16P3

Fri 15 May

L16

§4.6

Week 4 · Lesson 13 of 17

Factors that affect rate of reactions — temperature and surface area

Last lesson you met collision theory and the two requirements for a successful collision (sufficient energy + correct orientation). Today we take the first two of the four §4.6 rate factors: temperature (which changes how fast and how hard particles collide) and surface area (which changes how many surface atoms are exposed to react).

Learning Intentions + Success Criteria

LITo explain how temperature and surface area change reaction rate, using collision theory.

SC: I can:

01I can describe and predict the effect of increasing temperature on rate (more frequent and more energetic collisions).
02I can describe and predict the effect of increasing surface area on rate (e.g. lump vs powder).
03I can apply both factors to everyday examples (e.g. milk in fridge vs bench; Mg ribbon vs Mg powder).

Engage

5 min

Quick recap · from last class

L15 · §4.5 Rate of reaction: collision theory — application and consolidation

Try these 2questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.

ClickView video · school login↗

What Are the Methods to Measure Rate of Reaction?

Start with this — your school's ClickView video sets up today's chemistry.

TP Engage · Predict-Observe-Discuss

The TP hook is Speeding Up Reactions: three effervescent tablets, three beakers of water, and one changed variable — temperature. Before the tablets go in, predict which beaker will react fastest: cold, room-temperature, or hot water.

Observe: compare how quickly bubbles form and when fizzing stops.
Discuss: How does temperature affect particle motion and successful collisions?
Bridge: What other factors might change reaction rate? Next we look at surface area (Mg ribbon vs Mg powder); the remaining two — concentration and catalysts — come next lesson.

Demo — Surface area (Mg ribbon vs Mg powder + HCl)

YouTube · Magnesium and Hydrochloric Acid Demo, Surface Area · open in new tab

Same metal, same acid, same temperature. The only difference is the shape of the solid: a thin ribbon vs a fine powder. The powder fizzes much faster because more Mg atoms are exposed on the surface, so more collisions per second happen between Mg and the H⁺ ions (hydrogen ions) in solution. Same products either way (MgCl₂ (magnesium chloride) + H₂ (hydrogen gas)); only the rate changes.

Predict · your turn

Write before you watch

A bottle of milk left on the kitchen bench at room temperature goes sour within a day. The same milk in the fridge lasts over a week. Using collision theory, what do you think makes the difference?

Explicit

17 min

Today's procedure — predicting effect of T or surface area on rate

START — you change ONE factor; what happens to the rate?

1. Which factor are you changing?

↓ Temperature ↑

Two effects:
• Particles move faster → more collisions/sec
• Each collision more energetic → bigger fraction clears Eₐ

→ Rate ↑

↓ Surface area ↑

Solid broken into smaller pieces.
More exposed atoms on the surface → more collisions/sec at the surface.

→ Rate ↑
e.g. Mg powder fizzes in HCl much faster than a Mg ribbon

Decrease the factor → reverse the logic (rate ↓). Concentration and catalysts come next lesson (L14, Tue 19 May).

Four factors that change reaction rate

You met all four in L11 (collision theory). Today: temperature and surface area.

Factor	Easy to control?	Everyday handle
Temperature (today)	yes	fridge vs kitchen bench
Surface area (today)	yes	whole vs crushed solid
Concentration	yes	dilute vs concentrated acid (L14)
Catalyst	needs the right substance	enzymes, catalytic converters (L14)

Raising the temperature increases the reaction rate

Heating reactants has two effects (both push the rate up):

Particles have more kinetic energy → they move faster → more collisions per second.
Each collision is more energetic → a bigger fraction clear the activation energy Eₐ → more successful collisions per second.

Everyday handles:

Boil water for tea → tea brews faster.
Refrigerate food → spoilage reactions slow down.

Figure 4.29 — Same particles, more kinetic energy. At higher temperature they move faster, collide more often, and a larger fraction of collisions clear the activation-energy barrier.

Increasing the surface area increases the reaction rate

Surface area is the exposed area of a solid substance. If you increase the surface area of a substance, there are more particles at the surface that have the opportunity to react with another substance, so the reaction rate increases.

Consider the reaction of zinc metal with an acid (Figure 4.30). If the zinc is in the form of a large cube, the total surface area of the cube is the sum of the areas of the faces of the cube. If the zinc cube is broken up into many smaller pieces, the total surface area greatly increases, and more area is exposed to react with the acid. If the zinc is crushed into a powder, its surface area increases even further. This form of zinc reacts the fastest.

You meet the same rule in the kitchen: small pieces of food cook faster than large ones because heat has more surface to attack at once.

Figure 4.30 — (a) The large cube exposes only its outer atoms to H⁺ ions (hydrogen ions): slow reaction. (b) The same mass broken into smaller pieces exposes far more surface, giving H⁺ ions many more places to collide: faster reaction.

Pause · write in your notes§2 — Surface area rule, why, and Mg worked example

Rule: crushing or grinding a solid increases its surface area, which increases the rate of reaction.
Why (collision theory): reaction happens only at the surface of a solid (inside particles can't be reached). Smaller pieces ⇒ more surface particles exposed ⇒ more collisions/sec ⇒ faster rate.
Worked example — same reaction, different shape:

Form of Mg in HCl	Surface area	Observation
Ribbon (one strip)	small	slow stream of bubbles, takes minutes
Powder	large	vigorous fizzing, dissolves in seconds

Same chemistry; same HCl (hydrochloric acid); the powder reacts faster because every grain exposes its surface to acid.

Reading rate qualitatively from a graph

When you actually measure how fast a reaction goes — e.g. by catching the hydrogen gas from Mg + HCl in a syringe and reading the volume every 10 s — you get a volume-of-gas vs time curve. You don't need to calculate anything from it. You just read the shape:

The steepest part of the curve = the fastest rate at that moment. For most reactions this is right at the start (t = 0), when reactant concentration is highest.
The curve flattens as reactants get used up: lower concentration → fewer successful collisions per second → slower rate.
A flat line (gradient = 0) means the reaction has stopped — a reactant has run out.

The figure below shows the same Mg + HCl reaction run at two temperatures. The hot run is steeper at the start and reaches its plateau sooner; the cool run is shallower and gets to the same plateau later. Both finish flat at the same final volume because the same amount of Mg was used.

Volume-of-gas vs time for the same Mg + HCl reaction at two temperatures. Steeper = faster rate. Flat = reaction stopped. Both runs reach the same final volume because the same amount of Mg was used.

Key terms

Keywords

kinetic energy: Energy of motion. Higher temperature = higher average kinetic energy.
surface area: The area of the outermost layer of a solid that is exposed to react.
activation energy: The minimum energy a collision must have for the reaction to occur.

Watch out · common traps

Trap 1

“Temperature only matters for industrial reactions.”

Wrong — temperature changes the rate of every reaction. The fridge / kitchen-bench milk pair is a textbook example: same milk, same bacteria, but the warmer one spoils much faster.

In the lab, the same rule shows up the moment you heat the beaker — fizzing speeds up, colour changes happen sooner, gas evolves faster. There is nothing special about "industrial" reactions; temperature is a universal rate factor.

Rule: temperature ↑ → kinetic energy ↑ → rate ↑ for any reaction.

Trap 2

“Crushing the zinc gives more zinc, so it reacts faster.”

Wrong — same mass means the same number of zinc atoms. Crushing doesn't add more zinc, it just exposes more of the same atoms to the acid.

Reaction happens at the surface: H⁺ ions (hydrogen ions) can only collide with zinc atoms on the outside of the lump. A cube has very few atoms on the outside; a powder has the same atoms but most are now on a surface.

Rule: same mass ≠ same surface area. Surface area is what controls the collision frequency, not the total amount.

Trap 3

“Temperature and surface area do the same thing to rate.”

Wrong — both raise the rate, but by different mechanisms:

Change	Effect
Temperature ↑	frequency ↑ AND fraction of collisions clearing Eₐ ↑ (faster + more energetic)
Surface area ↑	frequency of surface collisions ↑ (more atoms exposed)

In a "justify with collision theory" answer, examiners look for the right mechanism — don't say "more energy" for a surface-area change, and don't forget the energy/Eₐ effect when temperature changes.

Rule: state the right mechanism for each factor.

Apply

25 min

Question 1Temperature effect

Fill in the blanks to complete the textbook explanation of how temperature increases reaction rate.

Question 2Surface area effect

Question 3Everyday example — temperature

Your turnShort answer · Have a go first

Use collision theory to explain why we keep food in the refrigerator. Refer to the temperature change and how it affects collisions between particles.

Have a go first:

Question 4Match the change to the effect on rate

Catch

5 min

Reflect

10 min

Your turnReflect · One thing you learned

One thing I now understand about how temperature or surface area changes reaction rate that I didn't understand before:

I've finished this lesson

Success criteria — where are you right now?

Next class (Tue 19 May, P4): the other two §4.6 factors — concentration and catalysts.