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H2 Chemistry > chemical energetics l > Flashcards

chemical energetics l Flashcards

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1
Q

ΔH (delta H) definition

A

change in energy content, whether energy released or absorbed, of a process in a system at constant pressure
unit: KJ mol-1

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2
Q

describe exothermic reaction

A
  1. energy is released to the surroundings
  2. temperature of surroundings increase
  3. delta H is negative, <0
  4. products are energetically more stable than reactants
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3
Q

describe endothermic reaction

A
  1. energy is absorbed from the surroundings
  2. temperature of surroundings decrease
  3. delta H is positive, >0
  4. products are energetically less stable than reactants
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4
Q

define standard enthalpy change of reaction

A

energy change in a chemical reaction when the molar quantities of reactants stated in the chemical equation react under standard conditions of 1 bar and 298K

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5
Q

standard enthalpy change of formation of a substance

A

energy change when 1 mole of the pure substance in a specified state is formed from its constituent elements in their standard state under standard conditions of 1 bar and 298K

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6
Q

what can standard enthalpy change of formation of a substance signify

A

used to predict the stability of a compound relative to its constituent elements

more negative its value, the more stable the compound relative to its constituent elements, l

less likely the decomposition of the cpd back into its constituent elements

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7
Q

standard enthalpy change of formation of an element

A

always 0 KJ mol-1

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8
Q

standard enthalpy change of combustion

A

energy released when 1 mole of the substance is completely burnt in excess oxygen under standard conditions of 1 bar and 298K

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9
Q

what can standard enthalpy change of combustion indicate

A

energy values of fuels
more heat liberated upon complete combustion, the bestter the fuel is

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10
Q

definition of standard enthalpy change of neutralisation

A

energy change when the an acid and base react to form 1 mole of water under standard conditions of 1 bar and 298K

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11
Q

what can reaction between a strong acid and a strong base

A

reaction between aqueous H+ and aqueous OH- to produce 1 mol of H2O
H+ is produced when a strong acid is completely dissociated
OH- is produced when a strong base is completely ionised

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12
Q

why is a neurtralisation reaction between a weak acid and strong base less exothermic than a neutralisation reaction between a strong acid and strong base

A

since some energy is consumed to bring about further dissociation of the weak acid to produce enough H+ for the reaction, the energy relreased from the reaction will be less, making the reaction less exothermic

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13
Q

what is the dissociation reaction made up

A

breaking of the O-H bond, which is endothermic
hydration of the H+ ion which is exothermic
overall dissociation reaction is endothermic

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14
Q

define standard enthalpy change of atomisation of an element

A

energy absrobed when 1 mole of gaseous atoms is formed from the element under standard conditions of 1 bar and 298K

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15
Q

define standard enthalpy change of atomisation of a compound

A

energy absorbed when 1 mole of the compound is converted to gaseous atoms under standard conditions of 1 bar and 298K

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16
Q

why is the standard enthalpy change of atomisation of element/cpd always positive

A

energy must be absorbed to break all the bonds between the atoms in the element/compound during atomisation reaction

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17
Q

define bond dissociation energy of a X-Y bond BDE

A

energy required to break 1 mole of that particular X-Y bond in a particular cpd in gaseous state
value is positive as bond breaking absorbs ebergy

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18
Q

delta change for the reverse of a BDE reaction

A

reverse reaction involves formation of 1 mole of X-Y bond
so it is negative

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19
Q

define bond energy of a bond

A

average energy required to break 1 mole of the X-Y bond in the gaseous stare

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20
Q

how can value of BE be calculated

A

for a diatomic gas X2, bond energy of the X-X bond = 2 x enthalpy change of atomisation of the diatomic gas

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21
Q

define first ionisation energy of an element

A

energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of singly positively charged gaseous ions

22
Q

define second IE of an element

A

energy required to remove 1 mole of electrons from one mole of singly positively charged ions to form 1 mole of doubly positively charged gaseous ions

23
Q

define 1st electron affinity of an element

A

energy change when 1 mole of gaseous atoms acquires 1 mole of electrons to form of singly negatively charged gaseous ions

24
Q

why is 1st EA always negative

A

energy released when the nucleus attracts the additional electron is usually larger than the energy taken in to overcome inter-electronic repulsion

25
Q

define 2nd EA of an element

A

energy absorbed when 1 mole of singly negatively charged gaseous ions acquires 1 mole of electrons to form 1 mole of doubly negatively charged gaseous ions

26
Q

why is th 2nd EA always positive

A

energy is required to overcome the repulsion between the two negatively charged species, one being the anion and the other being the electron

27
Q

what does infinitely dilute solution mean

A

a solution that does not produce any further enthalpy change when more solvent is added

28
Q

how to calculate heat change

A

q in J = mass of solution in g x specific heat capacity x change in temperature in celsius

29
Q

define specific heat capacity

A

quantity of heat required to raise the temperature of 1g of the substance by 1 degree celsius/1K
units: J g-1°C or J g-1 K-1

30
Q

define heat capacity

A

quanity of heat required to raise the temperature of the substance by 1°C/1K
units: J°C-1 or JK-1

31
Q

relationship between heat change and enthalpy change

A

enthalpy change in J/mol = - heat change/no of mol

32
Q

what can n be in different reactions

A

for enthalpy change of:
neutralisation: no of moles of water formed
combustion: no of moles of reactant combustion
reaction: no of moles of limiting reactant

33
Q

what are assumptions when calculating heat change

A
  1. negligible heat loss to surrounding due to insulation
  2. density of solution approximately that of water 1 gcm-3 unless otherwise stated
  3. specific heat capacity of solution is approximately that of water, 4.18Jg-1k-1 unless otherwise stated
34
Q

calculating enthalpy change using combustion data

A

sum of reactants minus sum of products

35
Q

calculating enthalpy change using formation data

A

sum of products minus sum of reactants

36
Q

calculating enthalpy change using bond energy data

A

sum of bond energy of bonds in reactants minus sum of bond energy of bonds in products

37
Q

define lattice energy (of an ionic compound)

A

energy released when 1 mole of the solid ionic compounds is formed from its constituents gaseous ions under standard conditions of 1 nar and 298K

38
Q

what is lattice energy used as a measure of

A

used as a strength of ionic bonding: the more negative/greater the magnitude of lattice energy of compound, the stronger the ionic bonding

39
Q

what are the factors affecting LE and how do they affect LE

A
  1. ionic charge: the bigger the q+ or q-, value of q+q- is larger so greater/more negative the magnitude of LE
  2. ionic size: smaller the r+ or r-, the smaller the sum of r+ and r-, so greater/more negative the magnitude of LE
40
Q

what is the purely ionic model of a lattice

A

assumes that all the ions are spherical and have their charge evenly distributed around them

41
Q

what are predominantly ionic cpds

A

experimental lattice energy values are in good agreement with theoretical lattice energy values

structure of lattice for these cpds are close to purely ionic, where the lattice consists of spherical ions with evenly distributed charge

42
Q

whtat are ionic compounds with partial covalent character

A

discrepancy btwn experimental and theoretical lattice energies
the ionic cpds have some covalent character due to substantial polarisation of anion by the cation present

43
Q

why can the experimental lattice energy values be more negative than the theoretical values

A

bonding in the ionic cpds with covalent character is stronger than that predicted by the purely ionic model

44
Q

define standard enthalpy change of hydration (of an ion)

A

energy released when 1 mole of gaseous ions is dissolved in water to form 1 mole of aqueous ions under standard conditions of 1 bar and 298K

45
Q

sdefine standard enthalpy change of solution of a substance

A

energy change when 1 mole of teh substance is completely dissolved in a solvent to form an infinitely dilute solution under standard conditions of 1 bar and 298K

46
Q

step 1 of dissolving ionic solid in water

A
  1. ionic crystal lattice breaks down to form monatomic gaseous ions
  2. since ionic bonds are broken, it is a endothermic process
  3. an amount of energy equals to lattice energy is absorbed to break the ionic bonds and force the ions apart to form gaseous ions
47
Q

step 2 of dissolving ionic solid in water

A
  1. ion-dipole interactions form between the gaseous ions and water molecules to produce hydrated ions in aqueous solution
  2. hydration process is exothermic
  3. enthalpy cahnge of hydration: energy released when gaseous ions dissolve in water
48
Q

calculating enthalpy change of solution with lattice energy

A

sum of enthalpy change of hydration of each of the gaseous constituent ions minus lattice energy

49
Q

solubility of ionic salt when enthalpy change of hydraation < 0

A

ionic salt is soluble in water
total hydration energy released is enough to compensate for energy required to break down solid ionic crystal lattice
the more negative the value enthalpy change of hydration, the more soluble the salt

50
Q

solubility of ionic salt when enthalpy change of hydraation > 0

A

ionic salt is not soluble in water
total hydration energy released is NOT enough to compensate for energy required to break down solid ionic crystal lattice

H2 Chemistry (14 decks)

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