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Chemistry L6 > Energetics > Flashcards

Energetics Flashcards

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1
Q

What is enthalpy change ΔH?

A

the heat change at a constant pressure

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2
Q

What is the ΔH in an exothermic reaction?

A

negative
an exothermic reaction gives out heat

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3
Q

What is the ΔH for an endothermic reaction?

A

positive
and endothermic reaction takes in heat from the surroundings

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4
Q

What is Standard enthalpy change?

A

reactions done under standard conditions

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5
Q

What are the standard conditions for enthalpy change?

A

temperature - 298K
pressure - 100kPa

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6
Q

What is the standard enthalpy change of combustion ΔcH?

A

The enpalthy change when 1mol of a substance is completely burned in oxygen
under standard conditions and states

ALWAYS exothermic

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7
Q

What is the standard enthalpy of formation ΔfH?

A

The enthalpy change when 1 mol of a compound is formed FROM its elements
under standard conditions and states

endo OR exothermic

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8
Q

What is the ΔfH for an ELEMENT?

A

Zero

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9
Q

What is Hess’s law?

A

the enthalpy change for a reaction is independent of the route taken

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10
Q

What is the equation for enthalpy of combustion?

A

Σ∆cH (reactants) - Σ∆cH (products)

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11
Q

What is the equation for enthalpy change of formation?

A

Σ∆fH (products) - Σ∆fH (reactants)

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12
Q

What is Calorimetry?

A

used to find the heat released in a reaction

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13
Q

How can enthalpy of combustion be found experimentally?

A
  • burning a known mass of a substance
  • use the energy released to increase the temperature of a known mass of water
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14
Q

What is the equation for heat energy?

A

q= mc∆T
specific heat capacity = 4.18

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15
Q

What is the calculation for ∆H using heat energy?

A

∆H = q/n

q must be divided by 1000

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16
Q

What happens in a combustion reaction?

A

a fuel is used to heat up a known amount of water

17
Q

Why will the value of ∆H be less than a data book value?

A

heat loss to surroundings
incomplete combustion

18
Q

What is bond dissociation energy?

A

Enthalpy change to break the bond in 1 mol of gaseous molecules to form gaseous atoms

19
Q

What are bond enthalpies?

A

bonds that only appear in one molecule and values would be accurate
eg Cl-Cl

20
Q

What is a mean bond enthalpy?

A

enthalpy required to break 1 mol of covalent bonds averaged over a range of compounds

21
Q

Which bonds are mean enthalpy bonds?

A

bonds that appear in many molecules
bind enthalpy is different in in diff molecules
values are not 100% accurate

22
Q

What is the equation for enthalpy change of a reaction?

A

∆H = Σ bonds broken - Σ bonds formed

Chemistry L6 (17 decks)

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