[Year 2] CI

Question 1

What might chemical companies consider in order to be as financially economical as possible?

Answer 1

• Raw materials: cheaper, but must be widely available to ensure good sustainable supply.
• Energy: required in manufacturing and transport, high TP increases costs. Purification of products also needs to be considered.
• Waste: from by-products, due to gov regulations costs. Co-products can be sold off (although at a lower price).
• High atom economic and percentage yields.

Question 2

What are the pros and cons of different reaction conditions that chemical companies consider?

Answer 2

Pressure:

• Pros: beneficial to gas reactions, as high pressure speeds up the rate of reaction.
• Cons: pumps, thicker pipes and vessels, and additional safety equipment are needed (which are expensive to run).

Catalysts:

• Pros: allows reactions to run quickly at lower temps, saving money. Can be reused.
• Cons: expensive, homogeneous catalyst must be separated after the reaction (additional step increases costs).

Temperature:

• Pros: Higher temps = faster rate of reaction, more product per unit time.
• Cons: More fuel needed, therefore more expensive.

Question 3

If you were to plot the Con - Time graph for both forward and backward reactions of a reversible reaction on the same graph, what would happen when they both plataue?

Answer 3

• Dynamic equlimbrium is met.
• This is where the rate (Con/Time) of forward reaction = rate of backwards.
• At this point the concentrations remain constant.
• ONLY occurs in a closed system.

Question 4

What effect does temperature have on Kc?

Answer 4

• Only valid for one temperature.
• Changing the temperature will change the equilibrium concentrations, and hence Kc will change too.
• If temp change causes equilibrium to shift right, Kc INCREASES.
• If temp change causes equilibrium to shift left, Kc
  DECREASES.
• This can be explained using the Kc expression of a reaction.

Question 5

What effect does concentration have on Kc?

Answer 5

No effect

Question 6

What effect does a catalyst have on Kc?

Answer 6

No effect.

• catalyst speed upi the rate of forward AND reverse reaction.
• so speed up the rate at which equilibrium is established.

Question 7

What are the conditions of the Haber Process? And why?

Answer 7

Temp: 400ᵒC
∵ although it shifted the equilibrium to the left (as the forward reaction is exo), this is so the rate of reaction isn’t so small that it takes too long.

200atm
∵ although a high pressure would favour the forward reaction (due to moles), high pressures are expensive and can be dangerous.

For these reasons both of these are compromised.

Question 8

What are the advantages and disadvantages of the uses of chemical products?

Answer 8

Adv:

• Parmaceutcals help prevent and cure disease.
• Fertilisers ensure food is produced in the most efficient way possible
• Plastics are widely used today.
• Dyes and cleaning procus have helped improve living environment and lifestyle.

Dis:
- Some chemicals can be harmful to the environment.
(gases = air pollution) (liquid/solid = land pollution).
- Some chemicals are flamalble and explosive (must be stored and handled correctly.
- Some chemicals can be harmful to our health (via inhalation or contact).

Question 9

What is the Kc expression of 2A + B ⇌ 2C + D?

Answer 9

Kc = [C]²[D] / [A]²[B]

Question 10

How can we find out the concentrations of a substance without affecting the equilibrium?

Answer 10

Colourimetry:

• Coloured solutions absorbs particular wavelengths of light.
• This can be used to determine the absorption of the wavelength of light of the reaction at equilibrium.
• A calibration curve of known concentrations can be plotted.
• Our test sample is compared to these.

pH:

• Useful if reactant and products have different pH values.
• pH can be measured at equilibrium.
• pH equation can then be used to calculate the concentration.

Question 11

How can you work out Kc from information about initial quantities and equilibrium?

Answer 11

Initial moles
Change in moles
Equilibrium moles

Equilibrium moles/vol = Equilibrium concentration

Kc = [products] / [reactants]

Question 12

What is the electron configuration of Nitrogen?

Answer 12

1s¹ 2s² 2p³

Question 13

Why is Nitrogen an unreactive gas at RTP?

Answer 13

Nitrogen is diatomic with triple covalent bonds which is required a lot of energy to break.

Question 14

How is ammonia made?

Answer 14

The Haber Process:
- the reaction between N₂ and H₂.

N₂ + 3H₂ ⇌ 2NH₃

Question 15

What is the significance of ammonia?

Answer 15

• It shares 3 of its valence electrons with hydrogen and a lone pair of electron remains.
• As a result it can form H-bonds with water, and so is very soluble.
• It can also form dative covalent bonds using its lone pair of electrons (although these are not as strong as standard covalent bonds).
• As a result of its lone pair, is also acts as a base to form ammonium ions (this N-H bonds is dative).

Question 16

List the oxides of nitrogens (and their properties)?

Answer 16

• N₂O (Nitrogen(I) oxide/ Dinitrogen oxide): colourless gas with sweet smell.
• NO (Nitrogen(II) oxide/ Nitrogen monoxide): colourless gas.
• NO₂ (Nitrogen(IV) oxide/B Nitrogen dioxide): Brown gas with sharp, pungent smell.

Question 17

How do you test for ammonium compounds (and hydroxides)?

Answer 17

Litmus paper.

• Add sodium hyrodxide
• Heat, if ammonium is present amonia gas will be produced.
• Use damp red litmus, this will turn blue as ammonia dissolves in the water.

Question 18

How do you test for Nitrate(V) ions?

Answer 18

Using sodium hydroxide and aluminium/ dervarda’s alloy.

• add sodium hydroxide and aliminium foil/Devarda’s alloy to the solution you want to test.
• Heat.
• If nitrate(V) ions are present they are reduced by aluminium to ammonia gas.
• Use damp red litmus, this will turn blue as ammonia dissolves in the water.

Question 19

What is the reaction that shows the production of Ammonia from Nitrogen gas?

Answer 19

N₂ + 3H₂ → 2NH₃

Question 20

What is the reaction that shows the production of Ammonium ions from Ammonia?

Answer 20

NH₃ + H⁺ → NH₄⁺

Question 21

What is the reaction that shows the production of Nirtate(III) ions from Ammonium ions?

Answer 21

NH₄⁺ + O₂ → NO₂⁻ + 4H⁺ + 2e⁻

Question 22

What is the reaction that shows the production of Nitrate(V) ions from Nirtate(III) ions?

Answer 22

NO₂⁻ + H₂O → NO₃⁻ + 2H⁺ + 2e⁻

Question 23

What is the reaction that shows the production of Nitrogen gas from Nitrate(V) ions?

Answer 23

2NO₃⁻ + 12H⁺ + 10e⁻ → N₂ + 6H₂O

Question 24

What are the reactions that shows the production of Nitrogen Oxides from Nitrogen gas?

Answer 24

N₂ + O₂ → 2NO
N₂ + 2O₂ → 2NO₂
2N₂ + O₂ → 2N₂O

Question 25

What is the definiton of the rate of reaction?

Answer 25

The change of concetration/ amount of a reactant or product, per unit time.

Question 26

What ways can be used to measure the rate of a reaction?

Answer 26

How long it takes for a precipitate to form:

• Place cross on paper and time how long it takes for it to disappear.
• Difficult to know when goes cloudy, use same observer to reduce error.

Mass lost:

• If reaction produces gas, place on balance and measure the mass lost as gas is lost per unit time.
• If gas is harmful or toxic must be done in a fume cupboard.

Volume produced:

• Measure amount of gas produced in a gas syringe.
• Done over a specified time.
• Ensure syringe is correct size.

Change of pH:

• Use pH meter to measure pH at regular intervals.
• Plot graph, and use it to calc [H⁺].

Titrations:

• take small samples at regular time intervals.
• quench reaction
• and titrate them against know con.

Colourimerty:

• Coloured solutions absorbs particular wavelengths of light.
• This can be used to determine the absorption of the wavelength of light of the reaction at equilibrium.
• A calibration curve of known concentrations can be plotted.
• Our test sample, at regular times, is compared to these.

Question 27

How might you slow a reaction down?

Answer 27

• Dilution with dionised water.
• Cool it down.
• Add a chemical to stop the reaction (quenching)

Question 28

How can you find the rate from a graph?

Answer 28

Plot con - time graph.

Find gadient.

If not linear: draw a tangent at specific points and plot on a rate - time graph. The gradient of this is the total rate of reaction.

Question 29

What is a clock reaction?

Answer 29

A reaction wher you can time how long it takes for a reaction to occur.

Question 30

What assumptions are made when we do a clock reaction/Initial rates method and want to find its rate?

Answer 30

• Temp remains constant.
• [reactants] doesn’t change significantly during time period of the reaction.
• Reaction hasn’t proceeded too far when the end point is observed.

Question 31

What does a quicker clock reaction mean for the rate?

Answer 31

Faster initial rate of reaction.

Question 32

Describe the Iodine clock experiment.

Answer 32

Equation: H₂O₂ (aq) + 2H⁺ (aq) + 2I⁻(aq) → H₂O (l) + I₂(aq)

• Add sodium thiosulfate and starch (indicator) to excess Hydrogen peroxide.
• Sodium thoisulfate reacts immediately with iodine produced in this reaction:

2S₂O₃²⁻(aq) + I₂ → 2I⁻(aq) + S₄O₆²⁻(aq)

• When there is no more sodium tiosulfate left, I₂ reacts with starch.
• This gives a deep blue/black colour.

∴ by varying the [I₂] and/or [H₂O₂] and keeping eveything else constant, the time take for blue/black colour change changes.

• We can then work out the order of reaction.

Question 33

Describe the initial rates method (for finding out the order of reaction).

Answer 33

• Find the rate of reaction for different concentrations over a set time period.
• Work out the initial rates at each concentration.
• Plot rate - concentration to work out orders.

Question 34

What are the different parts of a rate equation?

Rate = k[A]ᵃ[B]ᵇ

Answer 34

Rate = k[A]ᵃ[B]ᵇ

Rate = Rate/ moldm⁻³s⁻¹
k = rate constant/units vary
[A] = concentration of  A/ moldm⁻³
[B] = concentration of  B/ moldm⁻³
xᵃ = order with respects to a
xᵇ = order with repects to b

Question 35

What does the order of reaction tell us?

Answer 35

How the concentration of a substance affects the rate.

Question 36

Why does the rate constant increase when temperature does?

Answer 36

• When temperture increases so doe the rate as moleulces have more kinetic energy
• The concentration on substances remains the same.
• Therefore for rate to increase so does k.

Question 37

What do different graphs look like for differing rates?

Answer 37

0 order:

• linear concentration-time graph.
• horizonle line rate-concentration graph.

1st order:

• shallow curve concentration-time graph (changes by equal amounts).
• linear rate-concentration graph.

2nd order:

• steep curve concentration-time graph (changes by unequal amounts).
• curved rate-contration graph.

Question 38

What is the half life of a reaction?

Answer 38

The time it takes for half of the reaction to be used up.

Question 39

How can half life be used to calculate the order of reaction?

Answer 39

0 order:
- decreasing half-life.

1st order:
- constant half-life.

2nd order:
- increasing half-life.

Question 40

How can half life be used to work out the rate constant?

Answer 40

k = ln2/t₀.₅

Question 41

What does the arrhenius equation link?

Answer 41

The activation energy and temperature to the rate constant(k).

Question 42

What is the Arrhenius equation?

Answer 42

k = Ae⁻ᴱᵃ/ᴿᵀ

k: Rate constant
A: Arrhenius constant
e: Universal constant
Ea: Activation energy / J
R: Gas constant
T: Temperature /K

Question 43

What relations can be derived from the Arrhenius equation?

Answer 43

k = Ae⁻ᴱᵃ/ᴿᵀ

• As the activation energy (Ea) gets smaller, the rate constant (k) gets bigger: as many more particles have enough energy to react when they collide.
• As temperature (T) increases, so does the rate constant (k): as particles have more kinetic energy, and are more likely to collide with at least the Ea.

Question 44

Express the Arrhenius equation in the form y = mx +c :

Answer 44

k = Ae⁻ᴱᵃ/ᴿᵀ
lnk = lnA - Ea/RT
lnk = -Ea/RT + lnA

y: lnk
m: -Ea/R
x: 1/T
c: lnA

Question 45

What reactants (or substances derived from them) must appear in the rate determining step?

Answer 45

Those that apear in the rate equation.

Question 46

Is the rate determining step fast or slow?

Answer 46

Slow.

Question 47

Deduce the rate equation using the following information…

Step 1: A + B → 2C (fast)
Step 2: 2C → D (slow)
Step 3: D + E → F + G (fast)
OVERALL: A + B + E → F + G

Answer 47

Rate = k[A][B][C]²

Reasoning:

• C must be in the rate equation, as it is the rate-determining step.
• [C] is raised to a power of 2 as two moles of it are used to make 1 mole of D in the rate-determining step.
• A and B must also be in the rate equation as they make up C.