Unit 2.1 Thermochemistry

Question 1

Well it states to acknowledge the principle of conservation of energy (literally physics)

Answer 1

Energy cannot be destroyed or created only transferred from one form to another

Question 2

2 examples of an endothermic reaction?

Answer 2

• Photosynthesis
• Decomposition of CaCO₃

Question 3

3 examples of an exothermic reaction?

Answer 3

• Respiration
• Combustion
• Metal + acid

Question 4

I shall hope u already know how to create reaction profiles for exothermic and endothermic reactions
(either refer to book or the next unit)

Answer 4

worth it?
saves time.
meh

Question 5

If endothermic, △H = ?

Answer 5

Positive

Question 6

If exothermic, △H = ?

Answer 6

Negative

Question 7

In terms of △H, what makes the products of a chemical reaction more stable

Answer 7

More exothermic
More negative
More -△H

Question 8

Define bond enthalpy

Answer 8

The enthalpy required to break a covalent X-Y bond into X atoms and Y atoms, all in the gas phase

Question 9

Define average bond enthalpy

Answer 9

The average value of the enthalpy required to break a given type of covalent bond in the molecules of a gaseous species

Question 10

How to calculate standard enthalpy change

Answer 10

• I assume we are actually given a table of energy for each bond
• Doesn’t matter which side is products or reactants:
• Minus the left side with the right side
• Can determine whether it’s exothermic or endothermic
• What may be difficult:
  How to tell what bonds are there within a chemical compound?

Question 11

Explain standard enthalpy change of formation △H_f^θ or △_fH^θ

Answer 11

The enthalpy change when:
- 1 mole of product is formed
- in standard state under standard conditions
- Changes = negative
- Units = kJmol^-1

Question 12

a) C_(s) + ½O_2(g) -> CO_(g)
b) 2C_(s) + O_2(g) -> 2CO_(g)
c) C_(g) + ½O_2(g)) -> CO_(g)

WHICH of these reactions show standard molar enthalpy of formation. Explain.

Answer 12

a). You know why?:
- 1 mole
- C = solid
- Idk if it actually is negative, whatever u can figure that out

Question 13

a) H(g) + ½Br₂(l) -> HBr(g)
b) H₂(g) + Br₂(l) -> 2HBr(g)
c) ½H₂(g) + ½Br₂(l) -> HBr(g)

WHICH OF THESE reactions show standard molar enthalpy of formation. Explain.

Answer 13

c). YOU KNOW why?
- 1 mole again
- Show it’s also H₂ right?
but it’s smart cuz u can just write down ½ duhhh
- Could be negative? Nega

Question 14

Explain standard enthalpy change of combustion △H_c^θ or △_cH^θ

Answer 14

The enthalpy change when:
- 1 mole of a substance
- Completely combusted in oxygen
- Under standard conditions
- Negative kJmol^-1

Question 15

Ethane (C₂H₆) and carbon (C) as a thermochemical reaction for the standard molar enthalpy of combustion?

Answer 15

ethane (△H_c^θ = -1560kJmol^-1):
C₂H₆(g) + 3½O₂(g) -> 2CO₂(g) + 3H₂O(l)

carbon (△H_c^θ = -394kJmol^-1:
C(s) + O₂(g) -> CO₂(g)

Ya see how they’re all negative? (exothermic)

Question 16

Difference of standard molar enthalpy of formation of potassium oxide (K₂O) between △H_f^θ and △H_c^θ

Answer 16

△H_f^θ = 2K(s) + ½O₂(g) -> K₂O(s)

△H_c^θ = K(s) + ¼O₂(g) -> ½K₂O(s)

I think the trick here is:
△H_f^θ = 1 mole of product
△H_c^θ = 1 mole of substance (reactant perhaps?)

Question 17

What does the θ mean seen in the notation △H_f^θ or △H_c^θ?

Answer 17

Theta/plimsoll:
- Under standard conditions
- Temp: 298K, 25°C
- Pressure: 1 atm, 10100pa? [101325pa???]

Question 18

Explain enthalpy change of reaction △H_r or △_rH

Answer 18

The enthalpy change in a reaction between the number of moles shown in the reaction equation

After a long consideration, this is the exact same as finding the bond enthalpy change at the beginning. BUT IF IT’S GONNA STATE THE DIFFERENCE:
- Heat given off or taken up for the reaction

Question 19

State Hess’s Law

Answer 19

The total enthalpy change for a reaction is independent of the route taken from the reactants to the products

Question 20

Wait

Answer 20

Wait?