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AS Chemistry > 3.2.1 Enthalpy Changes > Flashcards

3.2.1 Enthalpy Changes Flashcards

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1
Q

What is enthalpy and how is it measured?

A
  • heat content of a chemical system
  • measured by the energy change
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2
Q

What are exothermic and endothermic reactions?

A
  • exo
  • energy is given out
  • ΔH is negative
    -e.g.combustion of fuels
  • endo
  • heat energy is absorbed
    ΔH is positive
  • e.g. thermal decomposition of CaCO3
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3
Q

Enthalpy change in exothermic reactions

A
  • heat loss from system to surroundings
  • ΔH is negative because heat is lost by system
  • enthalpy of products is smaller than enthalpy of reactants
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4
Q

Exothermic energy level diagram

A

See notes

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5
Q

Enthalpy change in endothermic reactions

A
  • require an input of heat energy
  • ΔH is positive because heat is gained by chemical system
  • enthalpy of products is greater than enthalpy of reactants
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6
Q

Enthalpy level diagram for endothermic reaction

A

See notes

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7
Q

What is the equation for photosynthesis?

A

6CO2 + 6H2O -> C6H12O6 + 6O2

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8
Q

What is activation energy?

A

The minimum energy required for a reaction to take place

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9
Q

What are standard conditions?

A

Temperature - 298K
Pressure - 100kPa
Solutions - 1 moldm-3
Standard states

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10
Q

What is the standard enthalpy change of reaction?

A

The enthalpy change that occurs between molar quantities of reactants as shown in the stated equation, under standard conditions to give products in their standard state.
ΔH°r

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11
Q

What is the standard enthalpy change of formation?

A

The enthalpy change when one mole of a compound is formed from its elements under standard conditions both compound and elements are in their standard states
ΔH°f
Learn

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12
Q

What is the standard enthalpy change of combustion?

A

The enthalpy change when one mole of an element or compound reacts by complete combustion with excess oxygen under standard conditions
ΔH°c
Learn

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13
Q

What is the standard enthalpy change of neutralisation?

A

The enthalpy change when 1 mol of water is formed from a neutralisation reaction
ΔH°n

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14
Q

What is the equation to calculate heat energy given out?

A

Q = mcΔt
Q is in Joules
M is mass of solution in grams

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15
Q

How to calculate enthalpy change from heat energy?

A

Convert to kJ
kJ / mol
Find mol of reactants (not normally solution)

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16
Q

Why do enthalpy changes take place?

A
  • bonds are broken and formed
  • energy (in the form of heat) is needed to overcome attractive forces between atoms
17
Q

What is bond breaking and bond making?

A
  • bond breaking is endothermic
  • bond making is exothermic
18
Q

What is exact bond enthalpy?

A
  • the amount of energy needed to break one mole of specific covalent bonds in the gaseous state
19
Q

What is the difference between average bond enthalpy and exact bond enthalpy?

A
  • bond energies are affected by other atoms in the molecule
  • an average of a number of the same type of bond but in different environments is calculated
20
Q

What is average bond enthalpy?

A
  • the mean energy needed for the breaking of 1 mol of bonds in gaseous molecules
21
Q

How to calculate enthalpy change from bond energies?

A

ΔH = E (bond enthalpies of reactants) - E(bond enthalpies of products)

22
Q

What is Hess’ law?

A
  • the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs
  • it is an indirect way of measuring enthalpy change
23
Q

Draw a Hess’ cycle for enthalpy of formation (rough)

A

See notes

24
Q

Draw a Hess’ cycle for enthalpy of combustion (rough)

A

See notes

25
Q

Draw and solve a Hess’ cycle for combustion

A

See notes

26
Q

What is the link between strong and weak acids and enthalpy change?

A

Strong acids: all negative values since complete dissociation occurs, reactions have high values
Weak acids: all values are slightly lower

27
Q

Describe how a simple calorimeter can be used to calculate enthalpy change

A
  • measured directly by monitoring temperature change
  • experiments carried out in polystyrene beaker which are good insulators and prevent heat loss and gain
  • calculate heat energy exchanged using Q = mcΔt
  • calculate enthalpy by dividing by moles
28
Q

Arrows for Hess’s cycle of combustion

A

Point down

29
Q

Arrows for Hess’s cycle of formation

A

Point up

AS Chemistry (23 decks)

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