Quiz 1 - Thermochemistry | Thermochemistry

General Chemistry 2 - Quiz 1 - Thermochemistry


Which statement correctly describes an endothermic chemical reaction?

In an endothermic chemical reaction, reactants absorbs heat energy
⇒ the products will have higher potential energy than reactants
⇒ Hproducts > Hreactants
⇒ ∆H = Hproducts – Hreactants > 0


What happens to the value of ∆H of a thermochemical reaction if the reaction is reversed?

Hess’s Law: enthalpy changes for chemical equations are additive
⇒ ΔHrxn = ΔHrxn (forward) + ΔHrxn (reverse) = 0
⇒ ΔHrxn (reverse) = - ΔHrxn (forward)


∆H = 114 kJ for the reaction: 2 NO2 (g) → 2 NO(g) + O2(g). What is the ∆H for NO(g) + ½ O2(g) → NO2(g)?

Reaction 1:   2 NO2 (g) → 2 NO(g) + O2(g),   Reaction 2:   NO(g) + ½ O2(g) → NO2(g)
Reaction 2 is the reverse of reaction 1 with the reactants and the products divided by 2

Hess’s Law: enthalpy changes for chemical equations are additive
⇒ ΔHrxn (reverse) = - ΔHrxn (forward)   
AND   ΔHrxn = n x ΔHrxn (A) when a chemical equation A is multiplied by a factor n
⇒ ΔHrxn (Reaction 2) = - 12 ΔHrxn (Reaction 1) = - 12 x 114 = - 57 kJ


Which of the following does not have a standard heat of formation equal to 0 at 298 K and 1.0 atm?

The standard enthalpy of formation of the most stable form of an element is 0.
The most stable element for hydrogen is H2, not H ⇒ ΔH0f of H2 = 0   but   ΔH0f of H  0


Knowing that the enthalpy of combustion of propane gas is – 2220.0 kJ.mol-1, which of the following statements is incorrect?

ΔH combustion (C3H8) = – 2220.0 kJ.mol-1 < 0 ⇒ exothermic process
Exothermic process ⇒ q < 0: reaction releases energy as heat
ΔH combustion (2 moles of C3H8) = 2 x ΔH combustion (C3H8) = – 4440.0 kJ


If a reaction is carried out at constant pressure, which of the following statements is correct?

ΔHrxn = ΔUrxn + PΔVrxn     (when P = cst, isobaric conditions)
AND   ΔUrxn =  q + w = q - PΔVrxn    with q = heat change
⇒ ΔHrxn = q - PΔVrxn + PΔVrxn = q     at constant pressure


Which of the following is true about the law of conservation of energy?

The law of conservation of energy states that energy cannot be created or destroyed; it can only be converted from one form to another. The total energy of an isolated system remains constant.


Which of the following statements is correct regarding the first law of thermodynamics?

The first law of thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. If no work is done and no heat is exchanged, the internal energy remains constant.


Which of the following represents the standard enthalpy of formation for CO2 ​(g)?

The standard enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states.


What is the heat capacity of a substance?

Heat capacity is defined as the amount of heat required to raise the temperature of a given amount of substance by one Kelvin. It depends on the quantity of the substance and is often given for a specific amount, such as per mole or per gram.