A catalyst increases rate and ends chemically unchanged
Definition
Catalyst
A substance that increases the rate of a reaction by providing an alternative pathway with a lower activation energy without being used up.
- A catalyst takes part in the reaction but is regenerated, so the same mass is left at the end.
- Because it is not used up, only a small amount of catalyst is needed and it can be used again.
- A catalyst does not change the amount of product formed, only how quickly it forms.
- A catalysed reaction reaches the same final volume of gas as an uncatalysed one, but sooner.
- Catalysts are usually specific, so a catalyst that speeds up one reaction may have no effect on another.
Common Mistake
Do not write that a catalyst does not take part in the reaction, because it does take part and is then re-formed.
Identifying a catalyst and writing it over the arrow
- In an equation a catalyst is written over the arrow, not as a reactant or product.
- In 2H2O2(aq)→MnO22H2O(l)+O2(g)2\text{H}_2\text{O}_2\text{(aq)} \xrightarrow{\text{MnO}_2} 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}2H2O2(aq)MnO22H2O(l)+O2(g), manganese(IV) oxide is the catalyst.
- In an experiment, a substance is a catalyst if the rate increases and the same mass of it can be recovered afterwards.
- Filter off the solid after the reaction, then wash, dry and reweigh it.
- Transition metals and their compounds are common industrial catalysts, such as iron in making ammonia and platinum in car catalytic converters.
Practical
Testing catalysts for hydrogen peroxide
- Aim: find out which of several solids catalyse the decomposition of hydrogen peroxide.
- Apparatus: a conical flask with a bung and delivery tube, a 100 cm3100\,\text{cm}^3100cm3 gas syringe, a clamp and stand, a 25 cm325\,\text{cm}^325cm3 measuring cylinder, a digital balance, a stop clock, a filter funnel and filter paper, hydrogen peroxide solution and solids to test, such as manganese(IV) oxide, copper(II) oxide and zinc oxide.
Method
- Measure 25 cm325\,\text{cm}^325cm3 of hydrogen peroxide solution into the conical flask.
- Weigh 0.5 g0.5\,\text{g}0.5g of the first solid, such as manganese(IV) oxide.
- Add the solid to the flask, replace the bung at once and start the stop clock.
- Record the volume of oxygen in the gas syringe every 10 seconds.
- Repeat with no solid and with each other solid, keeping the volume, concentration and temperature of the hydrogen peroxide the same.
- Filter off each solid at the end, then wash, dry and reweigh it.
Accuracy and safety
- The run with no solid is a control, which shows how fast hydrogen peroxide decomposes on its own.
- The solid that gives the steepest line at the start of the graph is the most effective catalyst.
- A catalyst has the same mass at the end as at the start, because it is not used up.
- Wear eye protection, because hydrogen peroxide is an irritant.
A catalyst gives a pathway with lower activation energy
Definition
Activation energy
The minimum energy that colliding particles must have for a reaction to happen.
- On a reaction profile, the activation energy appears as the height of the peak above the energy of the reactants.
- With a catalyst, the reaction follows a different route whose peak is lower.
- At the same temperature, more of the collisions now have energy equal to or greater than the lower activation energy.
- So there are more successful collisions per second, and the rate increases.
- The catalyst does not give the particles more energy, and it does not change the overall energy change of the reaction.
Note
On a reaction profile with and without a catalyst, the reactant and product levels stay the same, and only the height of the peak between them is lower for the catalysed pathway.
Enzymes are the catalysts in living things
Definition
Enzyme
A protein that acts as a catalyst in living organisms.
- Enzymes catalyse the reactions in living cells, which would otherwise be far too slow at body temperature.
- Like other catalysts, enzymes are not used up and work by lowering the activation energy.
- Each enzyme is specific, so it catalyses only one reaction or a small group of similar reactions.
- Catalase, found in liver and potato, catalyses the breakdown of hydrogen peroxide into water and oxygen.
- Amylase in saliva catalyses the breakdown of starch into sugars.
- Enzymes in yeast catalyse fermentation, which turns sugar into ethanol and carbon dioxide.
Example
Dropping a small piece of raw liver into hydrogen peroxide solution makes it froth with oxygen almost at once, because the catalase in the liver catalyses the decomposition.
Self review
- What happens to the mass of a catalyst during a reaction?
- Where is a catalyst written in a chemical equation?
- How would you show that manganese(IV) oxide is a catalyst for the decomposition of hydrogen peroxide?
- How does a catalyst increase the rate of a reaction?
- What are enzymes?
