2 Thermochemistry

Question 1

State the ‘first law of thermodynamics.’

Answer 1

the law of conservation of energy; the energy of the universe is constant

Question 2

define ‘internal energy (E).’

Answer 2

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

Question 3

State the formula for ΔE (change in system’s internal energy).

Answer 3

ΔE = q + w
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 a higher to a lower temperature

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 absolute zero equivalent to?

Answer 6

0°C and -273.15K

Question 7

Define ‘thermochemistry.’

Answer 7

the study of energy changes accompanying a chemical reaction
the amount of energy taken in and given out (ΔH and ΔS)

Question 8

Define ‘enthalpy,’ ΔH.

Answer 8

for a reaction, it is a measure of the amount of heat released or consumed by a reaction

Question 9

State the formula used to thermodynamically define ‘enthalpy.’

Answer 9

H = E + pV 
H - Enthalpy
E - energy of the system (J)
p - pressure of the system 
V - volume of the system

Question 10

For a ΔH < 0 state the:

a) relative stability of reactant and product bonds
b) the heat change during the reaction

Answer 10

a) products > reactants

b) heat is released

Question 11

For a ΔH > 0 state the:

a) relative stability of reactant and product bonds
b) the heat change during the reaction

Answer 11

a) reactants > products

b) heat is consumed

Question 12

For a ΔH = 0 state the:

a) relative stability of reactant and product bonds
b) the heat change during the reaction

Answer 12

a) products = reactants

b) no change in heat

Question 13

Define an ‘exothermic’ reaction

Answer 13

heat is released by the reaction

Question 14

Define an ‘endothermic’ reaction

Answer 14

heat is consumed by the reaction

Question 15

Define the ‘standard enthalpy change of formation, ΔHf .’

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 always the ΔHf of a pure element, in its standard state?

Answer 16

zero

Question 17

State the value of ΔHf for the two following equations in terms of ΔH.

1. 2C(s) + 2H₂(g) → C₂H₄(g)
2. N₂(g) + 2O₂(g) → 2NO₂(g)

Answer 17

1. ΔH = ΔHf (C₂H₄)

2. ΔH = 2ΔHf (NO₂)

Question 18

What does a bomb calorimeter do and why?

Answer 18

studies exothermic reactions

because heat is transferred to raise the temperature of surroundings (water)

Question 19

Fill in labels 1-9 on the figure of a bomb calorimeter.

Answer 19

1. Motorised stirrer
2. Electrical leads for igniting sample
3. Thermometer
4. Insulated container
5. Oxygen inlet
6. Bomb (reaction chamber)
7. Fine wire in contact with the sample
8. Cup holding sample
9. Water

Question 20

State the formula for measuring enthalpy changes using calorimetry, including units and the final step that needs to be taken.

Answer 20

ΔH = mCpΔT
ΔH - enthalpy change (J)
m - mass (kg)
Cp - specific heat capacity (J/kg/K)
Δt - change in temperature 
Final step - ensure that you don't forget to find the moles and divide by them into questions where substances are given

Question 21

State ‘Hess’ Law.’

Answer 21

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

Question 22

State the formula for ΔH by Hess’ Law, including units.

Answer 22

ΔH = ΣH(products) - ΣH(reactants)
ΔH - change in enthalpy (J)
ΣH - sum of enthalpies (J)

Question 23

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

Answer 23

the sum of the heat changes in its steps

Question 24

What is the ΔH for a process, involving the transformation of reactants into products?

Answer 24

It is not dependent on pathways that are taken

Question 25

Which 2 things can be used to calculate the ΔH for a reaction?

Answer 25

The heat of formation of all reactants and products

Bond energies of individual bonds

Question 26

What is breaking bonds and what type of value is it?

Answer 26

endothermic

positive value

Question 27

What is making bonds and what type of value is it?

Answer 27

exothermic

negative value

Question 28

State the official formula for ΔH.

Answer 28

ΔH = ΣH(broken) - ΣH(formed)

Question 29

State the formulas for ΔHr and when to use them.

Answer 29

Generally use:
ΔHr = ΣΔH(reactants) - ΣΔH(products)
But when a combustion reaction (any time O2 is added to a compound):
ΔHr = ΣΔH(products) - ΣΔH(reactants)

Question 30

Explain the steps taken to draw a Hess’ Law/Born-Haber Cycle from enthalpies of combustion/formation.

Answer 30

1. Write a balanced reaction equation for the enthalpy you need to find
2. Write the 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 another)
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.

Question 31

State the formula linking ΔHf and ΔHr.

Answer 31

ΔHr = ΣHf(products) - ΣHf(reactants)

Question 32

Give a common example of a compound used to draw Born-Haber Cycles.

Answer 32

NaCl

Question 33

State each of the following in basic terms:

a) first law of thermodynamics
b) second law of thermodynamics
c) third law of thermodynamics

Answer 33

a) conservation of energy (energy cannot be created or destroyed)
b) entropy tends to increase (temperature tends to flow from a higher to a lower temperature)
c) entropy is temperature-dependent (if all thermal motion removed from particles state called absolute zero would occur)