Reaction A:

• has the steeper initial curve
• forms product more quickly
• reaches the plateau sooner

Reaction B:

• has a shallower curve
• forms product more slowly
• takes longer to reach the same plateau

---

Same Product, Different Rate

This comparison illustrates an extremely important idea:

rate and yield are not the same thing

Both reactions eventually produce:

0.75 mol/L

But Reaction A gets there:

faster

Therefore:

Reaction A has the greater rate

but:

both have the same final product concentration

---

Faster Does Not Mean More

Students sometimes assume:

faster reaction = more product

That is not necessarily true.

Two reactions can produce exactly the same final amount of product.

One simply reaches that amount:

more quickly

So always distinguish between:

rate → how quickly

and:

final amount → how much

---

Different Plateau Heights

Now suppose:

Reaction A plateaus at 0.80 mol/L

while:

Reaction B plateaus at 0.50 mol/L

Then Reaction A has produced a:

greater final concentration of product

However, this alone does not tell us why.

Possible explanations could include differences in:

• starting quantities
• limiting reactants
• reaction conditions
• equilibrium position

The graph shows:

what happened

Additional information may be needed to explain:

why

---

Comparing Initial Rates

To compare the initial rates, examine the curves close to:

t = 0

The reaction with the:

steeper initial gradient

has the greater initial rate of product formation.

This is more useful than simply asking:

which curve is higher?

Rate depends on:

slope

not just height.

---

Effect of Temperature

Higher temperature usually increases reaction rate.

Particles have greater:

kinetic energy

This produces:

• more frequent collisions
• more energetic collisions
• a greater proportion of collisions exceeding activation energy

Therefore, a higher-temperature product graph will often:

rise more steeply

and reach its final plateau:

sooner

---

Temperature Comparison

If two reactions use identical quantities but different temperatures:

higher temperature → steeper initial slope → faster product formation

If temperature changes only the rate and not the overall reaction outcome:

both curves reach the same final plateau

---

Effect of a Catalyst

A catalyst increases reaction rate by providing an alternative pathway with:

lower activation energy

Therefore:

with catalyst → steeper curve

without catalyst → shallower curve

If both experiments begin with the same reactant quantities and proceed to the same completion:

both produce the same final amount

The catalyst changes:

how quickly product forms

not the stoichiometric amount available from the starting reactants.

---

Effect of Concentration

Increasing reactant concentration means there are:

more reactant particles per unit volume

This can produce:

more frequent successful collisions

and therefore:

faster product formation

The graph may initially rise:

more steeply

However, if the starting quantities themselves differ, the final product amounts may also differ.

Always check the experimental conditions.

---

Effect of Surface Area

When a solid reacts, increasing its surface area can increase reaction rate.

For example:

powdered solid

has more exposed surface than the same mass in:

large chunks

More exposed particles are available for collisions.

Therefore:

greater surface area → faster reaction → steeper product formation curve

---

Gas Production Graphs

Some reactions produce a:

gas

Instead of measuring concentration, scientists may measure the:

volume of gas produced

For example:

acid + carbonate → salt + water + carbon dioxide

The graph may show:

volume of CO₂ (cm³) vs time

The interpretation is almost identical to a product concentration graph.

---

Gas Volume Over Time

Suppose carbon dioxide production gives:

The graph would:

rise rapidly

then:

rise more slowly

and finally:

plateau at 58 cm³

Therefore:

final gas volume = 58 cm³

and the reaction reaches its final gas volume at approximately:

50 seconds

---

Measuring Gas Formation

Gas-producing reactions can be investigated using equipment such as a:

gas syringe

As gas forms:

gas volume increases

Measurements can be recorded at regular time intervals.

These data can then be plotted as:

gas volume vs time

This creates a product formation graph.

---

Mass-Loss Graphs

Some gas-producing reactions can also be followed by measuring:

mass

If gas escapes from an open reaction vessel:

total measured mass decreases

This produces a graph that slopes:

downward

even though a product is being formed.

Why?

Because the gaseous product:

leaves the apparatus

Therefore, always identify:

what the y-axis actually measures

before interpreting graph direction.

---

The Axis Determines the Meaning

Consider these three graphs for the same gas-producing reaction:

product concentration vs time → increases

gas volume vs time → increases

mass of reaction vessel vs time → decreases

All three can describe:

the same reaction

The direction of the graph depends on:

what is being measured

---

Reaction Progress

Product formation graphs provide a visual record of:

reaction progress

At the beginning:

very little product has formed

During the reaction:

product accumulates

Near the end:

product forms more slowly

At the final plateau:

no further net product accumulation occurs

The graph therefore summarizes the entire reaction in:

one curve

---

Percentage of Reaction Progress

If the final product amount is known, we can estimate how far the reaction has progressed.

Suppose:

final product = 80 cm³

At a certain time:

product formed = 40 cm³

Then:

40 / 80 × 100 = 50%

Approximately:

50% of the final product amount has formed

This can help compare how rapidly different reactions approach their final state.

---

Halfway Point

The time required to produce half the final amount can also be useful.

If the final product amount is:

60 cm³

half is:

30 cm³

Find 30 cm³ on the y-axis and determine the corresponding:

time

A faster reaction reaches the halfway point:

sooner

---

Reaction A vs Reaction B

Suppose both reactions produce a final gas volume of:

60 cm³

Reaction A reaches 30 cm³ in:

12 seconds

Reaction B reaches 30 cm³ in:

28 seconds

Reaction A therefore reaches half the final product amount:

more quickly

This is another way to compare reaction progress.

---

Graph Shape and Particle Behaviour

The macroscopic graph can be explained by microscopic particle behaviour.

Beginning

Many reactant particles are available.

Frequent successful collisions

↓

rapid product formation

↓

steep graph

Later

Fewer reactant particles remain.

Fewer successful collisions

↓

slower product formation

↓

shallower graph

Final stage

No further net product accumulation.

↓

horizontal graph

---

From Particles to Graphs

This connection is important:

particle behaviour → reaction rate → graph shape

A graph is not simply a mathematical picture.

Its shape represents what is happening among:

reacting particles

at the microscopic level.

---

A Useful Interpretation Method

For product formation graphs, use:

S-R-P

S — Slope

How steep is the curve?

This tells you about:

reaction rate

R — Rise

How much does the graph increase?

This tells you how much:

product has formed

P — Plateau

Where does the curve become horizontal?

This tells you:

the final product amount and when net product formation stops

---

Describing a Product Graph

Avoid:

"The graph goes up and then becomes flat."

Instead write:

"The amount of product increases rapidly at first, then increases more slowly before reaching a constant final value."

An even stronger answer:

"The steep initial gradient shows rapid product formation. As reactants are consumed, the reaction rate decreases, so the gradient becomes smaller. Eventually the graph reaches a plateau, indicating no further net product formation."

---

Using Numerical Evidence

Strong graph descriptions should include numbers when available.

Instead of:

"Product increases quickly."

write:

"Product concentration increases from 0.00 mol/L to 0.52 mol/L during the first 20 seconds."

Instead of:

"The reaction eventually finishes."

write:

"The graph becomes horizontal at approximately 50 s, with a final product concentration of about 0.72 mol/L."

Numerical evidence makes scientific explanations:

more precise

---

Worked Example 1

A reaction produces the following gas volumes:

Describe the graph.

The gas volume increases rapidly at first, then increases more slowly. It reaches a constant volume of:

52 cm³

at approximately:

50 seconds

---

Worked Example 2

Using the previous data, calculate the average rate of gas formation during the first 20 seconds.

Change in gas volume:

39 − 0 = 39 cm³

Change in time:

20 − 0 = 20 s

Average rate:

39 ÷ 20 = 1.95 cm³/s

Therefore:

average rate = 1.95 cm³/s

---

Worked Example 3

During the interval from 30 s to 50 s:

Gas volume increases from:

47 cm³ to 52 cm³

Change:

52 − 47 = 5 cm³

Time interval:

50 − 30 = 20 s

Average rate:

5 ÷ 20 = 0.25 cm³/s

Compare this with the first 20 seconds:

first 20 s = 1.95 cm³/s

30–50 s = 0.25 cm³/s

Therefore, the reaction has:

slowed considerably

---

Worked Example 4

Reaction A and Reaction B both eventually produce:

80 cm³ of gas

Reaction A reaches 80 cm³ in:

40 seconds

Reaction B reaches 80 cm³ in:

90 seconds

What can we conclude?

Both produce the:

same final amount of gas

but Reaction A produces it:

more rapidly

Therefore:

Reaction A has a greater overall rate of product formation.

---

Worked Example 5

Reaction X reaches a plateau at:

70 cm³

Reaction Y reaches a plateau at:

45 cm³

What can we conclude?

Reaction X produces:

25 cm³ more final gas

because:

70 − 45 = 25 cm³

However, we cannot determine why from the graph alone.

We would need information about:

• reactant quantities
• concentrations
• limiting reactants
• reaction conditions

---

Completion vs Equilibrium

A horizontal product graph does not always mean the chemical reaction has completely stopped.

For a simple irreversible reaction, a plateau often indicates:

effective completion

For a reversible reaction at equilibrium:

forward and reverse reactions continue

but:

product concentration remains constant

because the forward and reverse rates are equal.

Therefore:

flat graph = no net concentration change

not necessarily:

no molecular reactions occurring

---

Common Misconception: The Highest Curve Is Always Fastest

A curve being higher does not automatically mean the reaction is faster.

Rate depends on:

gradient

For example, a curve could have a high product concentration but be completely horizontal.

At that moment:

rate of net product formation = zero

Always examine:

slope, not simply height

---

Common Misconception: The Plateau Is the Reaction Rate

The plateau height represents:

final product amount or concentration

It does not represent:

reaction rate

Rate is determined by:

gradient

So remember:

slope → rate

height → amount

---

Common Misconception: A Flat Graph Means No Product

A horizontal graph means:

product amount is constant

The product concentration may actually be:

very high

For example, a horizontal line at 0.80 mol/L means:

0.80 mol/L of product remains present

It simply is not increasing further.

---

Common Misconception: A Catalyst Produces More Product

For a reaction that proceeds to the same completion:

catalyst → faster reaction

but not:

more final product

The catalyzed curve reaches the same plateau:

sooner

This distinction is frequently tested.

---

Common Misconception: Reaction Rate Is Constant

A curved product formation graph has a changing:

gradient

Therefore:

reaction rate changes

If the graph becomes progressively shallower:

reaction rate is decreasing

---

Common Misconception: Product Graphs Must Always Show Concentration

Product formation can be measured using several quantities.

A graph may show:

• concentration
• mass
• volume
• moles
• pressure in some experimental systems

Always examine:

what is actually being measured

---

Check Your Understanding

1. What does an upward-sloping product formation graph indicate?

2. Why is a product formation graph often steepest near the beginning of a reaction?

3. What does the gradient of a product formation graph represent?

4. Explain why the graph usually becomes less steep as the reaction proceeds.

5. What does a plateau represent on a simple product formation graph?

6. A product concentration increases from 0.20 mol/L to 0.60 mol/L in 20 s. Calculate the average rate of product formation.

7. Two reactions reach the same plateau, but one reaches it sooner. What does this tell you?

8. Two reactions have different plateau heights. What does this tell you about their final product amounts?

9. Explain why a catalyst can change the shape of a product formation graph without changing the final plateau in a reaction that proceeds to the same completion.

10. Explain how the changing gradient of a product formation graph can be explained using collision theory.

---

Key Terms

• Product: Substance formed during a chemical reaction.
• Product formation: Production of new substances as a chemical reaction proceeds.
• Concentration: Amount of a substance present per unit volume.
• Reaction rate: Measure of how quickly reactants are consumed or products are formed.
• Gradient: Slope of a graph representing rate of change.
• Average rate: Change in measured quantity divided by the corresponding time interval.
• Instantaneous rate: Reaction rate at a particular moment.
• Initial rate: Reaction rate at the beginning of a reaction.
• Tangent: Straight line used to estimate the gradient of a curve at one point.
• Plateau: Horizontal region where the measured quantity remains approximately constant.
• Reaction progress: Extent to which reactants have been converted into products.
• Completion: Stage at which a simple irreversible reaction can no longer produce significant additional product.
• Final product amount: Quantity of product present when no further net product formation occurs.
• Collision theory: Explanation of reaction rates based on collisions between reacting particles.
• Successful collision: Collision with sufficient energy and suitable orientation to produce a reaction.
• Activation energy: Minimum energy required for a successful reaction-producing collision.
• Catalyst: Substance that increases reaction rate by providing an alternative pathway with lower activation energy.
• Dynamic equilibrium: State where forward and reverse reactions continue at equal rates, causing concentrations to remain constant.

---

Key Takeaways

• Product formation graphs show how the amount or concentration of a product changes over time.
• Product amount usually increases as a reaction proceeds.
• Product graphs often rise rapidly at first and then more slowly.
• The gradient of the graph represents the rate of product formation.
• Steeper gradient = faster product formation.
• Shallower gradient = slower product formation.
• Horizontal graph = no further net product accumulation.
• Product formation is often fastest near the beginning because reactant concentrations are highest.
• As reactants are consumed, successful collisions generally become less frequent and reaction rate decreases.
• Average rate can be calculated using change in product amount ÷ change in time.
• Tangents can be used to estimate instantaneous rates.
• The plateau height represents the final measured amount or concentration of product.
• The time at which the plateau begins can indicate when a simple irreversible reaction effectively reaches completion.
• Two reactions can produce the same final amount at different rates.
• A faster reaction does not necessarily produce more product.
• Slope tells us how fast; plateau height tells us how much.
• Temperature, concentration, catalysts, and surface area can affect reaction rate and therefore graph shape.
• A catalyst can make the curve steeper and cause the plateau to be reached sooner without changing the final amount for a reaction that proceeds to the same completion.
• Product formation may be monitored using concentration, gas volume, mass, moles, or other suitable measurements.
• Always read the axes before interpreting a reaction graph.
• Graph features can be connected to particle behaviour using collision theory.
• For product graphs, a useful interpretation strategy is Slope → Rise → Plateau.
• A useful overall relationship is reactant particles → successful collisions → product formation → changing graph gradient → final plateau.