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energetics Flashcards

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1
Q

what is enthalpy

A

the heat energy that is stored in a chemical system

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2
Q

how is enthalpy shown

A

H

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3
Q

how is enthalpy measured

A

from temperature changes when a chemical reaction takes place

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4
Q

delta H

A

heat energy change at constant pressure

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5
Q

what are the units of heat energy change (delta H)

A

kJ per mole

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6
Q

what happens in terms of heat in an exothermic reaction

A

heat energy is given out to the surroundings

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7
Q

what happens to the temperature of the surroundings when an exothermic reaction taxes place

A

temperature of surroundings increases

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8
Q

why is delta H negative in exothermic reactions

A

the chemicals lose heat energy

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9
Q

important exothermic reactions

A

combustion of fuels and respiration

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10
Q

what happens in terms of heat in endothermic reactions

A

heat energy is taken in from the surroundings

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11
Q

what happens to the temperature of the surroundings in endothermic reactions

A

the temperature of the surroundings decreases

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12
Q

why is delta H positive in endothermic reactions

A

the chemicals gain heat energy

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13
Q

what do endothermic reactions require

A

the input of heat energy

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14
Q

important endothermic reactions

A

thermal decomposition of calcium carbonate

photosynthesis

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15
Q

what can enthalpy profile diagrams be used to illustrate

A

the enthalpy change for a reaction

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16
Q

what do enthalpy profile diagrams show

A

the products, reactants and enthalpy change

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17
Q

activation energy

A

the minimum energy required to start a reaction by the breaking of bonds

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18
Q

do the products or reactants have a higher enthalpy in exothermic reactions

A

reactants

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19
Q

do the products or reactants have a higher enthalpy in endothermic reactions

A

products

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20
Q

what is required to start the exothermic reaction

A

energy is required to break the bonds, even though the products have a lower energy than the reactants

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21
Q

what will provide the energy needed to continue to overcome the activation energy

A

the net energy, once the barrier has been overcome

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22
Q

why is energy needed in endothermic reactions

A

the break the bonds and start the reaction

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23
Q

what do most endothermic reactions need to provide the necessary energy

A

to be heated continuously

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24
Q

what is bond enthalpy

A

the energy required to break one mole of a given covalent bond in the molecules in the gaseous state

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25
Q

why are bond enthalpy values always positive

A

bond breaking is endothermic

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26
Q

What do bond enthalpies give an indication of

A

The relative strength of a covalent bond

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27
Q

How do bond enthalpies give an indication of the relative strength of a covalent bond

A

The stronger the bond, the more endothermic the bond enthalpy

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28
Q

Why do chemists use mean bond enthalpy values

A

The same covalent bond may appear in different compounds and the value of the bond will be slightly different in each compound

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29
Q

Mean bond enthalpy

A

The energy required to break a covalent bond, averaged for that type of bond in a range of different compounds

30
Q

What happens in terms of the bonds in a chemical reaction

A

Bonds in the reactants are broken and new bonds are formed to make the products

31
Q

What is required to break bonds

A

Energy

32
Q

What is released when new bonds form

A

Energy

33
Q

What is the enthalpy change of a reaction

A

Sum of bonds broken - some of bonds formed

34
Q

Limitations of bond energy calculations

A

Using mean bond enthalpies to calculate the enthalpy change of a reaction often leads to a value that is less accurate than a value obtained from Hess’s Law

35
Q

Why is using mean bond enthalpies to calculate enthalpy change less accurate than a value obtained from Hess’s law

A

Bond enthalpies used are only an average value- not specific to compound in question

We are assuming all species are in the gaseous state- lots of compounds won’t be eg. Ethanol

36
Q

What are the standard conditions for standard enthalpy of formation

A

298k and 100kPa

37
Q

Standard state

A

The physical state of a substance under standard conditions

38
Q

Standard enthalpy of formation

A

The enthalpy change that occurs when one mole of a compound is formed from its constituent elements with all reactants and products in their standard states

39
Q

What is the standard enthalpy of formation of an element in its standard state

A

0

40
Q

What do all products of standard enthalpy of formation need to be

A

1 mole

41
Q

What is the relationship between negativity of standard enthalpy of formation of a compound and the stability of it

A

More negative = more stable

42
Q

Standard enthalpy of combustions

A

The enthalpy change that occurs when one mole of a compound reacts completely in oxygen with all reactants and products in their standard states

43
Q

Calorimetry

A

The method used to determine enthalpy changes by experiment

44
Q

What does calorimetry involve

A

Measuring the temperature change of a given amount of water as the reaction occurs- and converting this to a quantity of heat energy

45
Q

Specific heat capacity of water

A

4.18 J required to hear 1g of water

46
Q

What does heat change =

A

Mass of water x specific heat capacity x temperature change

Q=mc DELTA T

47
Q

What is q

A

Heat energy released or absorbed in joules

48
Q

What is m

A

Mass of water in grams

49
Q

What is the mass of water in grams equal to

A

Volume in cm3

50
Q

What is c

A

The specific heat capacity of water

51
Q

What is delta t

A

Temperature change in K

Same as change in celcius

52
Q

What must you do to q when you’ve initially calculated it using mcDELTA T

A

Divide it by 1000 to convert J to kJ

53
Q

Heat given out or taken in by one mole=

A

Q(in kJ)/ no of moles

54
Q

What are the units of delta H for experimental determination of enthalpy changes

A

kJmol-1

55
Q

When is delta H negative in calorimetry

A

If temp increases during reaction (exothermic reaction)

56
Q

When is delta H positive during calorimetry

A

If temperature decreases during a reaction (endothermic)

57
Q

Why may the experimental value for enthalpy of combustion be different to the data book value

A
  • heat loss to surrounding
  • incomplete combustion
  • some methanol may evaporate
  • some water may evaporate
58
Q

What can cooling curves be used for

A

As a method of accounting for heat loss with reactions in solutions

59
Q

How to use data to find maximum temperature rise

A

Plot a graph of temperature against time using results and determine max temperature change accompanying reaction

Extrapolate back to where lines meet (ish) to establish max temperature rise

60
Q

First law of thermodynamics

A

Energy cannot be created or destroyed but it can be changed from one form to another

61
Q

Where does hess’s law apply the first law of thermodynamics

A

To chemical reactions

62
Q

Hess’s law

A

The enthalpy change for a chemical reaction is independent of the route taken

63
Q

Why is it not always possible to measure the enthalpy change of a reaction directly

A
  • reaction may have more than one possible product

- practically impossible to measure

64
Q

What is Hess’s law used for

A

To calculate enthalpy changes which cannot be measured directly using an enthalpy cycle

65
Q

What can we do using Hess’s law if we know two of the enthalpy changes

A

Calculate the third

66
Q

What does delta H1 equal

A

Delta H 2 + delta H 3

67
Q

How to remember enthalpy cycles of combustion

A

C=R-P

68
Q

Where should you always do the vectors for enthalpy of combustion

A

Products to element

69
Q

Where should you do vectors in calculations for reactions from enthalpy of formation values

A

Elements to products

70
Q

How to remember enthalpy cycles of formation

A

F=P-R

Chemistry (42 decks)

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