Energetics Flashcards

1
Q

All chemical reactions involve…

A

… changes in energy, usually heat energy

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2
Q

Enthalpy change

A
  • energy change during any change in a system under constant pressure
  • ΔH
  • dépendent on amount of material involved, so the values are standardised
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3
Q

When a bond is formed…

A

… one electron from each atom entered a new electron cloud of lower energy

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4
Q

Bond enthalpy

A
  • heat energy obtained when a bond is formed
  • heat energy taken in when a bond is broken
  • 1dp
  • approximation - lacks accuracy
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5
Q

Having a low bond enthalpy

A
  • allows for low Ea; faster rate

* broken first in a series of steps

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6
Q

Why do bond enthalpies lack accuracy?

A

The energy required to break each of the bonds in a molecule is different

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7
Q

Mean bond enthalpy

A
  • enthalpy needed to break the covalent bond into gaseous atoms, averaged over different molecules
  • positive, because is energy is required to break a bond
  • must start and end in gaseous state
  • other states are much less accurate

E.g. 1/4CH4 -> C + H

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8
Q

Why is the real bond enthalpy différent to the average bond enthalpy?

A

The environment of the bonds differ

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9
Q

Exothermic

A
  • -ΔH
  • à réaction in which energy is given out
  • more energy is given out from bonds being formed than energy is taken in from bonds being broken
  • temperature increase in surroundings
  • majority of reactions
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10
Q

Endothermic

A
  • +ΔΗ
  • a reaction in which heat energy is taken in
  • breaking of bonds requires more energy than is given out by the formation of new bonds
  • temperature decrease in surroundings
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11
Q

Standard enthalpy of ΔH

A
  • molar quantities
  • 298K
  • 1 atm pressure
  • normal state
  • solutions at 1moldm^-3
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12
Q

Elements in their common state…

A

… are assigned an energy value of zero

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13
Q

What is important in enthalpy change values?

A
  • state signs
  • allotropic form

Because values are specific to quantity

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14
Q

Standard enthalpy of formation

A
  • ΔHf
  • enthalpy change that takes place when one mole of substance is formed from its elements in their standard states at a temperature of 298K and a pressure of 100kPa
  • an element = 0kJmol^-1
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15
Q

Standard enthalpy of combustion

A
  • ΔHc
  • enthalpy change when one mole of substance is burned completely in oxygen at a temperature of 298K and 100kPa, in standard states
  • incomplète combustion will lead to carbon (soot), CO and water; less exothermic
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16
Q

Standard enthalpy of neutralisation

A
  • ΔHneut
  • the enthalpy change when one mole of water is formed in a neutralisation reaction between an acid and an alkali at a temperature of 298K, and a pressure of 100kPa, in standard states
  • always exothermic
17
Q

Standard enthalpy of reaction

A
  • ΔH
  • then enthalpy change when the number of moles of substances in the equation react at a temperature of 298K and a pressure of 100kPa, in standard states
18
Q

Calorimetry

A

Measures the energy given out in a reaction and absorbed by water, either as a solvent for the chemicals or in a metal calorimeter

19
Q

Measuring calorimetry

A

1) mixing substances in an insulated container

2) using a calorimeter in a combustion experiment

20
Q

Enthalpy in solution practical

A

1) place 30cm^2 of dilute HCl (measured with a measuring cylinder) into an expanded, insulated polystyrene cup in a beaker
2) record the temperature every 3 minutes using a thermometer
3) after 3.5mins, add 30cm^2 NaOH
4) stir continuously
5) take readings every minute for 10mins
6) plot s graph and extrapolate lines

21
Q

Accuracy when measuring the enthalpy of a solution

A
  • wash equipments with solutions before use
  • dry equipment
  • if solid reagent is used, weigh with balance
  • if the reaction is too slow, cooling occurs, decreasing accuracy of ΔT; take temp at regular intervals and extrapolate back to 0
  • measure initial temperatures of both for an average initial temperature
22
Q

Errors in enthalpy in solution

A
  • assuming volume (and mass) of water is equal to volume (and mass) of solution
  • assuming Cp to be the same as water
  • energy lost to surroundings over time (relatively small)
  • neglecting Cp of calorimeter; ignoring energy absorbed by apparatus
  • incomplète réaction
23
Q

Combustion enthalpy practical

A

1) measure 100cm^3 water (w/ measuring cylinder)
2) place water in calorimeter
3) place liquid in spirit burner and weigh (w/ balance)
4) note initial temperature of water (w/ thermometer)
5) set up calorimeter above spirit burner on tripod
6) it tie the liquid
7) extinguish the flame after ΔT = 20°C
8) re-weigh the burner to find out how much liquid has been burnt

24
Q

Error in the combustion calorimetry

A
  • energy lost to the surroundings (relatively large); flame heat escaping as convection current; using a thermometer which reads to less than 1°C is not justified
  • unlikely to give more than 2sf
  • incomplète combustion or energy transfer; évaporation of fuel
  • lack of special equipment to allow calorimeter to capture energy (e.g. draught excluder)
  • H2O (g) is not standard
25
Hess’ law
* for a given chemical change, overall energy change will always be the same where the change takes place in one or multiple steps * derived from first law of thermodynamics; energy is always conserved
26
Shorthand Hess’ law
Total enthalpy change for a reaction is independent of the rate by which the chemical reaction takes place
27
Hess law is used to...
... calculate enthalpy changes for reaction that cannot be measured directly by experiments; alternative reactions are carried out that can be measured experimentally
28
Examples of reactions that cannot be measured experimentally
* adding the right amount of water in hydration reactions * measuring temperature changes of solids * impossible to measure temperature to decompose a solid * reactions that do not readily occur
29
Dashed arrow show...
... indirect routes; signs must be observed so that the reactions measured point towards the product
30
When drawing a Hess cycle, remember to show;
* intermediates * data * calculation steps
31
Homologous series enthalpy changes
* enthalpy of combustion increases in constant increments | * because there is a constant amount and type of extra bonds being broken and made