Topic 5: Energetics and Thermochemistry

Question 1

What is thermodynamics?

Answer 1

Thermodynamics is the study of energy and how it is interconverted.

Question 2

What is the 1st law of thermodynamics and what is it called?

Answer 2

That energy can be converted from one form to another and that the total amount of energy for a given system will remain constant.

It’s called law of conservation of energy.

Question 3

What is the law of conservation of energy?

Answer 3

It states that energy can be neither created nor destroyed. It can only be converted between different forms.

Question 4

What is heat, q?

Answer 4

Heat is a form of energy that is transferred from a warmer body to a cooler body.
Can be transferred by the processes of conduction, convection, and radiation.

Question 5

What is bond dissociation energy?

Answer 5

Type of energy required to break a chemical bond, which is an endothermic process.

Question 6

What is thermochemistry?

Answer 6

It’s the study of heat changes that occur during chemical reactions.

Question 7

What is the unit for enthalpy change?

Answer 7

kJ mol^(-1)

Question 8

What is a calorimeter?

Answer 8

It’s any apparatus used to measure the amount of heat being exchanged with the surroundings.

Question 9

What is an exothermic reaction?

Answer 9

A chemical reaction in which heat is transferred from the system to the surroundings.

negative enthalpy change

the enthalpy of the products is lower that that of the reactants, i.e. the products are more energetically stable.

Question 10

What is an endothermic reaction?

Answer 10

A chemical reaction that absorbs heat from their surroundings.

positive enthalpy change

enthalpy of the products is greater than that of the reactants, i.e. the reactants are more energetically stable.

Question 11

What is the unit for specific heat capacity, c?

Answer 11

kJ kg^(-1) K^(-1)

Question 12

What is the specific heat capacity of water?

Answer 12

4.18 kJ kg^(-1) K^(-1)

Question 13

What is specific heat capacity of a pure substance?

Answer 13

The amount of heat needed to raise the temperature of 1g of the substance by 1 degree Celsius. or 1K.

The lower the specific heat capacity of a given substance, the higher the temperature rise achieved for the same amount of heat transferred to the sample.

does not vary in magnitude with the size of the system being described.

Question 14

What is the formula to calculate heat, q, of a system?

Answer 14

q = mc ∆T

where
m is mass in kg
c is specific heat capacity

Question 15

What are the standard conditions for the standard enthalpy change of a reaction?

Answer 15

temp: 25C/298K
pressure: 100kPa

Question 16

What is the enthalpy change during a reaction?

Answer 16

deltaH of reaction = sum(deltaH products) - sum(deltaH reactants)

all at standard conditions

Question 17

What is the standard enthalpy change of formation?

Answer 17

Energy change upon the formation of 1 mol of a substance from its constituent elements in their standard state.

Question 18

What is the standard enthalpy change of combustion?

Answer 18

The heat evolved upon the complete combustion of 1 mol of substance.

Question 19

What is Hess’ Law?

Answer 19

Builds up on the conservation of energy law, ie regardless of the route by which a chemical reaction proceeds, the enthalpy change will always be the same providing the initial and final states of the system are the same.

Question 20

Using Hess’s law, and the following information, calculate the enthalpy change ΔH4
for the reaction: C + 2H2+ 1/2O2 → CH3OH ∆H4

CH3OH + 1 1/2O2 → CO2 + 2H2O
∆H1= -676 kJ

C + O2 → CO2
∆H2 = -394 kJ

H2 + 1/2O2 → H2 O
∆H3 = -242 kJ

IB, May, 2006 (4)

Answer 20

● Look at the overall equation 4 for the enthalpy of formation of methanol.
● From equations 1–3, the reactant carbon and the product methanol should be the main focus of your methodology.
● For carbon, we require a reaction that uses 1 mol of carbon as a reactant. Carbon is a reactant in equation 2, so this equation can be used as written:
C + O2 → CO2 ∆H2 = -394 kJ
● For methanol we need to use equation 1 , but we need to reverse the equation so that methanol is a product not a reactant. When reversing the chemical equation we must change the sign of the enthalpy value:
CO2 + 2H2O → CH3OH + 1 1/2O2

∆H1 = +676 kJ

● Because oxygen is found in all three equations, the next point of focus should be hydrogen. We require 2 mol of hydrogen as a reactant. Therefore, equation 3 can be used in the direction as written but with double the number of moles. This means that the enthalpy value must be doubled:
2H2 + O2 → 2H2O
∆H3 = -484 kJ

● Now we can add the three equations together, eliminating those species common to both sides and summing the enthalpy values as shown in table 1 p. 159

The combination of these reactions can also be represented diagrammatically as shown in ?gure 3. You will notice that the enthalpy cycle diagram shows the combustion of methanol equation reversed, in the same way it was during the summation of equations method

Question 21

What is bond enthalpy?

Answer 21

The energy required to break 1 mol of bonds in gaseous covalent molecules under standard conditions.

Bond breaking is endothermic, thus has a positive enthalpy value.

Question 22

What is the formula to calculate enthalpy change of reaction using bond enthalpies?

Answer 22

∆Hstand = sum(BE bonds broken) - sum(BE bonds formed)