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A-Level Chemistry > Enthalpy > Flashcards

Enthalpy Flashcards

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1
Q

What is enthalpy change of reaction?

A

The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation under STD conditions

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2
Q

What is enthalpy change of formation?

A

The enthalpy change when 1 mole of a substance is formed from its elements under STD conditions

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3
Q

What is enthalpy change of combustion?

A

The enthalpy change when 1 mole of a substance reacts completely with O2 under STD conditions

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4
Q

What is enthalpy change of neutralisation?

A

The enthalpy change when solutions of an acid and an alkali react together under standard conditions to produce 1 mole of water

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5
Q

What are the standard conditions?

A

100KPa/ 1atm
298K (25degrees)
1moldm*3
STD states - C= solid H= gas H2O= liquid

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6
Q

How do you calculate standard enthalpy of reaction?

A 
Calc amount of heat lost/gained 
Calc no.of moles
q/n
/1000 to get kJ
Choose correct sign (-/+)
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7
Q

What must you do if the coefficient of the reactant NOT in excess is not one?

A

Multiply your answer of standard enthalpy by the number

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8
Q

What is the equation for heat energy q (J)?

A

Mass of water X Specific heat capacity of water

(4.18Jg-1k-1) X Change in temp

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9
Q

What is enthalpy?

A

The heat that is stored in a chemical system

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10
Q

What is enthalpy change?

A

The difference in enthalpy between the products and reactants in a reaction

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11
Q

What is exothermic?

A

Chemical reaction that transfers heat energy from the system to its surroundings

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12
Q

What is endothermic?

A

Chemical reaction that transfers heat energy from its surroundings into the system

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13
Q

What happens to the temperature in an exothermic reaction?

A

It increases

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14
Q

What are examples of exothermic reactions?

A

Combustion

Oxidation

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15
Q

What happens to the temperature in an endothermic reaction?

A

It decreases

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16
Q

What are examples of endothermic reactions?

A

Photosynthesis

Thermal decomposition

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17
Q

What does an exothermic reaction profile look like?

A

Products have a lower enthalpy

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18
Q

What is the sign for an exothermic enthalpy change?

A

Negative

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19
Q

What does an endothermic reaction look like?

A

Products have a higher enthalpy

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20
Q

What is the sign for an endothermic enthalpy change?

A

Positive

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21
Q

What are the units for enthalpy change?

A

kJmol*-1

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22
Q

What is activation energy?

A

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

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23
Q

What does the reaction profile lines look like when activation energy is included?

A

They are curved

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24
Q

What goes on the axis of a reaction profile?

A

X- Progress of reaction

Y- Enthalpy

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25
Which bit on an AE exo reaction profile tells you the AE and enthalpy change?
AE = Reactants line to curve | Enthalpy change = Reactants to products
26
Which bit on an AE endo reaction profile tells you the AE and enthalpy change?
AE = Reactants to curve | Enthalpy change = Reactants to products
27
Why would the standard enthalpy change of an element not be zero sometimes?
Because the element isn't in its STD state
28
What is average bond enthalpy?
The average enthalpy change when one mole of gaseous covalent bonds are broken
29
What is bond enthalpy?
The energy required to break a given covalent bond averaged over a range of compounds
30
What is enthalpy change?
Heat energy change, measured at a constant pressure
31
How do you calculate bond enthalpies?
Highlight bonds broken + bonds made Workout the bonds broken + bonds made Workout enthalpy change = (bonds broken) - (bonds made)
32
Why would the answer of a bond enthalpy be different to one in the textbook?
Not calculated under STD conditions | Used in calculations as averages
33
What is Hess' Law?
If a reaction can take place by more than one route, and the initial and final concentrations are the same, the total enthalpy change is the same regardless of route taken
34
Why is there no O2 value when calculating standard enthalpy change of combustion?
Since O2 can't combust with O2
35
What are the factors that influence rate?
``` Concentration Pressure (if a gas) Temperature SA (if a solid) Presence of catalyst ```
36
When does a reaction take place?
When particles collide
37
What will happen if there is more successful collisions?
Faster rate of reaction
38
How do you calculate rate?
Change in conc/ Time
39
What are the units for rate?
moldm-3/s
40
How can rate of reaction be calculated from a graph?
From the gradient | dy/dx
41
What does rate of reaction measure?
How fast a reactant is being used up or how fast a product is being formed
42
What does it tell you about rate if the graph is steeper?
Faster rate
43
What is rate of reaction?
The change in concentration of a reactant or product in a given time
44
How does rate change between the start and the end of the reaction?
``` Start = fastest rate as each reactant at highest conc Middle = starts to slow as reactant being used up so conc decreases End = zero as reactant used up so conc doesn't change ```
45
What does the slope of the graph look like when the reaction has stopped?
It plateaus
46
What is collision theory?
A theory that states that two reacting particles must collide for a reaction to occur
47
What happens in most collisions?
Molecules collide but then bounce off and remain unchanged
48
What makes a collision effective?
Particles collide with correct orientation | Particles have sufficient energy to overcome activation energy barrier of the reaction
49
Why does increasing the conc increases rate?
Increases no. of particles in same volume Particles closer together So more frequent collisions So more successful collisions
50
Why does increasing the pressure of a gas increase rate?
``` Increases conc of molecules As same no.of gas molecules occupy a smaller volume So closer together So collide more frequently So more effective collisions ```
51
What methods can be used to measure rate of reaction?
Monitoring the removal (decrease in conc) of a reactant | Following the formation (increase of conc) of a product
52
What properties can be used to measure rate?
Concentration Gas volume Mass of reactants/products Colour
53
Why does increasing temperature increase rate?
Particles have more KE So more frequent collisions So more collisions have the Ae
54
If a reaction produces a gas, what two methods can be used to determine rate?
Monitoring the volume of gas produced at regular time intervals using gas collection Monitoring the loss of mass of reactants using a balance
55
Why can these two methods (gas) be used?
Vol of gas produced + loss of mass are both proportional to change in conc of a reactant/product
56
What is breaking bonds?
Endothermic
57
What is making bonds?
Exothermic
58
Does bond breaking require or release energy?
Require
59
Does bond making require or release energy?
Release
60
What is a catalyst?
A substance that changes the rate of a chemical reaction without undergoing a permanent change. It lowers the activation energy of the reaction by providing an alternative reaction pathway
61
Is a catalyst used up in a chemical reaction?
NO
62
What may a catalyst do in a reaction?
May react with a reactant to form an intermediate OR Provide a surface on which the reaction can take place
63
What happens to a catalyst at the end of a reaction?
It is regenerated
64
What is a homogenous catalyst?
A catalyst that is the same physical state as the reactants
65
What does a homogeneous catalyst do?
Reacts with reactants to form an intermediate | Intermediate then breaks down to give the product + regenerate catalyst
66
What are some examples of homogeneous catalysts?
Used to make esters (H2SO4) both liquids | Used for ozone depletion (.Cl radical) both gases
67
What is a heterogeneous catalyst?
A catalyst that is a different physical state to the reactants
68
What state are heterogeneous catalysts normally?
Usually solids
69
What do heterogeneous catalysts normally come into contact with?
Gaseous reactants or solutions in reactants
70
What does a heterogeneous catalyst do?
Reactant molecules adsorbed onto surface of catalyst , where reaction takes place Product molecules leave the surface of catalyst by desorption
71
What is desorption?
The release of an adsorbed substance from a surface
72
What is adsorption?
The process by which a solid holds molecules of a gas or liquid or solute as thin film
73
What does a catalytic convertor do?
Oxidises CO to CO2 | Reduces NO to N
74
Why is a catalytic convertor good?
Converts harmful gases into less harmful products
75
What does a catalytic convertor do to any unburnt hydrocarbons?
Oxidises them into H2O + CO2
76
Where can a catalytic convertor be found?
Petrol vehicles
77
What does the catalyst in a catalytic convertor contain?
Platinum Rhodium Palladium
78
How does a catalytic convertor work?
Hot exhaust gases passed over heterogeneous catalyst
79
What does the catalytic convertor look like and why is this useful?
Honeycomb mesh | Provides large SA for reactions to take place
80
What are 4 advantages of using catalysts industrially?
Reduce energy requirements so less electricity or fossil fuels used Reduces temperature needed Uses less fossil fuels so cuts CO2 emissions Makes product faster + uses less energy so cuts costs
81
What is the Boltzmann distribution?
Some molecules move slowly with low energy Some molecules move fast with high energy Most molecules have average speed + energy This spread of molecular energies = Boltzmann distribution
82
What are the features of the Boltzmann distribution curve?
No molecules have zero energy = curve starts at origin Area under curve = total no. of molecules No maximum energy for molecule = curve doesn't meet X-axis
83
What does the curve look like of a low temperature on a Boltzmann graph?
Stays the same and to the left
84
What does the curve look like of a high temperature on a Boltzmann graph?
Lower and to the right
85
What does the curve of a higher temperature on a Boltzmann graph tell us?
More molecules have energy greater than or equal to Ea So greater proportion of collisions will lead to an increased rate Collisions more frequent as molecules have more KE But increased energy more important than increased frequency
86
What does the curve look like of the presence of a catalyst on a Boltzmann graph?
Curve doesn't change | But provides an alternative roue with a lower activation energy
87
What does the curve of the presence of catalyst on a Boltzmann graph tell us?
Greater proportion of molecules exceed new lower Ea More molecules now have a greater or equal energy to lower Ea On collision more molecules react to form a product So increased rate
88
What does the curve look like of a higher concentration on a Boltzmann graph?
Peak gets higher as increase no. of particles | Doesn't shift left or right since amount of energy stays the same
89
What is on the X-axis of a Boltzmann graph?
Energy
90
What is on the Y-axis of a Boltzmann graph?
No. of molecules with given energy

A-Level Chemistry (26 decks)

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