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A Level Chemistry - Physical Chemistry > 3.1.4 Energetics > Flashcards

3.1.4 Energetics Flashcards

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1
Q

Standard temperature

A

298 KELVIN

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2
Q

Standard pressure

A

100KPa

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3
Q

Standard definition of enthalpy formation

A

The standard enthalpy of formation is the enthalpy change, when one mole of a compound, is formed from its elements in their standard states and under standard conditions

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4
Q

Standard enthalpy of combustion definition

A

The standard enthalpy of combustion is the enthalpy change when one mole of a substance, is burned completely in excess oxygen, all substances in normal states under standard conditions.

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5
Q

What is standard concentration

A

1 mol dm -3

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6
Q

Standard unit for standard enthalpy

A

KJ mol -1

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7
Q

What is the value of the standard enthalpy change of neutralisation

A
  • 57 KJ mol -1
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8
Q

Enthalpy change for an Exothermic reaction is

A

Negative

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9
Q

Enthalpy change for an endothermic reaction is

A

Positive

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10
Q

Define standard enthalpy change of neutralisation

A

Enthalpy change when one mole of water is formed in a neutralisation reaction under standard conditions with all substances in their standard states

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11
Q

Define enthalpy change of a reaction

A

Enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions and all chemical in their standard states

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12
Q

Equations for energy change of water

A

Energy change of the water = mass of water (g) x specific heat capacity of water ( J g-q K-1) x temperature change of water (c)
= m x c x temp change

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13
Q

Equation for standard enthalpy change of combustion

A

Thermal energy (KJ) / number of moles - (mass of fuel / molar mass of ?)
AH = Q/ moles

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14
Q

Reasons why for calorimeters particular value may not be correct
X4

A

Unlit sprit burned uncapped fuel will evaporate - think burn more fuel then we actually did
Heat energy released does not all pass into water - lots air or metal calorimeter
Not all fuel combusts- incomplete combustion releases less thermal energy
Not carried out under standard conditions

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15
Q

Hydrogen exist as

A

H2

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16
Q

Where can you add numbers in the equation for standard enthalpy of combustion
This applies for all
Example equation
H2 + O2 —>H2O

A

You can’t change the initial product
So in the example you can’t change the number of H2
Answer H2 + 1/2O2 —-> H2O

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17
Q

Where can you add number when doing standard enthalpy of formation
Example equation
C + H2 —> C4H10

A

You can add number of the left side so the reactants but not the right can’t change the products
Answer 4C + 5H2 —-> C4H10

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18
Q

Define standard enthalpy of a reaction

A

Enthalpy change when a reaction takes place in the molar quantities shown by the balanced chemical equation under standard conditions and standard states

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19
Q

When constructing Hess cycles for standard enthalpy change of formation do you need to know the enthalpy change of a compound or element

A

Compound
Enthalpy change of an element is always zero

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20
Q

When using data for standard enthalpy change of formation which way do the arrows point

A

Up
When using formation data both arrows point upwards

21
Q

When using standard enthalpy of combustion data which way do the arrows go in a Hess cycle

A

Down
(Trees burn down)

22
Q

Do you use big numbers in Hess cycles

A

YES

23
Q

When doing an enthalpy cycle of formation (using combustion data) what are the molecules that go on the bottom of the cycle

A

E.g. if you have elements C and H
You would have CO2 + H2O
You don’t need to add oxygen to original equation

24
Q

When finding the standard enthalpy of combustion do you need the data for the elements or compounds

A

Both you need to use the enthalpy of formation for all the molecules/elements present
Excluding oxygen

25
Q

Why do you never use the standard enthalpy of combustion for oxygen

A

Oxygen cannot be combusted
Oxygen is used to combust chemicals

26
Q

Breaking bonds is an ….. reaction

A

Endothermic reaction
Putting energy in

27
Q

Forming bonds is an ….. reaction

A

Exothermic
Energy released

28
Q

Define average bond enthalpy

A

The mean bond enthalpy is the standard enthalpy change associated with breaking one mole of covalent bonds in a gaseous substance into individual gaseous atoms averaged over several compounds.

29
Q

Different molecules have the same/different bond enthalpy?

A

Different
That why it’s called average bond enthalpy

30
Q

When do the kangaroos finally come out of there mothers pouch

A

When they become an adult

31
Q

Nitrogen exists as

A

N2

32
Q

Bromines standard state is

A

Liquid

33
Q

What are these standard states
CH3Br
Na2O2

A

Liquid
Solid

34
Q

When writing combustion equations what is usually produced

A

Carbon dioxide and water
(Sometimes oxides as 1 product)

35
Q

Hydrocarbons eg. C6H14 in their standards states are

A

Gases

36
Q

Na + O2 =

A

Na2O
(Not balanced)

37
Q

When writing neutralisation equations what are the products + state symbols

A

X (aq)+ H2O (l)

38
Q

State the meaning of the term enthalpy change as applied to a chemical reaction

A

Heat energy change at a constant pressure

39
Q

What does oxygen always exist as

A

O2

40
Q

Exothermic sign

A

-

41
Q

Endothermic sign

A

+

42
Q

Units for enthalpy change

A

KJ mol-1

43
Q

Enthalpy equation for breaking and forming bonds

A

Enthalpy change = breaking bonds - forming bonds

44
Q

Do you use big numbers when finding bond enthalpy

A

YES

45
Q

Equation to find energy transferred equation

A

q=mc[delta]t
Important this is energy not enthalpy

46
Q

q=mc(delta)t
What each part means and values

A

q= heat energy joules
M= mass in grams
C=specific heat capacity (given)
T= temperature kelvin

47
Q

How to find enthalpy change once found energy changed

A

Enthalpy is KJmol-1 aka kJ/mol
So find the moles and divide by the energy found in KJ

48
Q

The cycle you draw is based off,….

A

What data your given
E.g. formation data = formation cycle

49
Q

How to improve accuracy without changing equipment to reduce percentage uncertainty

A

Increase concentrations

A Level Chemistry - Physical Chemistry (8 decks)

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