U3 AOS2: Optimising Chemical Reactions Flashcards
Rate/extent of reactions, electrolysis
1
Q
Collision theory
For a reaction to occur, reactants must…
A
- Collide with each other
- Have sufficient energy to break the reactants’ bonds
- Collide with the correct orientation
- Allows bonds to break & new bonds to form
- If the orientation is incorrect, particles bounce off each other (reaction does not occur)
NOTE: Most collisions do not result in chemical reactions.
2
Q
Transition state
A
- New arrangement of atoms when Ea is absorbed
- Occurs at the state of maximum energy potential
- Bonds are both breaking and forming at this stage
- Atoms in this state are highly unstable and rearrange into products as the reaction progresses
3
Q
Activation energy (Ea)
A
- Minimum amount of energy that must be absorbed to break bonds in reactants so a chemical reaction can occur
- Reactions only occur when the energy of the collision is equal to or greater than the activation energy
- Low activation energy = fast reaction rate
- High activation energy = slow reaction rate
4
Q
Rate of reaction
A
= change in concentration / time
- Affected by
- Surface area of solid reactants
- Concentration of reactants in solution
- Pressure of gaseous reactants
- Temperature
- Catalyst
5
Q
How does increasing surface area increase reaction rate?
A
- More particles are exposed on the solid’s surface
- More reactants collide with these exposed particles
- This increases the frequency of successful collisions
NOTE: Only the particles at the surface of the solid participate in the reaction.
6
Q
How does increasing concentration and pressure increase reaction rate?
A
- More reactant particles in the system per unit volume
- This increases the frequency of successful collisions
7
Q
How does increasing temperature increase reaction rate?
A
- Particles are moving faster and have higher kinetic energy
- This increases the proportion of collisions with E ≥ Ea (more collisions overcome the Ea barrier)
8
Q
How do catalysts increase reaction rate?
A
- Provide an alternative reaction pathway w/ a ↓ Ea
- Decrease the amount of energy required to break bonds in the reactants by temporarily binding to them
- This increases the proportion of collisions with E ≥ Ea
- Remember that catalysts
- Don’t alter the extent of reaction, therefore the △H value stays the same
- ↓ Ea of the forward and reverse reaction to the same extent, therefore percentage yield stays the same
- Aren’t used up in the reaction (can be collected/reused)
9
Q
Homogenous vs heterogenous catalysts
A
- Homogenous – same state as reactants & products
-
Heterogeneous – diff state to reactants & products
- Generally easier to use
- Can be easily separated from the products
- Can be used at high temperatures
10
Q
Extent of reactions
A
- The proportion of reactants that have been converted into products
- No information about how fast a reaction will happen
11
Q
Open vs closed systems
A
-
Open (e.g. bushfires)
- Matter & energy can be exchanged w/ the surroundings
-
Closed (e.g. a submarine under water)
- Only energy is exchanged w/ the surroundings
- Equilibrium can only be reached in closed systems
12
Q
Reversible vs irreversible reactions
A
-
Reversible – can only proceed in one direction
- Products can be converted back to reactants
- Changes in states are also reversible
- Do not proceed to completion, thus, yield of products is never equal to the theoretical yield
- Can reach dynamic equilibrium
-
Irreversible – can proceed both forward & backward
- Products can’t be converted back to reactants
- Cannot reach dynamic equilibrium
13
Q
Dynamic equilibrium
A
- Concentration of products & reactants remain constant
- Forward and backward reactions are occurring simultaneously at the same rate (no observable net change)
- Constant temperature, pressure and amount of Ps & Rs
- Reaction is ‘incomplete’
- At a molecular level
- Bonds are constantly being broken and formed
- Ps & Rs are constantly converted from one to the other
14
Q
Position of equilibrium
A
- Relative amounts of reactants & products at equilibrium
- Can be changed by:
- Adding/removing a reactant or product at constant V & T
- Changing volume at constant temperature
- Changing pressure at constant volume & temperature
- Changing temperature at constant volume & pressure
15
Q
Le Chatelier’s Principle
A
- If an equilibrium system is subject to change, the system will adjust itself to partially oppose the effect of the change
16
Q
Effect of changing reactants or products on the position of equilibrium
A
-
Adding reactants / removing products
- Forward reaction is favoured to form more products
- Position of equilibrium shifts to the right
-
Adding products / removing reactants
- Backward reaction is favoured to form more reactants
- Position of equilibrium shifts to the left
17
Q
Effect of changing pressure or volume on the position of equilibrium
A
-
Increasing pressure by decreasing volume
- System shifts in the direction that lowers pressure
- Side with less gas particles is favoured
- Position of equilibrium shifts to the side with less particles
-
Decreasing pressure by increased volume
- System shifts in the direction that increases pressure
- Side with more gas particles is favoured
- Position of equilibrium shifts to the side with more particles
- Position of equilibrium is not affected if the number of reactant and product particles are equal
NOTE: This only applies to gaseous systems. In liquids and solids, particles are too tightly for an increase in pressure to have a noticeable effect on volume.
18
Q
Effect of changing concentration on the position of equilibrium
A
-
Decreased concentration via dilution (+ water)
- System will favour the side with more particles
-
Increased concentration
- System will favour the side with less particles
- Position of equilibrium is not affected if the number of reactant and product particles are equal
19
Q
Effect of changing temperature on the position of equilibrium and equilibrium constant (K)
A
-
Exothermic reactions (– △H) release heat
- Increasing temp adds heat to the RHS
→ System wants to decrease heat by favouring the reverse reaction, (equilibrium shifts to the left)
→ Decreased K - Decreasing temp removes heat from the RHS
→ system wants to increase heat by favouring the forward reaction, (equilibrium shifts to the right)
→ Increased K
- Increasing temp adds heat to the RHS
-
Endothermic reactions (+ △H) absorb heat
- Increasing temp adds heat to the LHS
→ system wants to consume heat by favouring the forward reaction, (equilibrium shifts to the right)
→ Increased K - Decreasing temp removes heat from the LHS
→ system wants to increase heat by favouring the reverse reaction, (equilibrium shifts to the left)
→ Decreased K
- Increasing temp adds heat to the LHS
TIP: Think of heat as a product (exothermic) or reactant (endothermic).
20
Q
Effect of catalyst on equilibrium
A
- Decrease Ea of the forward & reverse reaction by same amount
- This will increase the rate in which equilibrium is reached (system can reach equilibrium faster)
- This has no effect on the position of equilibrium or K
21
Q
Equilibrium constant (K)
A
- Value indicates the extent of reaction at equilibrium
- Affected by temperature
- NOT affected by concentration, pressure or catalysts
- Small K value – more reactants than products
- Large K value – more products than reactants
- If K is close to 1 – concentrations of Rs & Ps are about equal
22
Q
Reaction quotient (Q)
A
- Extent of a reaction that is not at equilibrium
- Value of Q changes as the reaction progresses
- Q = K when the system at a particular temp reaches equilibrium
- If Q > K, the system will shift to the left to reach equilibrium
- If Q < K, the system will shift to the right to reach equilibrium
23
Q
How can equilibrium systems be designed to be more energy efficient?
A
-
Catalysts increase reaction rate w/o the need for high temp
- This decreases energy input
- Lower temp is less expensive and needs less fuel
-
Heat exchangers recover wasted heat from exo reactions
- Recover and reuse heat energy
- Reduces energy input requirements
24
Q
Electrolysis
A
- Passage of electrical energy from a power supply (e.g. a battery) through a conducting liquid
- Electrical energy is converted into chemical energy
- Allow non-spontaneous reactions to occur
25
Galvanic vs electrolytic cells
- **Galvanic cells**
- AN OIL RIG CAT
- Anode = negative, cathode = positive
- Spontaneous reactions
- Chemical → electrical energy
- Negative gradient
- **Electrolytic cells**
- AN OIL RIG CAT
- Anode = positive, cathode = negative
- Non-spontaneous reactions
- Electrical → chemical energy
- Positive gradient
26
Using the electrochemical series to predict electrolytic half-cell reactions
- **Oxidation** reaction is **above** the **reduction** reaction on the electrochemical series (positive gradient)
27
Competition at electrodes in electrolytic cells
- **Water** is a potential reactant when aqueous electrolytes are used (look for the word 'solution')
- **Reactive electrodes** may also participate in the reaction
28
Voltage required to operate an electrolytic cell
**= higher half-cell E⁰ – lower half-cell E⁰**
- GIVEN WITH A GREATER THAN SIGN e.g. >+1.20V
29
Limitation of the electrochemical series in predicting electrolytic half-cell reactions
| Electrolytic cells
- It is based on standard conditions and **most electrolysis reactions are not performed at standard conditions**
- Reactions are affected by electrolyte concentration, gas pressures, current, voltage and electrode types
30
Separation and continuous removal of products in electrolytic cells
- **Electrolytic reactions form products that are reactants of a spontaneous redox reaction**
- Must be separated to prevent them spontaneously reacting (undesirable and potentially dangerous)
- This can be done by...
- Using a semipermeable membrane to separate products
- Using a mesh to separate the electrodes
- Removing the products as they form
31
Aqueous vs molten electrolytes in electrolytic cells
- **Aqueous (aq)**
- Used in favour of a molten electrolyte when possible
- Contains water which can be a potential reactant
- Can easily operate at SLC
- **Molten (l)**
- An ionic compound heated to become a liquid
- No water present
- Requires much more energy
- Must operate at high temperatures (at its melting point)
- Expensive and often hazardous (due to high temp)
32
Chemical additives in electrolytic cells
- Can **lower the melting point** of molten electrolytes
- This allows the cell to operate at lower temperatures, making it **cheaper** and **safer** to run
- E.g. adding CaCl₂ to NaCl (l) lowers its melting point
33
Power supply and external circuit of electrolytic cells
- **Power supply** must be **direct current** (current that consistently flows in a single direction)
- **Wire** forms the external circuit, allowing electrons to flow from the anode to the cathode
34
Primary vs secondary cells
- **Primary** cells **cannot be recharged** (disposable)
- Go flat when the reaction reaches equilibrium
- Products move away from electrodes or are consumed by side reactions, preventing them from being recharged
- E.g. alkaline cells
- **Secondary** cells **can be recharged**
- Aka rechargeable cells or accumulators
- E.g. lithium-ion cells
35
Secondary cells
- Act as **galvanic** cells during **discharge**
- Chemical to electrical energy
- Spontaneous reaction
- Anode is negative
- Act as **electrolytic** cells during **recharge**
- Electrical to chemical
- Non-spontaneous reaction
- Anode is positive
## Footnote
TIP: R**ec**harge = **e**lectrical to **c**hemical energy.
36
Conditions required for the recharge of secondary cells
- Connection to a power supply with a **voltage higher than that produced by the cell during discharge**
- **Undamaged electrodes**
- Discharge **products remain in contact with electrodes** (so that they can be converted back into reactants via electrolysis)
37
Over time, batteries cannot be adequately recharged and will need replacing. Why?
- **Products may be unable to remain in contact with electrodes** after numerous recharge/discharge cycles
- **Unwanted side reactions** can occur at high temperatures, consuming the cell's reactants and products
- **Species can crystalise** (solidify) at low temperatures, preventing the flow of ions
- **Electrodes can be damaged**
38
Electroplating
- A commercial application of electrolysis
- Results in a **thin layer of metal over another surface**
- **Anode (+)** has a metal that loses electrons (oxidation)
- **Cathode (–)** has an object that gains electrons (reduction)
- Metal ions deposited on the object results in a thin metal layer over its surface
39
Factors that determine the amount of products that form in electrolytic cells
- **Charge** on the ion in the electrode reaction
- **Current** flowing through the cell
- Length of **time** that the current flows
## Footnote
NOTE: Faraday's laws describe the relationship between these factors.
40
Green hydrogen
- Hydrogen gas **produced using renewable energy sources**
- Cleanest form hydrogen (no carbon emissions)
- E.g. through PEM electrolysis or artificial photosynthesis
41
Advantages and disadvantages of green hydrogen
- **Advantages**
- Only product of its combustion is water
- High energy density
- Abundant (present in H2O & most carbon compounds)
- **Disadvantages**
- Limited infrastructure for production/storage/distribution
- Electrolysis process is energy intensive
- Need high pressures to efficiently store it as a gas (exp)
- Not found as an element (energy needed to produce it)
- Explosive (requires careful handling and storage)
42
Polymer electrolyte membrane (PEM) electrolyser
- Can be powered by solar/wind power, hydroelectricity, biomass
- The **PEM separates e⁻ & gases** produced during electrolysis
- Allows protons (H+) to flow, completing the internal circuit, while blocking other ions & e⁻
- Efficient as it ↓ the contamination/mixing of gases
- Electrodes are often covered with a **platinum catalyst** which increases the production rate of the gases
- Has an acidic electrolyte
- High rate of hydrogen production ✅
- Expensive due to expensive materials (e.g. catalysts) ❌
## Footnote
NOTE: Increasing the PEM thickness would decrease the efficiency due to added resistance to proton flow.
43
Reactions during PEM electrolysis
- **Water oxidation** at the anode
- 2H₂O(l) → 4H⁺(aq) + O₂(g) + 4e⁻
- **Proton reduction** at the cathode
- 4H⁺(aq) + 4e⁻ → 2H₂(g)
- **Overall equation**
- 2H₂O(l) → 2H₂(g) + O₂(g)
- Electric current passes through water, causing it to split into hydrogen and oxygen (can then be collected)
## Footnote
NOTE: This is non-spontaneous.
44
Artificial photosynthesis
- **Human-made materials** capture **sunlight** and **split water** molecules to **create hydrogen and oxygen**
- Electrodes are often covered with catalysts which increases the rate of production of the gases
- H₂(g) is produced instead of glucose
- Acidic
45
Reactions during artificial photosynthesis
- **Water oxidation** in acid using catalysts (anode)
- 2H₂O(l) → 4H⁺(aq) + O₂(g) + 4e⁻
- **Proton reduction** in the presence of catalysts (cathode)
- 4H⁺(aq) + 4e⁻ → 2H₂(g)
- **Overall equation**
- 2H₂O(l) → 2H₂(g) + O₂(g)
- Electric current is generated by sunlight
46
Advantages of artificial photosynthesis
- Does not create greenhouse gases (liquid water is produced)
- Does not require the use of fossil fuels
- Can remove carbon dioxide from the atmosphere
