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Chemistry: Module 3 > Enthalpy Changes > Flashcards

Enthalpy Changes Flashcards

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1
Q

what is enthalpy?

A

it measures the heat energy in a chemical system

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2
Q

what are the units for standard enthalpy change?

A

kJmol-1

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3
Q

how to calculate enthalpy change?

A

H(products) - H(reactants)

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4
Q

what type of reaction has a negative enthalpy change?

A

exothermic

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5
Q

what type of reaction is a positive enthalpy change?

A

endothermic

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6
Q

what is an exothermic reaction?

A

a chemical system that releases heat energy to the surroundings

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7
Q

what is an endothermic reaction?

A

a chemical system that takes in heat energy from its surroundings

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8
Q

what is activation energy?

A

minimum energy required for a reaction to take place

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9
Q

what happens to the temperature of the surroundings in an exothermic reaction?

A

increases

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10
Q

what is on the x axis of an energy profile?

A

progress of reaction

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11
Q

what is on the y axis of an energy profile diagram?

A

enthalpy, H

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12
Q

for an exothermic reaction, what has a higher enthalpy?

A

reactants

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13
Q

for an endothermic reaction, what has a higher enthalpy?

A

products

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14
Q

what would the speed of a reaction be if it has a small activation energy?

A
  • takes place quickly
  • because the energy needed to break bonds is readily available from the surroundings
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15
Q

what would the speed of a reaction be if it has a large activation energy?

A

the reaction would be slow or don’t occur at all

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16
Q

what is standard pressure?

A

100kPa

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17
Q

what is standard temperature?

A

298K (25 degrees celsius)

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18
Q

what is standard concentration?

A

1moldm-3

19
Q

what is standard state?

A

physical state of a substance under standard conditions (100kPa and 298K)

20
Q

what is enthalpy change of reaction?

A

the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions with all reactants and products in their standard states

21
Q

what is enthalpy change of formation?

A

the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

22
Q

what is enthalpy change of combustion?

A

the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions with all reactants and products in their standard states

23
Q

what is enthalpy change of neutralisation?

A

the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions with all reactants and products in their standard states

24
Q

what are the 3 methods to calculate enthalpy changes?

A
  • calorimetry
  • bond enthalpies
  • Hess’ law
25
Q

what is the equation for calorimetry?

A

q=mc^T

26
Q

what is the value of c?

A

4.18

27
Q

how do you work out enthalpy change using q=mc^t?

A

enthalpy change= q/ moles

28
Q

what mass and moles do you use when calculating enthalpy change of reaction?

A

mass: total mass
moles: limiting reagent

29
Q

what mass and moles do you use when calculating enthalpy change of neutralisation?

A

mass: total volume of acid and base
moles: moles of water

30
Q

what type of reaction has a rise in temperature?

A

exothermic

31
Q

what type of reaction has a fall in temperature?

A

endothermic

32
Q

in terms of bond breaking and bond forming, why are some reactions endothermic?

A

more energy is required for breaking bonds compared to the energy released when forming bonds

33
Q

in terms of bond breaking and bond forming, why are some reactions exothermic?

A

more energy is released when bond forming compared to the energy required when breaking bonds

34
Q

what affects the accuracy of the enthalpy change?

A
  • heat loss to surroundings
  • incomplete combustion
  • evaporation of methanol from the wick
  • non-standard conditions were used
35
Q

what is bond enthalpy?

A

when one mole of a specified type of bond in gaseous bonds is broken

36
Q

what is a limitation of bond enthalpy?

A
  • in different environments, the actual energy involved in breaking and forming individual bonds may differ slightly
37
Q

what does Hess’ law state?

A
  • route 1 = route 2
  • sum of clockwise arrows = sum of anticlockwise arrows
38
Q

how do you draw Hess’ cycle for an enthalpy change of formation?

A

reactants –> products
elements in standard states
- arrows point upwards from elements in standard states to the reactants and products

39
Q

what is the equation for enthalpy change of formation(Hess’s law)?

A

^H = sum of ^H products - sum of ^H reactants

40
Q

what is the enthalpy change of elements?

A
  • 0kJmol-1
41
Q

how do you draw Hess’ cycle for an enthalpy change of combustion?

A

reactants –> products
oxides
- arrows point downwards from reactants and products towards the oxides

42
Q

what is the equation for enthalpy change of formation(Hess’s law)?

A

^H = sum of ^H reactants - sum of ^H products

43
Q

what is the acronym for remembering endothermic and exothermic for bond breaking and bond forming?

A
  • bendo
  • mexo
44
Q

what is average bond enthalpy?

A

the breaking of 1 mol of bonds in gaseous molecules

Chemistry: Module 3 (12 decks)

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