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Chemistry > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

Explain system and surroundings

A

The system consists of the molecules which are reacting, the surroundings are everything else e.g. air, solution, glassware

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2
Q

What is the symbol for enthalpy change?

A

🔺️H

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3
Q

What happens during an exothermic reaction?

A

Products have lower enthalpy than reactants and energy is transferred from the system to the surroundings (surroundings get hot)

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4
Q

What is 🔺️H measured in?

A

kJ mol-1

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5
Q

What happens during an endothermic reaction?

A

Products have higher enthalpy than reactants and energy is transferred from surroundings to system (surroundings get colder)

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6
Q

Definition of activation energy

A

Activation energy (Ea) is the minimum energy required to start a reaction

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7
Q

Describe the arrows in an exothermic and endothermic profile

A
  • Ea arrow goes to the top of the curve from the reactants, single-headed
  • 🔺️H arrow goes from reactants to products (down exothermic, up endothermic), single-headed
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8
Q

What are the standard conditions?

A

Pressure of 100kPa
Temp of 298K (25°C)
Concentration of 1 mol dm-3

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9
Q

What must enthalpy change equations have?

A

State symbols

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10
Q

Definition of enthalpy change of combustion

A

The enthalpy change when ONE MOLE of a substance reacts COMPLETELY WITH OXYGEN under standard conditions

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11
Q

Definition of enthalpy change of formation

A

The enthalpy change when ONE MOLE of a compound is formed from its CONSTITUENT ELEMENTS in their STANDARD STATES

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12
Q

Definition of enthalpy change of neutralisation

A

The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form ONE MOLE OF WATER under standard conditions

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13
Q

At what enthalpy change is 🔺️neutH constant?

A

-57 kJ mol-1

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14
Q

What is the 🔺️fH of elements?

A

0 kJ mol-1

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15
Q

What are the 2 ways of determining enthalpy changes?

A

Polystyrene calorimeter and spirit burner

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16
Q

What is the equation for calculating energy exchanged?

A

q = mc🔺️T

17
Q

What is the equation for calculating 🔺️H from q?

A

🔺️H = q/n

18
Q

What are the 2 errors which can occur in calorimetry?

A
  1. Heat loss to surroundings

2. Not done under standard conditions

19
Q

What impact does heat loss to surroundings have on q and 🔺️H?

A

q: 🔺️T is smaller (heat lost to air instead of water) than it should be so q is smaller

🔺️H: magnitude of 🔺️H decreases so 🔺️H is less exothermic than the actual value

20
Q

How is heat lost for 🔺️cH specifically?

A

Liquid fuel might evaporate, heat capacity of beaker not taken into account, incomplete combustion

21
Q

What does Bendo Mexo mean?

A 
Bendo = breaking bonds is endothermic 
Mexo = making bonds is exothermic
22
Q

What defines an exothermic reaction?

A

The energy required to break bonds is less than the energy released when making new bonds

23
Q

What defines an endothermic reaction?

A

The energy required to break bonds is more than the energy released when making new bonds

24
Q

Why are bond enthalpies always positive?

A

Bond-breaking is endothermic

25
Q

What is the equation for calculating 🔺️H from bond enthalpies?

A

🔺️H = sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds made

26
Q

What are the 2 limitations of bond enthalpies?

A
  1. Only used in GASEOUS state so if there are substances which aren’t gaseous under standard conditions the calculated 🔺️H will be different to the standard value
  2. It’s an average of the bond enthalpy in different compounds so different compounds will have slightly different enthalpies
27
Q

Why can’t some enthalpy changes be determined DIRECTLY?

A
  1. High activation energy
  2. Slow reaction rate
  3. More than one reaction taking place
28
Q

Describe combustion cycles

A
  • Arrows point down (represent combustion reactions)
  • Combustion products at bottom of arrows
  • With the indirect route arrow = add, against the indirect route arrow = subtract
29
Q

Describe formation cycles

A
  • If formation data is given, use a formation cycle
  • Elements at the bottom
  • Arrows point up as we form those substances from elements

Chemistry (29 decks)

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