12.12Chapter 12: Chemical Reactions 3 and Rates

Surface area and reaction rate — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 13 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision

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Lesson summary

12.12 Surface area and reaction rate — Powder vs Lumps — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe and explain how surface area affects reaction rate and plan a fair test.Solids — in a reaction between a solid and a liquid or gas, the reaction can on…

Breaking up a solid
More exposed particles
Collisions
Total product
Hypothesis
Prediction

Part 1 of 6Surface area and exposed particles

Surface area and exposed particles

Solids

Solids in a reaction between a solid and a liquid or gas, the reaction can only happen where they meet,
- so only the surface particles of a solid can react.

Surface area

Surface area the total area of the outside of a solid.

Breaking up a solid

Breaking up a solid breaking a lump into smaller pieces exposes more particles, so the surface area increases greatly.
A 2 cm cube has a surface area of 24 cm², and cutting it into eight 1 cm cubes gives 48 cm², double the area for the same mass.

Ratio

Ratio a 1 cm cube has surface area to volume 6:1, while a 2 cm cube has 3:1.
Smaller pieces always have a bigger surface area compared with their volume.

Powder

Powder ten grams of powder has a greater surface area than one 10 g lump of the same substance.

Part 2 of 6Collision theory explanation

Collision theory explanation

More exposed particles

More exposed particles a larger surface area exposes more particles of the solid to the other reactant.

Collisions

Collisions the frequency of collisions increases, so there are more effective collisions per second and the rate rises.

Unchanged

Unchanged the particles do not move faster and the energy of each collision is unchanged.

Total product

Total product powdered calcium carbonate reacts faster with acid than marble chips of the same mass,
- but the same total amount of gas is released in the end.

Part 3 of 6Saffron's investigation

Saffron's investigation

Question

Question do powders react quicker than big lumps of solid?

Hypothesis

Hypothesis reactions happen when particles collide, only surface particles of a solid can react,
- and a mass of powder has more surface particles than the same mass of lumps,
so a powder reacts more quickly.

Reaction

Reaction calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide.
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.

Apparatus

Apparatus a conical flask on a balance with cotton wool in the neck, so the mass decreases as carbon dioxide escapes.

Prediction

Prediction the mass will decrease most quickly with the powder.

Method

Method add a big lump to dilute hydrochloric acid and measure the time for the mass to decrease by 1.0 g,
- then repeat with small lumps and with powder.

Variables

Variables the independent variable is the size of the calcium carbonate pieces and the dependent variable is the time for the mass to fall by 1 g.
Control the mass of calcium carbonate, the volume and concentration of the acid and the temperature.

Graph

Graph a bar chart is used because the independent variable, the size of the pieces, is discrete.

Conclusion

Conclusion the prediction was correct and the evidence supports the hypothesis.

Part 4 of 6Collecting gas to compare sizes

Gas syringe method
- add the same mass of each size, such as 5 g, to 50 cm³ of the same acid at the same temperature and measure the volume of carbon dioxide each minute.

Variable	Role
Size of pieces	Independent
Volume of gas at each time	Dependent
Mass of carbonate, acid volume and concentration, temperature	Controlled

Curves
- the powder gives the steepest curve, the small lumps a less steep curve and the big lumps the shallowest.
- All three level off at the same volume, since the same mass of carbonate is used and the acid is in excess.

Part 5 of 6Dust explosions

Flour mill explosion

Flour mill explosion in 1965 in London an explosion at a flour mill killed 4 people and injured 31.

How it happens

How it happens flour grains spread out in air and are surrounded by oxygen molecules, a match lights one grain,
which lights the grains near it, and a flame flashes through the cloud.

Other powders

Other powders sugar, sawdust, coal dust, custard powder and aluminium also form explosive mixtures with air.

Prevention

Prevention factories avoid dust build-up and sparks.

Part 6 of 6Industrial and everyday links

Grinding

Grinding solid reactants are ground to powder, and catalysts are made as fine meshes or pellets with pores.

Catalytic converters

Catalytic converters they use a honeycomb structure coated with a thin layer of metal to give a very large surface area.

Everyday

Everyday crushed tablets dissolve faster and logs split into kindling burn faster.

Other factors

Other factors concentration and temperature also affect the rate of reaction.

Key terms

Term	Meaning
Surface area	the total area of the outside of an object.
Collision frequency	how often particles collide.
Powder	very small pieces of solid with a large surface area.
Gas syringe	a syringe used to collect and measure a gas.
Catalytic converter	a device with a large surface area coated in metal catalysts.
Discrete variable	a variable with values that have no in-between.