L2 - Thermochemistry

Question 1

State the ‘First Law of Thermodynamics’ / Law of Conservation of Energy.

Answer 1

“The energy of the universe is constant.”

Question 2

Define ‘internal energy’.

Answer 2

The sum of the kinetic and potential energies of all the particles in the system. It can be changed by a flow of work, heat or both.

Question 3

What is the equation for ΔE?

Answer 3

ΔE = q + w

ΔE - change in a system’s internal energy
q - heat
w - work

Question 4

State the ‘Second Law of Thermodynamics’.

Answer 4

“Entropy will always tend to increase. Heat will always flow from higher temperatures to low temperatures.”

Question 5

State the ‘Third Law of Thermodynamics’.

Answer 5

“Entropy is temperature dependent. If all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would occur.”

Question 6

What is the value of absolute zero?

Answer 6

0 K = -273.15°C

Question 7

Define ‘thermochemistry’.

Answer 7

The study of energy changes accompanying a chemical reaction.

Question 8

Define ‘enthalpy’ (of a reaction).

Answer 8

A measure of the amount of heat released or consumed by a reaction.

Question 9

What is the equation for H?

Answer 9

H = E + PV

H - energy of the system
P - pressure of the system
V - volume of the system

Question 10

When ΔH < 0, what is the relative stability of bonds and the heat change during reaction?

Answer 10

Relative Stability of Bonds: Products > Reactants

Heat Change during Reaction: Heat is released

Question 11

When ΔH > 0, what is the relative stability of bonds and the heat change during reaction?

Answer 11

Relative Stability of Bonds: Products < Reactants

Heat Change during Reaction: Heat is consumed

Question 12

When ΔH = 0, what is the relative stability of bonds and the heat change during reaction?

Answer 12

Relative Stability of Bonds: Products = Reactants

Heat Change during Reaction: No change

Question 13

What happens during an exothermic reaction?

Answer 13

Heat is released by the reaction

Question 14

What happens during an endothermic reaction?

Answer 14

Heat is consumed by the reaction

Question 15

Define the ‘Standard Enthalpy of Formation’.

Answer 15

The change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states.

Question 16

What is the standard enthalpy of formation of a pure element in its standard state defined as?

Answer 16

Zero

Question 17

What studies exothermic reactions?

Answer 17

Bomb calorimeters

Question 18

What happens in bomb calorimetry?

Answer 18

Heat energy is transferred to raise the temperature of the surroundings (water)

Question 19

What is the equation for ΔH?

heat capacity

Answer 19

ΔH = m C.p ΔT

enthalpy change = mass x heat capacity x temperature change

Question 20

State ‘Hess’ Law’.

Answer 20

“The overall enthalpy change accompanying a chemical reaction is independent of the route taken in going from reactants to products (provided that in each case, the same initial and final states of temperature and pressure apply to the reactants and products).”

Question 21

What is the equation for ΔH?

products & reactants

Answer 21

ΔH = ΣH.products - ΣH.reactants

Question 22

What is the heat change of a whole reaction equivalent to?

Answer 22

The sum of heat changes in its steps.

Question 23

ΔH for a process involving the transformation of reactants into products is not dependant on _______.

Answer 23

pathway

Question 24

What can ΔH for a reaction be calculated from?

Answer 24

• Heat of formation of all reactants and products

- Bond energies of individual bonds

Question 25

What is the equation for ΔH.r?

Answer 25

ΔH.r = ΣH.broken + ΣH.formed

Question 26

What types of reactions are:

• bond breaking
• bond making

Answer 26

• bond breaking: endothermic (positive values)

- bond making: exothermic (negative values)

Question 27

What are the steps for forming a Born-Haber cycle?

Answer 27

1) Write a balanced reaction equation for the enthalpy you need to find
2) Write equations for the enthalpies you have been given
3) Draw the overall chemical reaction as an enthalpy diagram (with the reactants on one line, and the products on the other line)

4) Draw a reaction representing the intermediate step(s) by placing the relevant reactants on a line
(Remember to check for state changes/standard states)

5) Apply Hess’ law – add up the enthalpies for each step, taking direction of reaction into account.