TEAS Ready⚗️ Science · Chemistry · Lesson 4 of 6 · 6 min
Reaction types and what changes reaction rates
🔥 Your body runs thousands of reactions a second, and enzymes make them fast enough to keep you alive. Know the five reaction patterns and the four ways to speed one up.
💡 Key idea
Name the pattern by counting what goes in and what comes out, and remember: hotter, more concentrated, more surface or catalyzed means faster.
🧠 The idea
Most reactions fit a pattern. SYNTHESIS: two or more reactants combine into one product (A + B → AB). DECOMPOSITION: one compound breaks apart (AB → A + B). Single replacement: one element swaps into a compound (A + BC → AC + B). Double replacement: two compounds trade partners (AB + CD → AD + CB). COMBUSTION: a fuel reacts with oxygen, usually making carbon dioxide and water and releasing energy. An EXOTHERMIC reaction releases heat, so the surroundings warm up; an ENDOTHERMIC reaction absorbs heat, so the surroundings feel cold. Reaction rate rises with higher temperature (particles move faster and collide harder), higher concentration (more collisions), greater surface area (powder reacts faster than a lump) and a CATALYST. A catalyst lowers the activation energy, speeds the reaction and is not used up. Enzymes are biological catalysts.
✏️ Worked example
Classify 2KClO₃ → 2KCl + 3O₂. Step 1: count reactants: one compound, KClO₃. Step 2: count products: two substances, KCl and O₂. Step 3: one thing breaking into more than one is decomposition. Now compare a sugar cube and a spoon of granulated sugar dropped in hot tea: the granules dissolve faster because their total surface area is larger.
📌 Takeaways
- Synthesis builds up, decomposition breaks down, replacements swap partners.
- Exothermic releases heat; endothermic absorbs heat.
- A catalyst speeds a reaction by lowering activation energy and is not consumed.
📖 Terms in this lesson
Catalyst: A substance that speeds up a reaction without being used up; for example, enzymes are biological catalysts.
✅ Check yourself
Question 1 of 3
Which type of reaction is shown? Zn + 2HCl → ZnCl₂ + H₂
- Double replacement
- Combustion
- Single replacement
- Synthesis
Show the answer and rationale
Answer: C
Zinc, a lone element, replaces hydrogen in HCl, freeing hydrogen gas: that is single replacement. Double replacement needs two compounds trading partners, synthesis would combine reactants into one product, and combustion requires oxygen as a reactant.
Question 2 of 3
A cold pack gets cold when squeezed because the reaction inside
- absorbs heat from the surroundings
- releases heat out into the surroundings
- lowers the concentration of water
- uses a catalyst that cools it
Show the answer and rationale
Answer: A
An endothermic reaction absorbs heat from its surroundings, so your hand feels cold. Releasing heat describes an exothermic reaction (a hot pack), and catalysts change speed, not whether heat is absorbed or released.
Question 3 of 3
Which change would slow down a reaction between a metal and an acid?
- Grinding the metal into a fine powder
- Heating the acid
- Using a more concentrated acid
- Cooling the acid in an ice bath
Show the answer and rationale
Answer: D
Lower temperature means slower particles and fewer energetic collisions, so the reaction slows. Powdering the metal, heating the acid and raising acid concentration all increase collisions and speed the reaction up.
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