Reaction Rate Graphs and Applications
5. Energy, Rates, and Real-World Applications
Learning outcomes
- I can explain the relationship between energy changes and reaction rates.
- I can analyze real-world examples involving chemical energetics.
- I can evaluate the importance of catalysts in society.
- I can explain how reaction rates affect everyday products and processes.
- I can apply concepts from the course to unfamiliar situations.
Bringing Energy and Reaction Rates Together
Chemical reactions involve two closely related ideas:
energy changes
and:
reaction rates
Energy changes tell us whether energy is:
released or absorbed
Reaction rates tell us:
how quickly a reaction occurs
These ideas are connected, but they are not the same.
A reaction can:
- release a large amount of energy but occur slowly
- release a small amount of energy but occur quickly
- absorb energy and occur quickly
- absorb energy and occur slowly
To understand real chemical processes, we often need to consider:
both energetics and kinetics
Energetics and Kinetics
Two important branches of chemistry help us understand reactions.
Energetics
Concerned with:
energy changes during reactions
Questions include:
- Is energy released?
- Is energy absorbed?
- How much energy changes?
Kinetics
Concerned with:
reaction rates
Questions include:
- How quickly does the reaction occur?
- What factors affect its rate?
- What mechanism does it follow?
A complete understanding of a reaction often requires:
energetics + kinetics
Exothermic Reactions
An exothermic reaction transfers energy from the reacting system to the surroundings.
The surroundings may become:
warmer
Examples include:
- combustion
- many oxidation reactions
- acid-base neutralization
- some respiration-related reactions
For an exothermic reaction:
energy released forming new bonds > energy required to break bonds
The overall energy change is:
energy released to the surroundings
Endothermic Reactions
An endothermic reaction absorbs energy from the surroundings.
The surroundings may become:
cooler
Examples include:
- some thermal decomposition reactions
- reactions used in certain instant cold packs
- some dissolution processes
For an endothermic reaction:
more energy is required to break bonds than is released when new bonds form
The reaction therefore requires a net input of:
energy
Energy Profile Diagrams
An energy profile diagram shows the energy changes occurring during a reaction.
The vertical axis represents:
energy
The horizontal axis represents:
reaction progress
Important features include:
- reactant energy
- product energy
- activation energy
- overall energy change
Activation Energy
Even reactions that release energy usually require some energy to:
get started
This minimum energy is called:
activation energy
Symbol:
Eₐ
Particles must collide with sufficient energy to overcome this barrier.
Therefore:
collision + sufficient energy + suitable orientation → successful reaction
A Hill Analogy
Imagine pushing a ball over a hill.
The ball may eventually roll down to a:
lower-energy position
But first, you must provide enough energy to push it:
over the hill
The hill represents:
activation energy
This helps explain why an energetically favorable reaction may still:
occur slowly
Energy Released Does Not Determine Rate
Consider a piece of wood.
Wood can react with oxygen:
wood + oxygen → combustion products + energy
The reaction releases substantial energy.
Yet a piece of wood can sit in air for:
years
without suddenly bursting into flames.
Why?
Because the reaction requires sufficient:
activation energy
A flame or spark can provide this energy.
This demonstrates:
exothermic does not automatically mean fast
Collision Theory
Reaction rate can be explained using:
collision theory
Reacting particles must:
- collide
- collide with sufficient energy
- collide with suitable orientation
Only some collisions are:
successful collisions
The reaction rate depends on:
how frequently successful collisions occur
Temperature Connects Energy and Rate
Increasing temperature gives particles more:
kinetic energy
This affects reaction rate in two important ways.
Particles:
move faster
so collisions occur more frequently.
More importantly, a greater fraction of particles have enough energy to overcome:
activation energy
Therefore:
higher temperature → more successful collisions → faster reaction
Why Temperature Has Such a Strong Effect
Increasing temperature does not merely make particles:
slightly faster
It also increases the proportion of collisions that have enough energy to overcome the activation-energy barrier.
Therefore, even a moderate increase in temperature can sometimes produce:
a substantial increase in reaction rate
Catalysts Connect Energy and Rate
A catalyst increases reaction rate by providing:
an alternative reaction pathway
with a lower:
activation energy
This allows a greater proportion of collisions to become:
successful
Therefore:
lower Eₐ → more successful collisions → faster reaction
Catalysts and Energy Profile Diagrams
A catalyst changes the pathway between:
reactants and products
but does not change their starting and finishing energy levels.
Therefore, a catalyst:
lowers activation energy
but does not change the overall energy change of the reaction.
What a Catalyst Does NOT Change
A catalyst does not change:
- the energy of the reactants
- the energy of the products
- the overall energy change
- the stoichiometric amount of product possible from given starting amounts
For a reversible reaction, a catalyst also does not change:
the equilibrium position
It simply allows equilibrium to be reached:
more quickly
Why Catalysts Matter to Society
Catalysts are enormously important because many useful chemical reactions would otherwise be:
too slow
or require:
more extreme conditions
Catalysts are used in:
- fertilizer manufacture
- fuel processing
- polymer production
- pharmaceuticals
- food production
- vehicle exhaust systems
- biotechnology
- biological metabolism
Catalysts Can Save Energy
Suppose an industrial reaction is too slow at:
300°C
Without a catalyst, it might require:
500°C
to reach a useful rate.
If a catalyst allows an acceptable rate at 300°C, the factory may require:
less heating
This can reduce:
- energy consumption
- operating costs
- fuel use
- associated emissions
Catalysts can therefore contribute to:
more efficient chemical production
Catalysts Can Improve Selectivity
Some catalysts help favor a particular:
reaction pathway
This can increase production of the:
desired product
and reduce:
unwanted side products
Greater selectivity can mean:
- less waste
- easier purification
- better use of raw materials
This is especially important in:
pharmaceutical and fine-chemical manufacturing
Catalysts Also Have Limitations
Catalysts are extremely useful, but their use can involve challenges.
They may:
- be expensive
- require rare materials
- become contaminated
- lose activity
- require replacement
- require recovery after use
Therefore, evaluating a catalyst involves more than simply asking:
"Does it make the reaction faster?"
Catalytic Converters
Vehicle engines produce gases that can include harmful pollutants.
A catalytic converter contains catalysts that speed up reactions converting some harmful exhaust gases into:
less harmful products
Catalytic converters demonstrate how catalysts can be used for:
pollution reduction
Catalysts and Fertilizer Production
Ammonia is an important starting material for many:
fertilizers
It is manufactured industrially through the Haber process:
N₂ + 3H₂ ⇌ 2NH₃
An iron-based catalyst helps the system reach equilibrium:
more quickly
Without effective catalysis, ammonia production would be:
less practical and more energy-intensive
Biological Catalysts
Living organisms also depend on catalysts.
These biological catalysts are:
enzymes
Enzymes allow reactions to occur rapidly at the relatively mild temperatures found inside:
cells
Without enzymes, many reactions required for life would occur:
far too slowly
Energy and Respiration
Cells require energy for:
- muscle contraction
- active transport
- growth
- cell division
- protein synthesis
- nerve activity
Cells obtain usable energy through controlled biochemical pathways such as:
cellular respiration
The overall aerobic respiration reaction releases energy:
glucose + oxygen → carbon dioxide + water
But cells do not simply burn glucose in one uncontrolled step.
Instead, enzymes control:
a sequence of reactions
Why Controlled Energy Release Matters
Imagine releasing all the chemical energy in glucose:
instantly
Much of it would be released as:
heat
and could damage cells.
Biological pathways allow energy to be released through:
controlled reaction steps
This allows cells to capture useful energy for:
biological work
Everyday Example: Food Storage
Reaction rates explain why food is often stored in:
refrigerators
Lower temperatures reduce particle kinetic energy and generally slow:
- enzyme-controlled reactions
- microbial growth and metabolism
- many chemical reactions involved in food deterioration
Therefore:
lower temperature → slower deterioration → longer storage life
Refrigeration Does Not Stop Reactions
A refrigerator does not normally stop chemical and biological processes completely.
It:
slows them
This is why refrigerated food still eventually:
spoils
Freezing can slow many processes even further, although it also does not necessarily destroy all microorganisms or permanently stop all chemical change.
Everyday Example: Cooking
Cooking involves many chemical changes.
Increasing temperature increases reaction rates.
Processes include:
- protein denaturation
- browning reactions
- starch changes
- breakdown of plant tissues
- flavor-producing reactions
Higher temperatures can make these changes occur:
more quickly
But excessive temperature can cause:
- burning
- unwanted products
- nutrient loss
- undesirable textures
Again:
faster is not always better
Everyday Example: Baking
Baking depends on carefully controlled reaction rates.
For example:
baking soda or baking powder
can produce:
carbon dioxide
The gas expands and contributes to:
rising
If gas is produced too early, too late, too quickly, or too slowly, the final texture can be affected.
This demonstrates how reaction rate influences:
product quality
Everyday Example: Combustion
Combustion reactions release:
energy
Fuels include:
- natural gas
- gasoline
- diesel
- biomass
A combustion reaction needs:
- fuel
- oxygen
- sufficient activation energy
Once initiated, combustion may proceed rapidly because it releases:
heat
Why Fuels Need Ignition
Gasoline contains chemical energy.
Yet gasoline does not necessarily combust immediately simply because oxygen is present.
The reaction requires:
activation energy
A spark in an engine can provide the initial energy needed to begin:
combustion
This is another example showing the distinction between:
stored chemical energy
and:
reaction rate
Everyday Example: Hand Warmers
Some hand warmers use chemical reactions that:
release energy
A common type uses oxidation of iron.
The reaction is designed to occur:
slowly enough
to provide useful warmth over an extended period.
If the same energy were released almost instantly, the product would be:
less useful and potentially unsafe
This demonstrates:
usefulness depends on both energy released and rate of release
Energy Release: Slow vs Fast
Imagine two reactions release the same total amount of energy.
Reaction A
Releases the energy in:
5 seconds
Reaction B
Releases the energy in:
5 hours
The total energy change may be similar.
But their:
power and practical effects
are very different.
Reaction A releases energy:
much more rapidly
This could make it:
hotter or more difficult to control
Everyday Example: Instant Cold Packs
Some cold packs use processes that absorb:
thermal energy
from the surroundings.
The pack becomes:
cooler
This is an example of an:
endothermic process
The usefulness of the pack depends on both:
- the amount of energy absorbed
- how quickly that energy is absorbed
Everyday Example: Rusting
Rusting is an oxidation process involving:
iron
It is energetically possible under ordinary conditions, but usually occurs:
relatively slowly
Factors such as:
- water
- oxygen
- salts
- temperature
can affect the rate.
Rusting again shows that:
a reaction can be favorable but slow
Why Salt Can Accelerate Corrosion
Dissolved salts can increase the conductivity of water and help electrochemical corrosion processes proceed.
This is why corrosion can be particularly important in:
marine environments
or where roads are treated with:
de-icing salts
Understanding reaction rates helps engineers develop strategies to:
slow corrosion
Slowing Reactions Can Be Useful
We often discuss ways to:
increase reaction rate
But sometimes we want the opposite.
Examples include:
- slowing food spoilage
- reducing corrosion
- preventing unwanted oxidation
- slowing decomposition of medicines
- preventing fires
- extending product shelf life
Reaction-rate science is therefore about:
controlling rate
not simply increasing it.
Everyday Example: Medicines
Medicines can undergo chemical changes during:
storage
Temperature, light, moisture, and oxygen can affect these reactions.
This is why medicines may have instructions such as:
store below a specified temperature
or:
protect from light
The goal is often to:
slow unwanted chemical reactions
and preserve product quality.
Shelf Life
The shelf life of a product is the period during which it remains suitable for its intended use under specified storage conditions.
Reaction rates influence the shelf life of:
- foods
- medicines
- cosmetics
- batteries
- chemicals
Slowing unwanted reactions can:
extend shelf life
Everyday Example: Glow Sticks
Glow sticks produce light through:
chemiluminescence
A chemical reaction releases energy that ultimately appears partly as:
visible light
Temperature affects the reaction rate.
A warmer glow stick generally reacts:
faster and more brightly for a shorter time
A colder glow stick generally reacts:
more slowly and less brightly for longer
This is an excellent example of:
temperature → reaction rate → product performance
Batteries
Batteries rely on:
redox reactions
to convert chemical energy into electrical energy.
Reaction rates affect:
- current delivery
- performance
- charging and discharging
- temperature behavior
At low temperatures, some battery reactions proceed:
more slowly
This can reduce battery performance.
Reaction Rate and Safety
Some reactions can become dangerous if their rate increases too much.
For an exothermic reaction:
reaction releases heat
If heat cannot escape quickly enough:
temperature rises
Higher temperature can:
increase reaction rate
which releases:
even more heat
This positive feedback can lead to:
thermal runaway
Thermal Runaway
The basic pattern is:
reaction produces heat
↓
temperature increases
↓
reaction becomes faster
↓
more heat is produced
↓
temperature rises further
This is why temperature control is extremely important in:
industrial chemistry and battery systems
Surface Area and Energy Release
Surface area can dramatically affect reaction rate.
Consider:
a block of wood
and:
fine wood shavings
The shavings have a much greater:
surface-area-to-volume ratio
More material is exposed to oxygen.
Therefore, combustion can occur:
more rapidly
The chemical energy stored in the material may be similar per unit mass, but the:
rate of energy release
can be very different.
Concentration and Everyday Reactions
Increasing reactant concentration often increases:
collision frequency
This can increase reaction rate.
This principle is important in:
- cleaning products
- industrial manufacturing
- laboratory reactions
- combustion
- biological processes
However, higher concentration can also increase:
- cost
- hazards
- corrosiveness
- environmental impact
Therefore, concentration must be:
optimized
Pressure and Gas Reactions
For gaseous reactants, increasing pressure places particles:
closer together
This generally increases:
collision frequency
and can increase reaction rate.
Industrial gas reactions may therefore use:
elevated pressures
But higher pressure requires:
- stronger equipment
- more energy
- greater safety controls
Again, the practical question is:
What conditions provide the best balance?
Applying Ideas to an Unfamiliar Situation
Suppose you are told:
A company produces a chemical using a reaction that is too slow at room temperature. Heating greatly increases the rate, but temperatures above 150°C produce unwanted products.
How could the process be improved?
A strong response might consider:
Use a catalyst
because it may increase the rate without requiring such a high temperature.
Also consider:
- moderate heating
- optimized concentration
- improved mixing
- increased surface area if solids are involved
The goal is not simply:
maximum rate
but:
useful rate + desired product + acceptable cost and safety
Another Unfamiliar Situation
Suppose a food manufacturer discovers that a product spoils rapidly at:
25°C
but much more slowly at:
5°C
Explain why.
At lower temperature:
- particles have less kinetic energy
- collisions occur less energetically
- fewer collisions overcome activation energy
- enzyme and microbial processes generally slow
Therefore:
spoilage reactions occur more slowly
This applies reaction-rate theory to:
food preservation
Another Unfamiliar Situation
A manufacturer wants a hand warmer to remain warm for:
eight hours
rather than becoming extremely hot for:
ten minutes
What should engineers consider?
They need to control:
reaction rate
Possible approaches might involve controlling:
- oxygen availability
- reactant concentration
- surface area
- catalyst conditions
- heat transfer
The desired product requires:
controlled energy release
rather than simply a large energy change.
Another Unfamiliar Situation
A biological reaction works well at 35°C but becomes extremely slow at 5°C and almost stops at 70°C.
What might explain this?
At 5°C:
low kinetic energy reduces enzyme-substrate collision frequency
At 70°C:
the enzyme may be denatured
Therefore, the reaction has an:
optimum temperature range
This applies ideas about:
energy + collisions + enzyme structure
Evaluating a Catalyst
When evaluating whether a catalyst should be used, consider:
Benefits
- faster reaction
- lower activation energy
- potentially lower operating temperature
- lower energy use
- possible improved selectivity
- increased production rate
Limitations
- catalyst cost
- availability of catalyst materials
- possible toxicity
- catalyst deactivation
- recovery and recycling requirements
A strong evaluation considers:
both benefits and limitations
Evaluating Reaction Conditions
Suppose increasing temperature doubles production rate.
Is that automatically worthwhile?
Not necessarily.
We must also ask:
- How much additional energy is required?
- Does product yield change?
- Are unwanted reactions increased?
- Does the catalyst degrade?
- Does equipment need upgrading?
- Are safety risks increased?
- What is the environmental cost?
Real chemistry involves:
trade-offs
Rate, Energy, and Sustainability
Modern chemical processes increasingly aim to reduce:
- energy consumption
- waste
- hazardous materials
- greenhouse gas emissions
- unnecessary resource use
Catalysts can help because they may allow reactions to proceed efficiently at:
lower temperatures or pressures
Enzymes can sometimes allow industrial reactions to occur under:
mild conditions
These ideas contribute to:
green chemistry
Green Chemistry
Green chemistry focuses on designing chemical products and processes that reduce environmental harm.
Reaction-rate control can contribute through:
- efficient catalysts
- lower-energy pathways
- reduced waste
- greater selectivity
- renewable raw materials
- safer reaction conditions
The goal is not merely to make chemistry:
faster
but to make it:
more efficient, safer, and more sustainable
The Central Connection
We can now connect the major ideas of chemical energetics and reaction rates.
Reactants
↓
must overcome
activation energy
↓
through
successful collisions
↓
to form
products
The frequency of successful collisions determines:
reaction rate
The energy difference between reactants and products determines:
overall energy change
A catalyst:
lowers the activation-energy barrier
without changing the overall energy difference.
Energy Profile Summary
For an exothermic reaction:
reactants → activation-energy barrier → lower-energy products
For an endothermic reaction:
reactants → activation-energy barrier → higher-energy products
With a catalyst:
activation-energy barrier becomes lower
but:
reactant and product energy levels remain unchanged
This distinction is one of the most important concepts in:
chemical energetics
A Powerful Problem-Solving Framework
When given an unfamiliar reaction-rate situation, ask:
1. What reaction is occurring?
Identify:
reactants and products
2. What energy changes are involved?
Is the process:
exothermic or endothermic?
3. What limits the rate?
Consider:
- temperature
- concentration
- pressure
- surface area
- activation energy
- catalysts
4. What could change the rate?
Apply:
collision theory
5. What are the consequences?
Consider:
- cost
- safety
- yield
- quality
- energy use
- environmental effects
This approach allows you to apply familiar chemistry to:
new situations
Common Misconception: Exothermic Means Fast
False.
Exothermic describes:
energy change
It does not describe:
reaction rate
Rusting is an example of an oxidation process that can occur:
slowly
while combustion can occur:
rapidly
Both can release energy.
Common Misconception: Endothermic Means Slow
Also false.
Endothermic describes:
energy transfer
not:
speed
An endothermic process can occur rapidly if conditions allow a sufficiently high:
reaction rate
Common Misconception: Catalysts Add Energy
Catalysts do not supply the reaction with extra energy.
They provide:
an alternative pathway with lower activation energy
This allows more collisions to be:
successful
Common Misconception: Catalysts Change the Energy Released
A catalyst does not change the overall energy difference between:
reactants and products
Therefore, it does not change the overall reaction energy change.
It changes:
the pathway and rate
Common Misconception: Higher Temperature Is Always Better
Higher temperature often increases reaction rate.
But it may also:
- increase energy costs
- cause unwanted reactions
- damage products
- denature enzymes
- increase safety risks
The best temperature depends on:
the purpose of the process
Common Misconception: We Always Want Faster Reactions
Sometimes we deliberately want reactions to be:
slower
Examples include:
- food spoilage
- corrosion
- medicine degradation
- oxidation
- battery self-discharge
Reaction-rate science is fundamentally about:
control
Check Your Understanding
1. Explain the difference between chemical energetics and chemical kinetics.
2. Why can an exothermic reaction still occur slowly?
3. Define activation energy.
4. Explain why increasing temperature usually increases reaction rate.
5. How does a catalyst affect activation energy?
6. Does a catalyst change the overall energy change of a reaction? Explain.
7. Explain why refrigeration slows food spoilage.
8. Why does a glow stick usually glow more brightly but for less time when warmed?
9. Explain why wood shavings may burn faster than a large block of wood.
10. Explain why catalysts can reduce industrial energy requirements.
11. Why might an industrial chemist deliberately avoid the highest possible reaction rate?
12. Explain how enzymes allow living organisms to carry out chemical reactions efficiently.
13. A reaction releases a large amount of energy but occurs very slowly at room temperature. Suggest two ways its rate might be increased.
14. Explain why slowing reaction rates can sometimes be useful.
15. Evaluate why catalysts are important for modern society.
Key Terms
- Energetics: Study of energy changes associated with chemical reactions.
- Kinetics: Study of reaction rates and the factors affecting them.
- Exothermic reaction: Reaction that transfers energy to the surroundings.
- Endothermic reaction: Reaction that absorbs energy from the surroundings.
- Activation energy: Minimum energy required for a successful reaction.
- Reaction rate: Measure of how quickly reactants are consumed or products are formed.
- Collision theory: Model explaining reactions through collisions between particles.
- Successful collision: Collision with sufficient energy and suitable orientation to produce a reaction.
- Catalyst: Substance that increases reaction rate by providing an alternative pathway with lower activation energy.
- Enzyme: Biological catalyst.
- Energy profile diagram: Diagram showing energy changes during the progress of a reaction.
- Reaction pathway: Sequence of steps through which reactants become products.
- Combustion: Rapid reaction with oxygen that releases energy.
- Corrosion: Chemical deterioration of a material through reactions with its environment.
- Shelf life: Period during which a product remains suitable for its intended use under specified storage conditions.
- Thermal runaway: Self-accelerating process in which heat increases reaction rate, causing still more heat production.
- Selectivity: Tendency of a reaction or catalyst to produce a particular desired product.
- Green chemistry: Design of chemical products and processes to reduce hazardous substances, waste, and environmental impact.
- Optimization: Selection of conditions that provide the best practical balance among competing factors.
Key Takeaways
- Chemical energetics describes energy changes, while kinetics describes reaction rates.
- Energy change and reaction rate are related but are not the same thing.
- Exothermic does not mean fast.
- Endothermic does not mean slow.
- Most reactions require particles to overcome an activation-energy barrier.
- Reaction rate depends on the frequency of successful collisions.
- Increasing temperature generally increases particle kinetic energy and the proportion of collisions that overcome activation energy.
- Catalysts provide alternative reaction pathways with lower activation energies.
- Catalysts increase reaction rate without changing the overall energy difference between reactants and products.
- Catalysts are essential in many industrial, environmental, and biological processes.
- Catalysts can reduce energy requirements by allowing useful rates under less extreme conditions.
- Enzymes are biological catalysts that allow life-sustaining reactions to proceed rapidly under mild conditions.
- Combustion demonstrates how activation energy and energy release interact.
- Refrigeration preserves food by slowing chemical and biological processes.
- Cooking uses increased temperature to accelerate chemical changes.
- Hand warmers depend on controlled rates of energy-releasing reactions.
- Cold packs depend on processes that absorb thermal energy.
- Glow sticks demonstrate how temperature can change reaction rate and product performance.
- Corrosion demonstrates why slowing reactions can be useful.
- Surface area, concentration, temperature, pressure, and catalysts can all influence reaction rates.
- The amount of energy released and the rate at which it is released are different quantities.
- Very rapid exothermic reactions can create safety risks, including thermal runaway.
- Industry must balance reaction rate with yield, cost, energy use, safety, quality, and environmental impact.
- Green chemistry uses catalysts and optimized reaction conditions to improve efficiency and sustainability.
- When faced with an unfamiliar situation, consider reaction → energy → rate factor → collision theory → practical consequences.
- The central relationship is activation energy → successful collisions → reaction rate, while the difference between reactant and product energy determines the overall energy change.