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(A-level) Chemistry > Module 3.2 > Flashcards

Module 3.2 Flashcards

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1
Q

What is enthalpy

A

A measure of heat energy in a chemical system

It can be thought of the energy stored in bonds

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2
Q

What is the law of conservation of energy

A

The law that:

energy cannot be created or destroyed only transferred

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3
Q

How is enthalpy change measured

A

By measuring the energy transferred from the system to the surroundings

And the energy transferred from the surroundings to the system.

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4
Q

What is an exothermic reaction

A

A reaction with a negative change in energy between products and reactants

  • The temperature of the surroundings increase
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5
Q

What is an endothermic reaction

A

A reaction where enthalpy change is positive between products and reactants

  • the temperature of the surroundings decreases
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6
Q

What is activation energy

A

The minimum energy required for a reaction to begin

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7
Q

How do you draw an enthalpy diagram

A

write the reactants and products on the diagram

Draw the activation energy from the reactants to the top of the curve

Draw the enthalpy change from the reactants to the products

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8
Q

What are the standard conditions for change in enthalpy

A

100kpa pressure
298k temperature
1mol/dm3 concentration

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9
Q

What is the standard state of a substance

A

the physical state of the substance under standard conditions

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10
Q

What is the standard enthalpy change of reaction (ΔrH)

A

The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions and in standard states

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11
Q

What is the standard enthalpy change of formation (ΔfH)

A

The enthalpy change that takes place when 1 mol of a substance IS FORMED FROM ITS PRODUCTS under standard conditions, with all products and reactants in their standard states,

e.g

C(s) + 2H2(g) → CH4(g)

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12
Q

What is the standard enthalpy change of combustion (ΔcH)

A

The change in enthalpy that takes place when 1 mole of a substance reacts with oxygen under standard conditions with all reactants and products in their standard states

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13
Q

how is energy calculated in a reaction under standard conditions (calorimetry)

A

q = mcΔT

q = energy (J)
m = mass (g)
c = specific heat capacity (J/g/k)
ΔT = change in temperature (k)

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13
Q

What is the standard enthalpy change of neutralisation (ΔneutH)

A

The enthalpy change that accompanies the reaction of an acid and base to form 1 mole of H2O under standard conditions whith all reactants and products in their standard states

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14
Q

How is the enthalpy change of combustion in a reaction calculated (equation)

A

Find the q=mcΔT of the solution

Then divide the energy (which must be converted to KJ) by the moles of a specific substance
q(KJ) / mol = ΔcH (KJ/mol)

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15
Q

What is Hess’s Law

A

The law that enthalpy changes in a chemical reaction are independent of the route they take

∆H1 + ∆H2 → ∆H

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16
Q

How is a regular Hess cycle drawn

A

The direct route is between the products and reactants

The indirect route is from the products to the alternative route (alternative products / reactants) to the products

In the indirect route an arrow points down to the alternative products from the reactants
And an arrow points up from the alternative products to the products

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17
Q

What is the ∆Hf of elements

A

0

If the element is on both sides of the reaction its enthalpy change is 0

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18
Q

How do you draw the Hess cycle for enthalpy change of formation

A

Write the constituent elements below the reaction.

Draw arrows pointing up from the constituent elements to the reactants and the products

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19
Q

How do you draw the Hess cycle for enthalpy change of combustion

A

Write the equation
Underneath, write the products of combustion (usually H2O and CO2)

Draw arrows pointing to the reactants of combustion, from the products and the reactants

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20
Q

What are average bond enthalpies

A

The mean energy needed for 1 mole of a given gaseous bond to undergo homolytic fission (breaking covalent bonds)

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21
Q

What type of reaction is bond breaking

A

Endothermic
- energy is absorbed to break bonds

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22
Q

What type of reaction is bond forming

A

Exothermic
- energy is released when bonds are formed

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23
Q

Why are reactions exothermic

A

If more energy is released when forming bonds than energy is absorbed to break bonds, there is a negative energy change

This means the enthalpy change is negative

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24
Why are reactions endothermic
If more energy is absorbed when breaking bonds, than is released forming new bonds there is a positive enthalpy change. The reaction is endothermic
25
How do you calculate ∆H using average bond enthalpies
∆H of reactants - ∆H products (Break - make)
26
When calculating using average bond enthalpies do moles matter
Yes - multiply the average bond enthalpy by the number of moles
27
In Hess cycle calculations, do you need to do anything with moles
Yes Multiply the correct enthalpy changes by the correct moles
28
What is Calorimetry
A quantitative study of energy in a chemical reaction A mathematical equation is then used to calculate enthalpy change ( q = mc∆T)
29
Describe the experiment allows the direct measurement of enthalpy of combustion (Specifically copper calorimetry)
Combusting a fuel can be used to increase the temperature of a known mass of water -Measure the fuels starting mass -Add a known mass of water to the copper calorimeter -Mount the copper calorimeter above the fuel and measure the starting temperature of fuel -Combust the fuel using a spirit burnerfor a few minutes (e.g 5) and record the waters final temperature -Take the mass of the unused fuel and calculate the fuel used Calculate the energy change of water Find the moles of fuel burnt Energy ÷ moles = enthalpy
30
How can enthalpy of reaction be measured (experiment - the simpler one)
-Place an insulated polystyrene cup into a beaker, with a hole for a thermometer -Add a known measurement of a known concentration liquid and record its temperature (every minute) until its stable -Add the second reactant and do not record the temperature - Record the temperature every minute for 5 minutes Plot a graph to calculate the temperature change in the reaction Then calculate the change in enthalpy
31
How do you calculate density
Density = mass / volume
32
Why might the calculated enthalpy change of an experiment be different to the actual value
- Heat energy could be lost to the surroundings - The reaction may not go to completion ( e.g incomplete combustion) - Non - standard conditions
33
Why might the calculated enthalpy change of an experiment be different to the actual value WHEN USING AVERAGE BOND ENTHALPY
It is an average bond enthalpy - the values are not exact Bond energy / enthalpy assumes all molecules are gasseous
34
What is the collision theory
For a reaction ot occur, reactants must possess energy that is equal to or greater than the activation energy
35
Are reactions with a high activation energy faster than thoes with a low activation energy
No -as it will take longer to reach the required energy
36
How does tmeperature effect rate of reaction
AS temperature increases, the kientic energy of the particles increases. More particles have Ek greater than or equal to the activation energy, so there are more frequent successful collisions Rate of reaction is greater
37
How does increasing concentration effect the rate of reaction
increasing concentration means there are more molecules in the same volume, particles are more likley to collide as there are more of them There are more frequent successful collisions, so the rate of reaction is greater
38
How does increasing pressure effect the rate of reaction
The molecules are pushed closer together, the same number of molecules occupy a smaller volume. More frequent successful collisions, so the rate of reaction is greater
39
How do you measure the rate of reaction
Monitor the physical changes of the reaction, and the time, E.g concentration changes, change in mass You can pot this on a graphy, where the gradient is rate of reaction
40
What is the boltzman distribution
The distribution of molecular energies in a gas at a concent temperature
41
How does increasing temperature change the boltzman distribution
The peak is lower, and the curve shifts to the right
42
How does decreasing temperature change the boltzman distribution
The peak is higher and the curve shifts to the left
43
How does increasing pressure change the boltzman distribution
The graph has the same steepness but is higher than the original
44
How does decreasing pressure change the boltzman distribution
The graph has the same steepness but is lower
45
How do you lable / draw the boltzman distribution
The peak is labled as the most probable energy To the right of the peak lable the mean energy To the end of the curve lable the activation energy Draw a curve that quickly peaks, fall and then gradually lowers, do not touch the axis y axis = number of particles with kinetic energy x axis = kinetic energy
46
How do catalysts effect rate of reaction
Catalysts lower the activation energy of a reaction by providing an alternative reaction pathway. More particels have grreater than or equal to the activation energy, so there are more frequent successful collisons The rate of reaction is faster
47
How does using a catalyst change the boltzman distribution
The activation energy is moved to the left, the curve does not change at all
48
What is a catalyst
A substance that increases the rate of reaction, by providing an alternative reaction pathway, without being useed up in the process
49
What is a homogenous catalyst
A catalyst in the same phase (state) as the reactants
50
What is a hetrogenous catalyst
A catalyst in a different phase from the reactants
51
What are the advantages and disadvantages of using a homogenous catalyst
A: More uniform reaction conditions Can be highly specific to certain reactions D: HArder to seperate the product from the mixture Can be less stable and may degrade over time
52
Advantages and disadvantages of using a hetrogenous catalyst
A: Easy to seperate from the products Often more stable and durable D: Requires specific conditions for effective surface contact MAy suffer from catalyst poisoning (where impurities block active sites)
53
How do hetrogenous catalysts work
Adsorption - A reactant aproches the catalyst and settles onto an active site - Another reactant approches the catalyst - Rearangement of electrons takes place, to form the product Deadsorption - The products are released from the surface
54
What is La chatliers principle
If a dynamic equillibrium is disurbed by changing conditions, the reaction moves to counteract the change
55
What is a closed system
A system / container that reactants / products cant leave
56
What is dynamic equillibrium
Equillibrium in a reversible reaction where the products and reactants are being produced at the same rate
57
How does changing temperature effect the equillibrium contant
Increasing tempertature = The equillibrium position moves in the endothermic direction (positive direction( Decreasing temperature = The equillibrium position moves in the exothermic direction
58
What does the equillibrium position show
The equilibrium position highlights that ratio of the yeield of products / reactants If the equillibrium position moves to the backwards reaction, the yield of th ebackwards reaction increases
59
How does changing pressure effect the equillibrium constant
Increasing pressure = The equillibrium position moves to the side of fewest moles Decreasing pressure = The equillibrium position moves to the side with most moles
60
Do catalysts effect equillibrium position
no
61
How do you calculate equillibrium constant
Using the Kc formula Kc =[product]×[product]/ [reactant] × [reactant] Subsitute in the concentration If there are any moles in the reaction, they are put as a power
62
in the reaction N2O4 = 2NO2 write the equillibrium equation
kc = [NO2]^2 / [N2O4]
63
How do you find the units of the equillibrium constant
Put the moles and th eunits (mol dm^-3) and calculate the moles
64
What does it mean if the Kc is greater than 1
The reaction favours the products (there is a greater yield of products)
65
What does it mean if the Kc is lower than 1
The reaction favours the reactants (There is a great yield of reactants in the dynamic equilibrium)
66
How do you find the Kc, when you are give the moles of the reactants (not in equillibrium) and the moles of the products (in equillibrium) E.g 1.6mol No 1.4mol O2 At equillibrium 1.2 mol 02 is formed 2NO + O2 = 2NO2
Finfd the ratio of moles (in the reaction) Write out the intitial actual moles Divide the equillibrium moles, by the ratio Subtract the values to obtain the moles in equillibrium, for each molecule 2 + 1 = 2 1.2 + 0.6 = 1.2 1.6-1.2 = 0.6 (equillibrium moles for NO 1.4-0.6 = 0.8 (equillibrium moles for O2)

(A-level) Chemistry (9 decks)

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