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Chem Miss D > Energetics > Flashcards

Energetics Flashcards

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1
Q

Enthalpy definition

A

Energy constant at constant pressure

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2
Q

Bond enthalpies

A

Enthalpy change on breaking or making a bond (per mole)

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3
Q

Delta H calculation

A

Delta H = En (break broken) - Em (bonds formed)

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4
Q

What does positive delta h mean

A

Bonds breaking (putting energy in)

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5
Q

Negative delta h

A

Making bonds (getting energy out)

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6
Q

What does En / Em mean?

A

Number of moles of each types of bonds

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7
Q

Enthalpy change definition

A

Change in heat at constant pressure content under standard conditions

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8
Q

Combustion general equation

A

Fuel(CnHn+2) + O2 -> CO2 + H2O

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9
Q

Thermochemistry definition

A

Heat chemistry

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10
Q

Endothermic reaction definition

A

Energy must be put in to break a bond

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11
Q

Exothermic reaction definition

A

Every given out when bonds are formed

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12
Q

What does delta mean?

A

Change in

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13
Q

What does endothermic energy change mean?

A

Energy is absorbed from surroundings by reactants to form products

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14
Q

What are the x + y axis on energy change graphs?

A

X- reaction progress,t

Y- enthalpy,H

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15
Q

What is the delta h for endothermic enthalpy change?

A

Positive - gets more energy

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16
Q

Energy change graph for exothermic

A

Reactants higher enthalpy than products enthalpy

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17
Q

What does exothermic energy change mean?

A

Energy is released to surroundings by reactants to form products

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18
Q

What is the delta h for exothermic enthalpy change?

A

Negative

Enthalpy of products is decreased

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19
Q

Energy change graph for endothermic

A

Products higher enthalpy than reactants enthalpy

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20
Q

What is the standard enthalpy change or formation? Delta f H degrees

A

Enthalpy change when 1 mole of a substance is formed form it’s constituent elements with all reactants + products in a standard states

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21
Q

Standard enthalpy change of combustion? Delta c H degrees

A

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

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22
Q

What is the standard enthalpy change of neutralisation? Delta neut H degrees

A

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

23
Q

Combustion general equation

A

CnH2n+2 + O2 -> H2O

24
Q

How to measure enthalpy change?

A

1) check significant figures
2) calculate q using q = mc^T
3) calculate moles of whatever is reacting to raise the temp (mass/mr)
4) delta h = q/moles (divide q by 1000 + put - in front before dividing by moles)
6) answer to sf in step 1
7) each cm3 is 1g

25
Q

Delta h calculation

A

q/moles in reaction

26
Q

Measuring enthalpy change equation

A

quantity of heat = mass x specific heat capacity x change in temp
q = mc^t

27
Q

Enthalpy change units

A

Jg-1 k-1

28
Q

Average bond enthalpies definition

A

Exact energy needed to break a bond depends on the compound + bond being broken

29
Q

CH4 -> CH3 + H

Delta h value

A

+ 425

30
Q

CH3 -> CH2 + H

Delta h value

A

+ 470

31
Q

CH2 -> CH + H

Delta h value

A

+ 410

32
Q

CH -> C + H

Delta h value

A

+ 335

33
Q

Delta H calculations using average bond enthalpies

A

Energy to break bond - energy to make bonds

34
Q

Bond dissociation energy definition

A

Bond energy for diatomic molecules is breaking the only bond in the substance

35
Q

What state must the compound be in when calculating bond enthalpies?

A

Gaseous

36
Q

What can be included to change the state to a gas?

A

vaporisation value

37
Q

Practical aim for calorimetry - alcohols

A

Measure enthalpy of combustion (delta H c) of 3 alcohols (methanol, ethanol + propan-1-ol)

38
Q

Results of calorimetry - alcohols

A

Record as you progress through practical

39
Q

Hazards of calorimetry - alcohols

A
  • alcohols are flammable
  • methanol is toxic
  • remember to cap all burners when not using
40
Q

Calorimetry cup practical method

A
  • take temp for few mins before reaction
  • mix/combine reactants
  • take emo for another few mins
41
Q

Which results of the calorimetry cup do we use?

A

Only use results which are after highest temp once reacted
Or
If temp decreases only use results which are after lowest temp once reacted

42
Q

What is Hess’ law definition for enthalpy change of formation?

A

Enthalpy change of any reaction is independent of route taken

44
Q

Hess’ law definition

A

If a reaction takes place by more than one route, the initial + final conditions are the same , the total enthalpy change is the same

45
Q

What is the delta h for reactants?

A

Delta H f

46
Q

What is the delta h for reaction?

A

Delta h

47
Q

What is the delta h for products?

A

Delta h f

48
Q

Enthalpies change of reaction calculation

A

Products - reactants

49
Q

What is the definition for Hess’ law enthalpy change of combustion?

A
  • The enthalpy change when 1 mole of substance burns completely in oxygen
  • reactants + products in standard state/conditions (100 kPa)
50
Q

Calculations for Hess’ law enthapies of combustion

A 
Products = reaction - reactants 
Reaction = reactants - products 
Reactants = reaction + products
51
Q

What is bond energy?

A

The amount of energy needed to break a bond in a gasworks state + form a bond

52
Q

Is bond breaking exo or endothermic?

A

Endothermic as it requires energy

53
Q

What is bond dissociation energy?

A

Bond energy for diatomic molecules is breaking the only bond in substance

54
Q

Calculation for Hess’ law for enthalpies of formation

A

Reactants + reaction = products
Products - reactants = reaction
Products - reaction = reactants

Chem Miss D (8 decks)

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