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Chemistry A Level > Energetics > Flashcards

Energetics Flashcards

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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
Q

Hess’ law

A
  • 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
Q

Shorthand Hess’ law

A

Total enthalpy change for a reaction is independent of the rate by which the chemical reaction takes place

27
Q

Hess law is used to…

A

… calculate enthalpy changes for reaction that cannot be measured directly by experiments; alternative reactions are carried out that can be measured experimentally

28
Q

Examples of reactions that cannot be measured experimentally

A
  • 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
Q

Dashed arrow show…

A

… indirect routes; signs must be observed so that the reactions measured point towards the product

30
Q

When drawing a Hess cycle, remember to show;

A
  • intermediates
  • data
  • calculation steps
31
Q

Homologous series enthalpy changes

A
  • enthalpy of combustion increases in constant increments

* because there is a constant amount and type of extra bonds being broken and made

Chemistry A Level (12 decks)

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