CHM 2045C Module 7 Thermochemistry Name: __________________

 

Module Seven: Part B Discussion Questions 10 points

 

1. What is the standard state of an element or compound substance and give an example?

 

The standard state of an element or a compound is defined as the most stable form of the substance in the physical state that exists at a pressure of 1 bar and specified temperature (usually 25oC or 298 K).

 

For example

Hof for CO2 (g):

 

At 25 oC and 1 bar, the standard state of carbon is solid graphite, the most stable form of this element and the most stable form of oxygen is O2 (g)

 

C(s) + O2(g) CO2(g) Hof = -393.5 kJ

 

2. Why does water have a high specific heat capacity? What does this mean?

 

The specific heat of water is much larger than for most substances because of the unusually strong bonds between the water molecules (Look up the hydrogen bond in later chapters). These intermolecular bonds are progressively broken as more and more heat is added. What this means is that a considerable quantity of heat is required to heat water and considerable amount of heat must be transferred out of the water before it cools down appreciably.

 

3. Write four different mathematical expressions for the 1st Law of Thermodynamics. How are they related?

 

Some expressions for the 1st law are:

E = q + w where E refers to the system

qin = qout heat gained = heat lost

E = zero where E refers in this case to the entire universe

Hreaction = Σ( Hf (products) - Hf (reactants) )

 

All these expressions represent an energy balance, reflecting the fact that energy can neither be created nor destroyed.

 

4. Where does the energy come from in an endothermic process? And where does it go?

In an endothermic process energy is required. There are two sources for this energy: the energy may come from the surroundings if the system is heated; or the energy could come from the system itself if the kinetic energy of the atoms and molecules of the system is reduced. In this case, the temperature of the system decreases. Unless the system is isolated (well-insulated), there will be a movement of the energy between the system and its surroundings to reestablish thermodynamic equilibrium.

 

5. Define standard molar enthalpy of formation Hf . Why is the standard enthalpy of formation of a pure element in its most stable form defined as zero (page 230 and see table 6.2 p 271)?

 

The standard molar enthalpy of formation of a substance is the enthalpy change for a reaction in which one mole of the substance in its standard state is made from its constituent elements in their standard states.

 

For a substance that is an element, such a reaction represents no change, and therefore then enthalpy change must be zero because the element (or atom) already exists in nature and can not be assembled by man from its building blocks of subatomic particles. Elements are defined as the smallest unit of matter that has the chemical properties of that matter. It can not be subdivided into it building blocks by any chemical means. Therefore, we state energy change begins with putting atoms together to make molecules of compounds.

 

 

6. Define a spontaneous reaction. How can you tell whether a reaction is spontaneous?

 

A spontaneous reaction is a reaction that happens by itself. It may happen quickly or very very slowly but it does happen. Calculations in thermodynamics can be done to determine whether or not a reaction is spontaneous. However, if a process or reaction does happen itself, you can be certain it is spontaneous.

 

7. A system can exchange energy with its surroundings either by transferring heat or by doing work. This is expressed by the following equation: Δ E = q + w

Fill in the chart with correct signage:

Change Sign Conversion Effect of Esystem

Work done on the system by surroundings w > 0 (+) E increases (+)

Work done by the system on surroundings w < 0 (-) E decreases (-)

Heat transferred to system from surroundings q > 0 (+) E increases (+)

Heat transferred from system to surroundings q < 0 (-) E decreases (-)