Enthalpies Flashcards

Energy changes and rules in Chemistry!!!

1
Q

What is the Law of Conservation of Energy?

A

That energy can never be destroyed or created, but only transfered from one store to another…

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2
Q

What is Enthalpy?

A

The measure of Thermal Energy involved in a chemical reaction

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3
Q

How can we find the Enthalpy Change?

A

H(products) - H(reactants) = delta H

delta H will be positive when products having more energy than reactants….

and opposite goes the same……….

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4
Q

What happens during exothermic and endothermic reactions?

A
  1. EXO = Energy transfered from the sytem to surroundings, with TEMP INCREASE
  2. ENDO = Energy transfered to sytem from system to surroundings, with TEMP DECREASE.
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5
Q

What are the Standard Enthalpy Conditions?

A
  1. Pressure = 100 Kpa (1 atm)
  2. Temperature = 25.C (298K)
  3. Concentration = 1moldm-3
  4. Physical State = its state at RTP
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6
Q

What is the Standard Enthalpy Change of Reactions?

A
  • Enthalpy change that accompanies a reaction in molar quantities shown in chemical equations, under standard conditions!
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7
Q

What is the Standard Enthalpy Change of Formation?

A
  • Enthalpy change taking place when one mole of a compound is formed from its own elememts, under standar conditions
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8
Q

What is the Standard Enthalpy Change of Combustion?

A
  • Enthaply change taking place when 1 mole of a compound is completely burned with oxygen, under standard conditions
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9
Q

What is the Standard Enthalpy Change of Neutralisation?

A
  • Enthalpy change that accompanies a neutralisation reaction, with a base and an acid, to form 1 mole of liquid water, under standard conditions!
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10
Q

What is Hess’ Law?

A

“If a reaction can occur in 2 or multiple routes with the exact same initial and final conditions, the enthalpy change will remain the same for all routes!!

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11
Q

How can we construct enthalpy change for formation?

A
  • Using arrow POINTING UP, labelling delta H 1,2 and 3 in correspondong directions
  • Using Route 1 and Route 2 rule equations to find enthalpy changes

ROUTE 2 = -delta H2 +delta H3

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12
Q

How can we construct enthalpy change for combustion?

A
  • Using arrow POINTING DOWN, labelling delta H 1,2 and 3 in correspondong directions
  • Using Route 1 and Route 2 rule equations to find enthalpy changes

ROUTE 2 = delta H2 - delta H3

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13
Q

What is the 2 equations needed for Bond Enthalpy Calculations?

A
  1. Energy = Mass * SHC * Temp Change
  2. Enthalpy Change = Energy / Moles
  3. Water SHC: 4.18 j/kg/
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14
Q

Why may the enthalpy change of combustion be inaccurate at times?

A
  1. Heat lost to surroundings apart from water = more energy required to heat the same mass
  2. Incomplete combustion of methanol = methanol producing CO than CO2
  3. Evaporation of combustion chemical = if evaporated from the combustion vessel, lost mass of chemical unburned!
  4. Non-Standard conditions = official values are always under standard conditions, ensuring consistency and control variable!
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