Unit 2.1 Thermochemistry Flashcards

1
Q

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

A

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

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2
Q

2 examples of an endothermic reaction?

A
  • Photosynthesis
  • Decomposition of CaCO3
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3
Q

3 examples of an exothermic reaction?

A
  • Respiration
  • Combustion
  • Metal + acid
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4
Q

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

A

worth it?
saves time.
meh

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5
Q

If endothermic, △H = ?

A

Positive

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6
Q

If exothermic, △H = ?

A

Negative

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7
Q

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

A

More exothermic
More negative
More -△H

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8
Q

Define bond enthalpy

A

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

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9
Q

Define average bond enthalpy

A

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

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10
Q

How to calculate standard enthalpy change

A
  • 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?
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11
Q

Explain standard enthalpy change of formation △Hfθ or △fHθ

A

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

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12
Q

a) C(s) + ½O2(g) -> CO(g)
b) 2C(s) + O2(g) -> 2CO(g)
c) C(g) + ½O2(g)) -> CO(g)

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

A

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

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13
Q

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.

A

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

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14
Q

Explain standard enthalpy change of combustion △Hcθ or △cHθ

A

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

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15
Q

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

A

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

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

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

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16
Q

Difference of standard molar enthalpy of formation of potassium oxide (K2O) between △Hfθ and △Hcθ

A

△Hfθ = 2K(s) + ½O2(g) -> K2O(s)

△Hcθ = K(s) + ¼O2(g) -> ½K2O(s)

I think the trick here is:
△Hfθ = 1 mole of product
△Hcθ = 1 mole of substance (reactant perhaps?)

17
Q

What does the θ mean seen in the notation △Hfθ or △Hcθ?

A

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

18
Q

Explain enthalpy change of reaction △Hr or △rH

A

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

19
Q

State Hess’s Law

A

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

20
Q

Wait

A

Wait?