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OCR (A) GCSE Chemistry > Topic 5: Monitoring and Controlling Chemical Reactions > Flashcards

Topic 5: Monitoring and Controlling Chemical Reactions Flashcards

1
Q

What happens in a reversible reaction?

A

The product molecules can themselves react with each other or decompose and reform the reactant molecules

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2
Q

Can a reversible reaction occur in both directions?

A

Yes, can occur in the forward reaction (which forms the products) and the reverse, or backward, direction (which forms the reactants)

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3
Q

What is the symbol for a reversible reaction?

A

⇌

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4
Q

What is a dynamic equilibrium?

A

A state in a reversible reaction where the forward and backward reactions occur at the same rate, and the concentrations of reactants and products remain constant.

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5
Q

What are the conditions required for dynamic equilibrium?

A
  • The system must be closed (nothing can enter or leave).
  • Temperature and pressure must remain constant.
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6
Q

Why is equilibrium called “dynamic”?

A

Because the forward and backward reactions are still occurring, but there is no net change in the concentrations of reactants and products.

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7
Q

What is the Haber process?

A

A reaction where nitrogen and hydrogen react to form ammonia, which is a reversible reaction.

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8
Q

What is the balanced equation for the Haber Process?

A

N₂ ⁽ᵍ⁾ +3H₂ ⁽ᵍ⁾⇌2NH₃ ⁽ᵍ⁾

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9
Q

What happens to the forward reaction rate at the start of the Haber process?

A

The forward reaction rate is highest because the concentrations of N₂ and H₂ are at their maximum.

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10
Q

What happens to the forward and backward reaction rates as the Haber process proceeds?

A
  • The forward reaction rate decreases as
    N₂ and H₂
    are used up.
  • The backward reaction rate increases as
    NH₃ is formed and starts to decompose.
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11
Q

When is equilibrium reached in the Haber process?

A

Equilibrium is reached when the forward and backward reaction rates become equal, and the concentrations of N₂, H₂, and NH₃ remain constant.

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12
Q

Can equilibrium be reached in an open system?

A

No, equilibrium can only be reached in a closed system where nothing can enter or leave.

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13
Q

How can you identify when equilibrium is reached on a concentration vs. time graph?

A

Equilibrium is reached when the concentrations of reactants and products become constant (the graph lines flatten out).

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14
Q

What is a common mistake when identifying equilibrium on a graph?

A

Misidentifying the point where the lines start to flatten. Use a ruler to check where the lines become horizontal.

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15
Q

What does the relative amounts of all reactants and products at equilibrium depend on?

A

Conditions of reaction

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16
Q

What does Le Chatelier’s Principle state?

A

When a change is made to the conditions of a system at equilibrium, the system automatically moves to oppose the change

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17
Q

What is the principle used to predict?

A

Changes to the position of equilibrium when there are changes in temperature, pressure or concentration

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18
Q

What does knowing the energy changes, states and concentrations allow us to do with the principle?

A

Manipulate the outcome of reversible reactions to increase the percentage yield

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19
Q

How can you increase the percentage yield in an equilibrium?

A

The position of the equilibrium needs to move towards the right side, which is the product side, and therefore make a higher concentration of product at the point of dynamic equilibrium

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20
Q

What happens when you increase the concentration of the reactant in the system?

A

Equilibrium shifts to right to reduce effect of increasing concentration of a reactant

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21
Q

What happens when you decrease the concentration of the reactant in the system?

A

Equilibrium shifts to left to reduce the effect of a decrease in reactant

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22
Q

What happens when you increase the temperature of the reactant in the system?

A

Equilibrium moves in endothermic direction to reverse change

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23
Q

What happens when you decrease the temperature of the reactant in the system?

A

Equilibrium moves in exothermic direction to reverse change

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24
Q

What state doe changes in pressure only affect and what must we check in a system beforehand?

A
  • Only affects gases
  • Identify all gaseous reactants and products
  • If there are the same number of moles of gases on either side of the equation, then there is NO effect on the position of equilibrium when the pressure is changed
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25
What happens when you increase the pressure of the reactant in a system?
Equilibrium shifts in direction that produces smaller number of molecules of gas to decrease pressure again
26
What happens when you decrease the pressure of the reactant in a system?
Equilibrium shifts in direction that produces larger number of molecules of gas to increase pressure again
27
What is the equation for concentration in g/dm³ ?
Concentration = Mass of solute (g) / Volume of solution (dm³)
28
How do you convert from mol/dm³ to g/dm³?
To go from g/dm³ to mol/dm³ : - Divide by the molar mass in grams To go from mol/dm³ to g/dm³ : - Multiply by the molar mass in grams
29
What must you divide by to go from cm³ to dm³?
Divide by 1000
30
What is the equation for concentration in mol/dm³?
Concentration = Number of moles of solute (mol) / Volume of solution (dm³)
31
What is the concentration moles formula triangle?
Moles / Concentration x Volume
32
What are titrations?
Method of analysing concentration of solutions
33
What are some examples where titration is used?
- Acid base titration - Determine exact amount of alkali to neutralise quantity of acid - Prepare salts or other precipitates in redox reactions
34
What are used to show the endpoint in a titration?
Indicators, with phenolphthalein being popular
35
Why are some indicators not suitable for titration?
Litmus and other indicators are not suitable as they don't give a sharp enough colour change Universal Indicator also unsuitable as it is a mix of indicators and has too many subtle colour changes
36
What are the hazards and safety procedure we can take in a titration?
- Dilute HCl can harm eyes or skin - Acids and alkalis are corrosive - Avoid contact with skin and use safety goggles - Pipette with a safety filler to avoid corrosive liquids
37
Describe a titration of an unknown conc. of sulfuric acid and 0.1 mol/dm³ of NAOH solution
- Use pipette and pipette filler to place exactly 25 cm³ NaOH solution into conical flask - Place flask on white tile so tip of burette is inside - Add few drops of indicator to solution in flask - Perform rough titration by taking burette reading and running in solution in 1-3 cm³ portions, swirling flask vigorously - Close tap when end-point, sharp colour change, is reached - Record volume from meniscus at eye-level - Repeat with fresh NaOH but add one drop at time until colour change when rough end-point volume reached - Swirl and rinse with distilled water and finish at first sign of colour change that persists - Repeat until concordant (within 0.1cm³) results to increase accuracy
38
What is the volume occupied by one mole of any gas found to be?
24 dm³ or 24,000 cm³ - Molar gas volume at RTP
39
What is the gas volume triangle?
Volume / Moles x 24 dm³
40
What is yield?
Describe the amount of product you get from a reaction
41
Why is it that in practice, you never get 100% yield in a chemical process?
- Some reactants may be left behind in equipment - Reaction may be reversible - Lost during separation and purification - Side reactions - Impurity in substance - Lost during transfer
42
What is the actual yield?
Recorded amount of product obtained
43
What is the theoretical yield?
Amount of product that would be obtained under perfect practical and chemical conditions
44
What is the percentage yield?
- Measures how successful a chemical process is Percentage Yield = Actual Yield / Theoretical Yield X 100
45
What is atom economy?
- Efficiency of reactions - Studies amount of reactants that get turned into products Atom Economy = Total Mᵣ of reactants / Total Mᵣ of all products
46
Why are reactions with low atom economies problematic?
They use up many resources, produce substantial waste material requiring expensive disposal, making them unsustainable and economically unattractive.
47
What makes a reaction economically unattractive?
Raw materials are typically expensive, as is waste disposal (which requires chemicals, equipment, space, and transport).
48
What factors do companies analyze to improve chemical process efficiency?
Atom economy, percentage yield, rates of reaction, and equilibrium position.
49
Why are high percentage yields desirable in industrial processes?
They maximize the conversion of reactants to desired products, minimizing waste and improving economic efficiency.
50
Why are fast reaction rates important in industrial processes?
They allow for greater production volume in less time, improving operational efficiency and reducing costs.
51
How can equilibrium position be optimized in reversible reactions?
By altering reaction conditions to shift the equilibrium in favor of the products.
52
How can waste products improve a reaction's overall efficiency?
If waste products can be sold or reused, the overall atom economy and economic viability of the process improves.
53
What approach might companies take when a reaction pathway is inefficient?
Consider alternative methods of production that may produce more useful by-products or have better atom economy.
54
What is atom economy and why is it important?
Atom economy measures how many atoms from the reactants end up in the desired product. Higher atom economy means less waste and more efficient use of resources.
55
How does sustainability relate to chemical reaction pathways?
Sustainable reaction pathways minimize resource consumption and waste production, making them better for the environment and often more economical long-term.
56
What does the rate of reaction measure?
The rate of reaction measures how fast a reactant is used up or how fast a product is made, calculated using the formula: Rate = Amount of substance ÷ Time
57
What units are commonly used for rate of reaction?
Rate of reaction can be measured in g/s (for mass changes) or cm³/s or dm³/s (for volume changes, especially with gases)
58
How can you measure the rate of reaction when a gas is produced?
You can measure the loss in mass over time (if gas escapes) or collect and measure the volume of gas produced over time
59
What is the precipitation method for measuring reaction rate?
Place a flask with reacting solutions over a cross on paper and measure the time it takes for the cross to disappear as precipitate forms (e.g., Na₂S₂O₃ + HCl reaction forming sulfur precipitate)
60
How do you calculate the mean rate of reaction?
Calculate the overall change in quantity (e.g., volume of gas produced) and divide by the total time taken for the reaction
61
How do you determine the rate of reaction at a specific point?
Draw a tangent to the curve at that point and calculate the gradient of the tangent (change in y ÷ change in x)
62
How does increased concentration affect reaction rate?
Increasing concentration increases reaction rate as there are more reactant particles in a given volume, leading to more frequent successful collisions
63
How does increased temperature affect reaction rate?
Increasing temperature increases reaction rate as particles have more kinetic energy, exceeding the activation energy more often and causing more successful collisions
64
What happens to reaction rate when a solid reactant's surface area is increased?
Reaction rate increases as more surface area is exposed to the other reactant, producing a higher number of collisions per second
65
What is collision theory?
Collision theory states that chemical reactions occur only when reactant particles collide with sufficient energy (activation energy) to react
66
What is the approximate effect of a 10°C temperature increase on reaction rate?
For aqueous and gaseous systems, a 10°C temperature increase approximately doubles the reaction rate
67
What is a catalyst?
A substance that speeds up the rate of a reaction without being altered or consumed in the reaction
68
How do catalysts affect activation energy?
Catalysts lower the activation energy by providing an alternative reaction pathway, allowing more particles to have enough energy to react
69
Name three important industrial catalysts
Iron (Haber Process), Vanadium(V) oxide (Contact Process), Manganese(IV) oxide (hydrogen peroxide decomposition)
70
What are enzymes?
Biological catalysts that control reactions in cells and biological systems
71
What is an advantage of using enzymes in industrial processes?
Enzymes allow industrial reactions to happen at lower temperatures and pressures than usually needed, saving money and energy
72
What factors affect the rate of a chemical reaction?
Concentration of reactants, temperature, surface area of solid reactants, and presence of catalysts
73
Why is increasing reaction rate economically important?
Higher reaction rates imply a higher rate of production, making processes more efficient and sustainable
74
How does surface area affect collision frequency?
Increasing surface area increases the number of exposed reactant particles, increasing collision frequency and reaction rate
75
What is the relationship between reaction time and rate of reaction?
Rate of reaction is inversely proportional to reaction time (1/time = rate of reaction)

OCR (A) GCSE Chemistry (6 decks)

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