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chemistry paper 1 > energetics > Flashcards

energetics Flashcards

1
Q

what is enthalpy change

A

heat energy change measured in a reaction at a constant pressure + temp

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2
Q

what are standard conditions of enthalpy change reactions

A

100kPa
298K (25 degrees celcius)

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3
Q

what is an endothermic reaction

A

reaction that absorbs energy from surroundings
delta H = positive

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4
Q

what is an exothermic reaction

A

reaction that releases energy to the surroundings
delta H= negative

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5
Q

what is ^rH

A

standard enthalpy change of reaction

change of enthalpy during formation of 1 mole of the substance from its constituent elements under standard conditions

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6
Q

what is ^neutH

A

standard enthalpy change of neutralisation

the enthalpy change when an acid and alkali react to form 1 mole of water under standard conditions

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7
Q

what is ^cH

A

standard enthalpy change of combustion

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

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8
Q

what is ^fH

A

standard enthalpy change of formation

the enthalpy change when 1 mole of a compound is formed from its constituent elements in standard states under standard conditions

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9
Q

what is bond breaking

A

energy is absorbed
endothermic process
^H is positive

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10
Q

what is bond making

A

energy is released
exothermic process
^H is negative

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11
Q

what is Hess’s Law

A

the enthalpy change for a chemical reaction is the same, regardless of the route taken from reactants to products

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12
Q

how to work out overall enthalpy change

A

energy to break bonds + energy to make bonds

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13
Q

what is calorimetry

A

an experimental method for finding enthalpy change by measure temp change over time, data can be extrapolated

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14
Q

what is specific heat capacity

A

the energy required to raise 1g of substance by 1K without a change of state

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15
Q

what is the equation for energy change

A

q=MC delta T

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16
Q

what is the equation for enthalpy change per mol

A

delta H = q/moles

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17
Q

why is calorimetry prone to errors

A

heat can be lost from reaction by conduction, convection or inaccuracies in measuring temperatures

specific heat capacity=4.18 however this is of just the water and not the actual solution
the shc of calorimeter is not taken into account

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18
Q

how can you reduce heat loss to surroundings

A

put lid on calorimeter
insulating outsides of calorimeter with polystyrene

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19
Q

what ways do the arrows go in enthalpy change of formation hess cycle

A

both pointing upwards

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20
Q

what ways do the arrows point in enthalpy change of combustion

A

both downwards , with products always being H2O and CO2

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21
Q

what is the definition of bond enthalpy

A

the energy required to break one mole of the stated bond in a gaseous state under standard conditions

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22
Q

why are calculated bond enthalpies different to databook values

A

the same covalent bond but in different environments often have slightly different bond enthalpies, and the databook is averaged values

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23
Q

what is standard enthalpy of atomisation

A

the enthalpy change when one mole of gaseous atoms are produced from an element in its standard state under standard conditions

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24
Q

what is 1st ionisation energy

A

the enthalpy change when one mole of gaseous atoms is converted into one mole of gaseous +1 ions

25
Q

what is 2nd ionisation energy

A

the enthalpy change when 1 mole of gaseous +1 ions is converted into 1 mole of gaseous +2 ions

26
Q

what is 1st electron affinity

A

the enthalpy change when a mole of electrons is added to a mole of gaseous atoms to form ions with a -1 charge

27
Q

what is 2nd electron affinity

A

the enthalpy change when a mole of electrons is added to a
mole of gaseous 1- ions to form ions with a -2 charge

28
Q

what is lattice enthalpy of formation

A

the enthalpy change when one mole of a solid ionic compound is formed from its gaseous ions

29
Q

what is lattice enthalpy of dissociation

A

the enthalpy change when one mole of a solid ionic compound is dissociated into its gaseous ions

30
Q

what is enthalpy of hydration

A

the enthalpy change when water molecules surround one mole of gaseous ions

31
Q

what is enthalpy of solution

A

the enthalpy change when one mole of a solute dissolves completely in a solvent to form a solution in which the ions are far enough apart not to interact with each other

32
Q

what is mean bond enthalpy

A

the enthalpy change when one mole of covalent bonds are broken in a gaseous molecule, averaged over a range of compounds

33
Q

what factors affect the strength of an ionic bond

A

charge on ion- bigger, stronger attraction
size of ionic radius- smaller, stronger attraction
=higher charge density

34
Q

why is first electron affinity exothermic

A

you are forming an attraction between the negative electron and the ions positive nucleus so this releases energy

35
Q

what is a perfect ionic model

A

-ions that are perfectly spherical
-the charge is evenly distributed in the sphere

36
Q

why are some experimental lattice energy values different to theoretical (experimental more exothermic)

A

theoretical assumes the compound is purely ionic however the compound may not follow the perfect ionic model and have some covalent characteristics

37
Q

how do ionic bonds have covalent character (distortion)

A

the positive ion will distort the charge distribution in the negative ion (the positive ion polarises it)

38
Q

what makes cations more polarisable

A

smaller- higher charge density so can pull electrons towards it more readily

39
Q

what makes anions more polarisable

A

large ions with large charge are polarised more easily because electrons are further away from the nucleus

more repulsion of electrons from the ion

40
Q

what is electronegativity

A

the ability for an atom to attract a pair of electrons towards itself in a covalent bond

41
Q

for a substance to dissolve (enthalpy of solution) what must happen

A

1) substance bonds must break (endothermic)
2) new bonds formed between substance and solvent (exothermic)

42
Q

what must happen for a substance to actually dissolve in the solvent

A

the new bonds must be the same strength or stronger than the original ones broken, this makes the enthalpy of solution exothermic

43
Q

what 2 things affect enthalpy change of hydration

A

charge- ions with a higher charge can attract water molecules easier as the electrostatic attraction is stronger, releasing more energy when bond is made giving it a more exothermic enthalpy of hydration

smaller ions- larger charge density so attract water molecules easier, same as top

43
Q

how can you calculate enthalpy of solution

A

lattice dissociation followed by enthalpy of hydration= solution

44
Q

what is entropy

A

a measure of the disorder of a system (number of ways energy can be shared out between particles)

45
Q

what has more entropy, solids or gas

A

gas as the particles are arranged spread apart and free to move so they have more disorder than solids, where the particles vibrate around a fixed position

46
Q

what else affects entropy other than states

A

moles of substance- more moles=more disorder

47
Q

how can an endothermic reaction become spontaneous (feasible)

A

these require a larger activation energy to start as products have more energy
increasing entropy is energetically favourable therefore if changes in entropy overcome changes in enthalpy the reaction can happen spontaneously (if more moles in products/ state changes in products)

48
Q

equation for total entropy change

A

system + surroundings = total

49
Q

equation for delta surroundings

A

enthalpy change / temperature

50
Q

equation for delta system

A

products - reactants

51
Q

what does positive entropy change show

A

products are more disordered than reactants (natural direction of change)

52
Q

what does Gibbs free energy have to be for a reaction to be feasible

A

less than or equal to zero

53
Q

if a reaction is thermodynamically feasible why might it still not occur

A

due to kinetic factors eg activation energy

54
Q

what is mean bond enthalpy

A

enthalpy change needed to break the covalent bond into gaseous atoms, averaged over different molecules

55
Q

what does lattice energy measure

A

ionic bond strength

56
Q

equation for Gibbs free energy

A

enthalpy change - temp x sys

57
Q

what is T in delta G equation

A

T= delta H / delta sys

chemistry paper 1 (7 decks)

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