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Chem Miss D > Energetics > Flashcards

Energetics Flashcards

(53 cards)

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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

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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

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A

CnH2n+2 + O2 -> H2O

24
Q

How to measure enthalpy change?

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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
Delta h calculation
q/moles in reaction
26
Measuring enthalpy change equation
quantity of heat = mass x specific heat capacity x change in temp q = mc^t
27
Enthalpy change units
Jg-1 k-1
28
Average bond enthalpies definition
Exact energy needed to break a bond depends on the compound + bond being broken
29
CH4 -> CH3 + H | Delta h value
+ 425
30
CH3 -> CH2 + H | Delta h value
+ 470
31
CH2 -> CH + H | Delta h value
+ 410
32
CH -> C + H | Delta h value
+ 335
33
Delta H calculations using average bond enthalpies
Energy to break bond - energy to make bonds
34
Bond dissociation energy definition
Bond energy for diatomic molecules is breaking the only bond in the substance
35
What state must the compound be in when calculating bond enthalpies?
Gaseous
36
What can be included to change the state to a gas?
vaporisation value
37
Practical aim for calorimetry - alcohols
Measure enthalpy of combustion (delta H c) of 3 alcohols (methanol, ethanol + propan-1-ol)
38
Results of calorimetry - alcohols
Record as you progress through practical
39
Hazards of calorimetry - alcohols
- alcohols are flammable - methanol is toxic - remember to cap all burners when not using
40
Calorimetry cup practical method
- take temp for few mins before reaction - mix/combine reactants - take emo for another few mins
41
Which results of the calorimetry cup do we use?
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
What is Hess’ law definition for enthalpy change of formation?
Enthalpy change of any reaction is independent of route taken
44
Hess’ law definition
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
What is the delta h for reactants?
Delta H f
46
What is the delta h for reaction?
Delta h
47
What is the delta h for products?
Delta h f
48
Enthalpies change of reaction calculation
Products - reactants
49
What is the definition for Hess’ law enthalpy change of combustion?
- The enthalpy change when 1 mole of substance burns completely in oxygen - reactants + products in standard state/conditions (100 kPa)
50
Calculations for Hess’ law enthapies of combustion
``` Products = reaction - reactants Reaction = reactants - products Reactants = reaction + products ```
51
What is bond energy?
The amount of energy needed to break a bond in a gasworks state + form a bond
52
Is bond breaking exo or endothermic?
Endothermic as it requires energy
53
What is bond dissociation energy?
Bond energy for diatomic molecules is breaking the only bond in substance
54
Calculation for Hess’ law for enthalpies of formation
Reactants + reaction = products Products - reactants = reaction Products - reaction = reactants

Chem Miss D (8 decks)

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