enthalpy, reaction rates and equilibria Flashcards

1
Q

enthalpy definition

A

the total internal energy inside a chemical system
- this includes thermal and chemical energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

chemical system definition

A

all the atoms, ions and molecules that make up all the chemicals in a space

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

why does the enthalpy of a chemical system change during reactions

A

when a reaction takes place, energy is transferred between the system and its environment, causing enthalpy changes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

exothermic definition

A

when products have less energy than reactants, therefore energy is given off in the form of heat (this can be measured with a thermometer)
- overall enthalpy decrease

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

endothermic definition

A

when products have more energy than reactants so energy is taken into system in the form of heat
- overall enthalpy increase

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what are standard conditions
-temp
-pressure
-concentration
-state

A

temp - 298K / 25 C
pressure - 100 kPa
concentration - 1 moldm-3
state - the physical state of a substance under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

enthalpy change of formation definition

A

the enthalpy change that occurs when 1 mole of a substance is formed from its raw elements under standard conditions and states

remember the enthalpy change of formation of a raw element is always 0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

enthalpy change of combustion definition

A

the enthalpy change that occurs when 1 mole of a substance burns completely in excess oxygen under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

enthalpy change of neutralisation definition

A

the enthalpy change that occurs when 1 mole of water is formed in a reaction between an acid and a base under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what are the units for enthalpy change

A

kJmol-1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

activation energy definition

A

the minimum amount of energy needed for a reaction to take place

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

enthalpy change formula

A

= H(products) - H(reactants)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

bond enthalpy definition
(also known as bond energy/bond dissociation energy)

A

the amount of energy needed to break and separate 1 mole of a specific bond in gaseous molecules so that the resulting gaseous particles exert no force upon each other, under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is the difference between bond enthalpy and average bond enthalpy

A

bond energies are affected by other atoms in the molecule

average bond enthalpy is an average for many bonds taken from a wide range of compounds containing the bond

e.g. an O-H bond in water will have a different bond enthalpy to an O-H bond in methanol

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

why might listed average bond enthalpy for a specific bond differ from the average measured bond enthalpy of a specific bond in a specific molecule

A

the listed figure is an average taken from many compounds with that specific bond

the measured value is specific to the bond within a specific molecule, although this is also an average of breaking many of this type of bond

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

how do you determine bond enthalpies

A

bond enthalpies cannot be determined directly so enthalpy cycles are used to calculate the average

bond breaking = exothermic (-) as energy is needed to break bonds
bond forming = endothermic (+) as energy is released when they are made so

enthalpy change of a reaction = (+ enthalpy of bonds broken) + (- enthalpy of bonds formed)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

why might calculations for the same bond enthalpy from 2 different reactions produce different answers

A

the other bond enthalpies are averages so they will differ from the actual enthalpies involved in the reactions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

why does enthalpy change of combustion of alkanes become more exothermic as chain length increases

A

as chain length increases there are more atoms and bonds burned so more energy is released, so more exothermic

19
Q

what is hess’ law

A

the idea that products can be formed directly from raw elements or indirectly from raw elements

20
Q

outline collision theory

A

collision theory states that for a chemical reaction to take place, the particles need to collide with each other in the correct orientation AND with sufficient energy
- ineffective collisions occur when particles collide in the wrong orientation or without enough energy, bouncing off each other without causing a chemical reaction

21
Q

collision frequency definition

A

the number of collisions per unit of time

22
Q

what is the relationship between collision frequency and reaction rate

A

as collision frequency increases, the number of particles with energy greater than the Ea increases, so reaction rate increases

23
Q

3 factors that affect the rate of reaction and how

A

concentration
high conc = large no. of particles = high collision freq, etc

temperature
high temps = lots of movement = high collision freq, etc

pressure
high pressure = less space between molecules = high collision freq, etc

24
Q

catalyst definition

A

a substance which increases the rate of a reaction by facilitating an alternative mechanism with a lower activation energy, without being used up in the process of the reaction

25
difference between homogenous and heterogenous catalysts
homogenous catalysts are in the same phase as reactants heterogenous catalysts are in different phase to reactants
26
4 benefits of using catalysts
- allows for less extreme conditions which saves money - also saves energy, resulting in fewer CO2 emissions from burning fossil fuels - enables different mechanisms to be used with better atom economy + reduced waste + less use of hazardous reactants - many can be enzymes, which operate effectively close to room temperature and pressure
27
example of a reaction with a homogenous catalyst
CH3OH(aq) + CH3COOH(aq) >> CH3COOCH3(aq) + H2O(l) catalyst = conc H2SO4(aq)
28
example of a reaction with a heterogenous catalyst
haber process H2(g) + 3H2(g) >> 2NH3(g) catalyst = iron (s)
29
dynamic equilibrium definition
this occurs when a reversible reaction takes place in a closed system (or reactions in solution), when the forwards and backwards reactions are occurring at the same rate, so product and reactant concentrations remain constant products and reactants constantly reacting together, so dynamic
30
equilibrium position definition
refers to the relative amount of product and reactant in a reaction mixture
31
what is le chatilier's principle
if a change is made to a system in dynamic equilibrium, the position of equilibrium will move to counteract this change
32
what 3 factors influence the position of equilibrium
- pressure - concentration - temperature
33
how does concentration affect equilibrium position
if the concentration of reactants/products increases, the position of equilibrium will move to favour the other side if the concentration of reactants/products decreases, the position of equilibrium will move to favour that side
34
how does pressure affect equilibrium position
when pressure increases, position of equilibrium shifts to the side with less gaseous moles when pressure decreases, position of equilibrium moves to favour the side with more gaseous moles
35
how does temperature affect equilibrium position
when temperature increases, position of equilibrium moves to favour the endothermic reaction when temperature decreases, position of equilibrium moves to favour exothermic reaction
36
what is the effect of catalysts on dynamic equilibrium
catalytsts increase the rates of both the forwards and backwards reactions equally, so they have no effect on equilibrium position, although they can cause equilibrium to be reached faster
37
describe the process by which equilibrium can be formed for a reaction in a closed system
- at the start, reactant concentration is high so the position of equilibrium moves to the right, favouring forward reaction - as the reaction progresses, products concentration increases and products start reacting to reform reactants - the rate of reaction of reactants decreases as reactant conc decreases, and the rate of reaction of the products increases as concentration increases - eventually the rates of the forwards and backwards reaction will be equal, and the concentrations of reactants and products will be constant
38
what conditions are used in the haber process and why
pressure = 200atm high pressures move equilibrium position to side with less moles, increasing yield of ammonia, very high pressures are expensive/energy intensive to produce- compromise temperature = 400C forward reaction is exothermic, favoured by equilibrium at low temps, but this decreases reaction rate, so equilibrium wouldn't be reached- compromise catalyst = iron catalyst speeds up reaction rate
39
what 2 other features are used to improve yields, costs and sustainability in the haber process
- a heat exchanger warms incoming gas mixture to maintain an optimal temp so reaction rate is high, can also remove excess heat to help keep NH3 yield high - excess/reproduced H2 and N2 can be re-reacted to make more NH3, so high yield of ammonia
40
substances in what states should be included in the kc expression
(g) + (l) + (aq) are included solids (s) should be ignored
41
what does it mean if kc>>1
equilibrium lies to the RHS so reaction mixture is mostly products
42
what does it mean if kc<<1
equilibrium lies to the LHS so reaction mixture is mostly reactants
43
what does it mean if kc is close to 1
there is a similar concentration of both reactants and products in the reaction mixture, equilibrium is somewhere in the middle