.

True

The temperature of a substance that has a specific heat of 1.75 J/(g • °C) will decrease by 1 °C for every 1.75 J of heat removed per gram.

True

Thermodynamics can be used to answer the question "Can two substances react together?"

True

Does the following chemical equation represent an endothermic or an exothermic reaction? 2H₂O (g) + 483.6 kJ → 2O₂ (g) + O₂ (g)

endothermic

Is the following reaction endothermic or exothermic: (ΔH = 66.4 kJ)? N₂ (g) + 2O₂ (g) → 2NO₂ (g)

endothermic

Is the reaction for the decomposition of sodium bicarbonate (baking soda) endothermic or exothermic? 2NaHCO3 (s) → Na2CO3 (s) + H2O (g) + CO2 (g) ΔH°f of NaHCO₃ = -947.7 kJ/mol ΔH°f of Na2CO₃ = -1130.9 kJ/mol ΔH°f of H₂O = -241.8 kJ/mol ΔH°f of CO₂ = -393.5 kJ/mol

endothermic

the quantity represented by ΔH

enthalpy

What is the term for the measure of randomness or lack of orderliness?

entropy

Is the following reaction endothermic or exothermic, given that the ΔH°f(PCl₃) = -319.7 kJ/mol? P₄ + 6Cl₂ → 4PCl₃

exothermic

A reaction in which both ΔH and ΔS are negative is

favorable at low temperatures.

a single criterion for favorability for a reaction to occur

free energy

If the warming curve of a substance has a gentle slope, what is the level of its specific heat?

high

How does the entropy change during the following reaction? C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (g)

increasing

On a melting curve, the heat energy represented by the horizontal portion of the curve is called

latent heat

Is the reaction 4Fe (s) + 3O2 (g) 2Fe2O3 (s) more or less spontaneous at 1000 °C than it is at 25 °C, assuming that ΔH and ΔS remain constant with temperature? ΔH = -1648.4 kJ, and ΔS = -0.55 kJ/(mol • K).

less spontaneous ΔG = ΔH − TΔS At 25 °C, ΔG = (-1648.4 kJ) − (298 K)(-0.55 kJ/[mol • K]) = -1485 kJ. At 1000 °C, ΔG = (-1648.4 kJ) − [1273 K][-0.55 kJ/(mol • K)] = -948 kJ. The reaction is less spontaneous at the higher temperature.

The specific heat of copper is 0.39 J/(g • °C), and that of silver is 0.23 J/(g • °C). Why does one metal have a higher temperature if the same amount of thermal energy were added to 50.0 g of each metal?

lower specific heat

The specific heat of copper is 0.39 J/(g • °C), and that of silver is 0.23 J/(g • °C). Which metal would have the higher temperature if the same amount of thermal energy were added to 50.0 g of each metal?

silver

the amount of heat required to raise the temperature of 1 gram of a substance 1 °C

specific heat

Using ΔG, determine whether the reaction 4Fe (s) + 3O₂ (g) 2Fe₂O₃ (s) is spontaneous at 25 °C, given that ΔH = -1648.4 kJ and ΔS = -0.55 kJ/(mol • K).

spontaneous ΔG = ΔH − TΔS At 25 °C, ΔG = (-1648.4 kJ) − [298 K][-0.55 kJ/(mol • K)] = -1485 kJ.

What does the degree symbol in ΔH°f represent?

standard state

the heat that, when applied to a substance, results in a temperature change in the substance

the sensible heat

What kind of chemical equation is the following? 2H₂O (g) + 483.6 kJ → 2O₂ (g) + O₂ (g)

thermochemical equation

Which branch of science studies the transfer of energy during chemical reactions or phase changes?

thermochemistry

A reaction that has a positive ΔH and a negative ΔS is

unfavorable at all temperatures.

Is a positive ΔH an indication that chemical bonds in the products are stronger or weaker than those in the reactants?

weaker

The equation that relates enthalpy, entropy, and free energy is

ΔG = ΔH - TΔS.