C Unit 2.1 Thermochemistry

Question 1

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

What may that mean?

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)
(I might implement a certain habit)

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
(outdated type shi right?)

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

How to calculate enthalpy change using bond enthalpies?
(4 long steps, def want that implementation yano?)

Answer 11

1. Draw out each molecule to show the bonds (if not already given)
2. Calculate the energy required to break all the bonds in the reactants
   (endothermic, therefore +ve value)
3. Calculate the energy released in forming all the bonds in the products
   (exothermic = -ve)
4. Add together all the enthalpy values
   △H = reactants - products

Question 12

Explain standard enthalpy change of formation (△H_f^θ or △_fH^θ)
4 things

Answer 12

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

Question 13

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 13

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

Question 14

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 14

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 15

Explain standard enthalpy change of combustion (△H_c^θ or △_cH^θ)
4 things

Answer 15

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

Question 16

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

Answer 16

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 17

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

Answer 17

△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 18

What does the θ mean seen in the notation △H_f^θ or △H_c^θ?
(stick with 1st)

Answer 18

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

Question 19

Explain enthalpy change of reaction △H_r or △_rH
(outdated)

Answer 19

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 20

Explain enthalpy change of reaction △H_r or △_rH
(same as standard enthalpy change)

Answer 20

• Heat given off or taken up for the reaction

Question 21

a) H₂(g) + ½O₂(g) -> H₂O(g)
^^ △H_r = -242 kJ mol^-1
b) 2H₂(g) + O₂(g) -> 2H₂(g)
^^ △H_r = -484 kJ mol^-1

Question 22

What’s the enthalpy change for this reaction?:

2H₂O(g) -> 2H₂(g) + O₂(g)

Answer 22

2H₂O - 463 x 2 = -926

+436 x 2 = +872
+ 269 x 2 = +538
+ together = +1410

-926 + 1410 = +484 kJ mol^-1

Huge Catch:
It is stated, “cuz it’s endo, swap signs”, what does that mean….

Question 23

State Hess’s Law
(3 way)

Answer 23

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

Question 24

We start here for the next time

Answer 24

Dude… I just been staring it’s sad.

Question 25

Wait

Answer 25

Wait?

Question 26

I think this was incomplete crap

Answer 26

Crap

Question 27

What’s Hess law the chemical version of what?

Answer 27

The principle of conservation of energy

Question 28

I rlly need fucking pictures

Answer 28

Not completing until i find a different study method