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AS Chemistry > Developing Fuels: Thermochemistry > Flashcards

Developing Fuels: Thermochemistry Flashcards

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1
Q

Definition enthalpy

A

Measure of heat content of a substance

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2
Q

Definition enthalpy change ( delta H )

A

Change in heat content at constant pressure

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3
Q

Symbol for enthalpy change

A

Delta H

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4
Q

Standard conditions (delta H°) (pressure and temp )

A

10^5 Pa

298 K

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5
Q

Describe reaction profile / energy level diagram for EXOthermic reactions

A
  • products = lower energy than reactants

- ve delta H

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6
Q

Describe reaction profile / energy level diagram for ENDOthermic reactions

A
  • products = higher energy than reactants

- +ve delta H

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7
Q

Definition of standard enthalpy change of reaction ( delta(r) H°)

A

Enthalpy change for a reaction with the quantities shown in the chemical equation

= value should always be quoted along with the equation

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8
Q

Symbol for standard enthalpy change of a reaction

A

Delta(r) H°

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9
Q

Definition of standard enthalpy change of formation ( delta(f) H°)

A

Enthalpy change when 1 mole of a substance is formed from its constituent elements with all reactants and products in standard states under standard conditions

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10
Q

Symbol for standard enthalpy change of formation

A

Delta(f) H°

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11
Q

Write equation to show standard enthalpy change of formation ( delta(f) H°) of H2O(l)

A

H2(g) + 1/2 O2 (g) —> H2O(l)

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12
Q

Definition of standard enthalpy change of combustion ( delta(c) H°)

A

Enthalpy change when 1 mole of a substance is completely burned in oxygen with all reactants and products in standard states under standard conditions

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13
Q

Symbol for standard enthalpy change of combustion

A

Delta(c) H°

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14
Q

Write equation to show standard enthalpy change for combustion of CH4

A

CH4(g) + 2 O2(g) —> CO2(g) + 2 H2O(l)

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15
Q

Definition of standard enthalpy change of neutralisation ( delta(neut) H°)

A

Enthalpy change when 1 mole of WATER is formed in a reaction between an acid and alkali under standard conditions

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16
Q

Symbol for standard enthalpy change of neutralisation

A

Delta ( neut) H°

17
Q

Write equation to show standard enthalpy change of neutralisation ( delta(neut) H°) of sulfric acid and sodium hydroxide

A

1/2 H2 SO4 (aq) + NaOH (aq) —>

1/2 Na2 SO4 ( aq) + H2O(l)

18
Q

Equation for heat energy transferred ( given out / absorbed )

A

q= mcAT

19
Q

What does each letter stand for in q= mcAT

A
  • q= heat energy transferred ( J )
  • m= mass of substance ( liquid / solution ) heated
  • c = specific heat capacity ( 4.18)
  • AT (delta T ) = change in temp ( in K but no conversions required )
20
Q

Equation for ( molar ) enthalpy change (per mole )

A

delta H = q ( KJ) / no. of moles reacting

21
Q

What does enthalpy change always have to have

A

+ or -

22
Q

In a calorimetry experiment if temp INCREASES what kind of reaction is it

A

EXOthermic ( -ve delta H )

23
Q

In a calorimetry experiment if temp DECREASES what kind of reaction is it

A

ENDOthermic ( +ve delta H )

24
Q

Describe steps to carry out dissolving / displacement / neutralisation calorimetry

A
  1. Using a measuring cylinder pour a known volume of a known concentration of acid into an insulated vessel
  2. Measure temp of acid before adding the alkali
  3. Using a measuring cylinder add a known volume of a known concentration of alkali to the acid
  4. Stir
  5. Record highest temp reached
  6. Calculations: q=mcAT, mols of acid/alkali ( one which is not in excess ) , delta H
25
Q

Describe steps to carry out combustion calorimetry

A
  1. Using a measuring cylinder pour a known volume of water into copper calorimeter
  2. Weigh spirit burner before lighting
  3. Measure temp of water before lighting
  4. Light the wick to heat the water & stir
  5. Extinguish burner & record temp of water after heating
  6. Reweigh the spirit burner
  7. Calculations:q=mcAT,mols fuel burnt, delta H
26
Q

Define average bond enthalpy ( 2 marker )

A
  • av. enthalpy change when 1 mole of bonds

- of gaseous covalent bonds is broken

27
Q

BENDO MEX

A
  • Breaking bonds is endothermic

- Making bonds is exothermic

28
Q

Energy change= ???

A

Break - Make

( Reactants - Products )

( energy needed to break bonds - energy released making bonds )

29
Q

Sign for EXOthermic reaction

A

-ve

30
Q

Sign for ENDOthermic reaction

A

+ve

31
Q

Explain what happens in an EXOthermic reaction

A

More energy is released making bonds than is needed to break bonds

( heat energy given out = increase temp)

32
Q

Explain what happens in an ENDOthermic reaction

A

More energy is needed to break bonds than is released making bonds

( heat energy taken in = decrease temp)

33
Q

Why are many bond enthalpies describes as averages

A

They are averaged out over different compounds containing the same bond

AS Chemistry (43 decks)

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