Temperature and Catalysts
In 2021, CSIRO Food and Nutrition found that raising bread-proving temperature from 27°C to 37°C cut yeast fermentation time from 90 minutes to just 45 minutes, a 10°C rise that doubled the rate.
Printable Worksheets
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Q1 · Why do you need to refrigerate food to stop it going off, even though it isn't "reacting" with anything visible?
Q2 · If a catalyst helps a reaction go faster but isn't used up, how is that even possible, what do you think is actually happening?
● Know
- How increasing temperature affects particle movement and collision energy
- What a catalyst is and how it speeds up reactions without being used up
- That enzymes are biological catalysts found in living organisms
● Understand
- Why hotter reactions are faster in terms of collision theory
- How a catalyst provides an alternative pathway with lower energy requirements
- Why very high temperatures can damage enzymes
● Can do
- Predict how temperature changes will affect a given reaction
- Identify catalysts in everyday and industrial contexts
- Design an investigation into the effect of temperature or catalysts on reaction rate
Leave bread dough in a warm kitchen at 35°C and it doubles in size in 45 minutes; put the same dough in the fridge at 4°C and it barely rises in 8 hours, the same yeast, the same sugar, but temperature has changed the reaction rate by a factor of more than ten. Temperature is one of the most powerful factors affecting reaction rate. Raising temperature increases the kinetic energy of particles, which affects reactions in two ways:
1. More frequent collisions: Faster-moving particles collide more often.
2. More energetic collisions: A greater proportion of collisions exceed the activation energy. The distribution of molecular energies shifts toward higher values, so the fraction of molecules with E > Ea increases dramatically.
For many reactions, a 10C increase approximately doubles the rate. This rule of thumb arises because the Boltzmann distribution is exponential - small temperature changes produce large changes in the high-energy tail.
Enzymes are protein catalysts in living organisms. Each enzyme has an optimal temperature where its catalytic activity is highest. Above this temperature, the protein structure denatures (unfolds), destroying the active site. This is why enzymes in the human body work best around 37C and are irreversibly damaged above 60C.
Bread rises because yeast enzymes convert glucose to ethanol and CO2. At 25C, bread takes 2 hours to rise. At 35C, it takes 1 hour. At 45C, the dough might rise in 30 minutes but then collapse because the yeast enzymes are denaturing. At 60C, the yeast is killed and the bread does not rise at all. Bakers exploit this temperature sensitivity: they use warm water to activate yeast but avoid hot water that would kill it. This is why recipes specify lukewarm water (30-40C) for yeast.
Australian enzyme research: CSIRO scientists study enzymes from extremophile organisms found in Australian hot springs and salt lakes. These enzymes function at temperatures or pH values that would destroy ordinary proteins. Taq polymerase, used in PCR DNA amplification, was originally isolated from hot spring bacteria. Australian researchers are discovering new extremozymes with industrial applications in biofuel production, waste treatment, and food processing.
Catalysts increase the energy of reactant particles. This is false. Catalysts do not heat up the reaction or give particles more kinetic energy. They provide an alternative reaction pathway with lower activation energy. The particles still have the same energy distribution; more of them simply have enough energy to react because the required threshold is lower. A catalyst is like lowering a high-jump bar - the athletes do not jump higher, but more of them clear the bar.
A reaction takes 4 minutes at 20C. Predict how long it takes at 40C, assuming rate doubles for every 10C rise.
How close was your prediction?
Nice calibration, your intuition is good for this kind of problem.
Good, being surprised is the point. This answer is worth remembering.
Catalysts participate in reactions but emerge chemically unchanged. They work by providing an alternative reaction mechanism with lower activation energy.
Homogeneous catalysts are in the same phase as the reactants. For example, iron(III) ions catalyse the decomposition of hydrogen peroxide in solution. The catalyst forms temporary intermediate compounds with reactants, then releases products and regenerates itself.
Heterogeneous catalysts are in a different phase, typically solid catalysts for gas or liquid reactions. The reactants adsorb onto the catalyst surface, react there, and the products desorb. Most industrial catalysts are heterogeneous because they are easier to separate from products and can operate continuously.
Enzymes are the most efficient and specific catalysts known. Each enzyme has an active site shaped to fit specific substrate molecules, like a lock and key. This specificity allows living organisms to run thousands of different reactions simultaneously without interference.
Catalytic converters in cars use platinum, palladium, and rhodium coated on a ceramic honeycomb structure. Exhaust gases pass through the honeycomb, where the metals catalyse three reactions: oxidation of carbon monoxide to carbon dioxide, oxidation of unburned hydrocarbons to CO2 and water, and reduction of nitrogen oxides to nitrogen and oxygen. The precious metal catalysts are not consumed and last for years. The honeycomb structure maximises surface area. A single catalytic converter contains only a few grams of these metals, but their value makes used converters targets for theft.
Australian catalytic converter recycling: Australia recovers platinum group metals from end-of-life catalytic converters, reducing the need for imported raw materials. Companies in Melbourne and Brisbane process thousands of converters annually. The recovered metals are refined and reused in new catalytic converters or other applications. This circular economy approach conserves scarce resources and reduces environmental impact from mining.
Enzymes work on any substrate. This is false. Enzymes are highly specific due to the shape of their active site. An enzyme that catalyses starch breakdown will not work on proteins or fats. This specificity is both a strength and a limitation. In biotechnology, scientists modify enzyme structures to change their specificity, creating enzymes that can process new substrates. But naturally occurring enzymes are exquisitely selective, which is why our bodies need hundreds of different enzymes for digestion alone.
Catalysts are essential to modern civilisation. Without them, many everyday products would be impossibly expensive or unavailable.
Fertiliser production: The Haber-Bosch process produces over 150 million tonnes of ammonia annually, using iron catalysts. About half the nitrogen atoms in your body passed through a Haber process reactor. Without this catalytic process, global agriculture could not support the current population.
Petroleum refining: Catalytic cracking breaks large hydrocarbon molecules into smaller, more valuable ones using zeolite catalysts. This process converts heavy crude oil fractions into petrol, diesel, and jet fuel.
Environmental protection: Catalytic converters, selective catalytic reduction (SCR) systems for power stations, and wastewater treatment all depend on catalysts to transform pollutants into harmless substances.
Emerging technologies: Single-atom catalysts, where individual metal atoms are dispersed on supports, promise unprecedented efficiency. Nanocatalysts with precisely controlled sizes and shapes are being developed for fuel cells and solar fuel production.
Modern laundry detergents contain enzyme cocktails: proteases break down protein stains (blood, grass), lipases break down fats (oil, grease), amylases break down starch (pasta, sauce), and cellulases remove microfibrils from cotton to restore colour brightness. These enzymes work at low temperatures (30-40C), allowing energy-efficient washing. Before enzyme detergents, clothes had to be washed at 60C or higher to achieve similar cleaning. Enzyme technology has saved enormous amounts of energy and water globally.
Australian nanocatalysis: Researchers at the University of Queensland and Monash University are developing nanostructured catalysts for hydrogen production from water splitting. These catalysts use earth-abundant materials like nickel and iron instead of expensive platinum. If successful, they could make green hydrogen economically competitive with fossil fuels, supporting Australia ambition to become a major hydrogen exporter. The Australian Renewable Energy Agency (ARENA) has funded multiple projects in this area.
Catalysts make reactions go to completion that would otherwise be impossible. This is false. Catalysts affect the rate at which equilibrium is reached, not the position of equilibrium. A reaction that is thermodynamically unfavourable will not proceed even with the best catalyst. Catalysts cannot change delta G. They only provide a faster pathway to the same equilibrium state. The thermodynamic feasibility is determined by enthalpy and entropy, not by catalysis.
- Iron (Haber process)
- Vanadium(V) oxide (Contact process)
- Lipase (biological)
- Platinum (catalytic converter)
- Sulfuric acid production from sulfur dioxide
- Converts toxic exhaust gases to harmless products
- Breaks down fats in digestion and detergents
- Ammonia synthesis from nitrogen and hydrogen
Wrong: "Catalysts are used up in the reaction." No � catalysts are not consumed. They participate in the reaction mechanism but are regenerated in their original form. Industrial catalysts can be used for years before they need replacing.
Right: Catalysts are not consumed, they participate in forming an intermediate but are fully regenerated by the end of the reaction. The same catalyst can be used over and over, which is why industrial catalysts can last for years.
Wrong: "Higher temperature always makes reactions faster." Not for enzyme-catalysed reactions. While most chemical reactions speed up with heat, enzymes denature at high temperatures and stop working. Biological washing powders work at 30 degrees C but not at 90 degrees C.
Right: Higher temperature increases the rate of most chemical reactions, but enzyme-catalysed reactions are an exception. Enzymes are proteins that denature above their optimum temperature, losing their shape and ceasing to function as catalysts.
Wrong: "Catalysts change the products of a reaction." No � catalysts only change the rate. The reactants and products remain exactly the same. A catalyst cannot turn hydrogen peroxide into something other than water and oxygen.
Right: Catalysts only change the rate of a reaction, they do not alter the reactants or products. The same products are formed, just more quickly. A catalyst cannot change what a reaction produces, only how fast it gets there.
Australian Winemaking and Catalysis
Australia is one of the world's largest wine exporters, and fermentation is at the heart of winemaking. Yeast enzymes catalyse the conversion of grape sugars into alcohol. Australian winemakers carefully control temperature during fermentation: too cold and the yeast enzymes work too slowly; too warm and the enzymes denature, producing off-flavours.
In the Barossa Valley and Margaret River, winemakers use temperature-controlled stainless steel vats to keep fermentation within the optimal range of 20 to 30 degrees C. This precise control of reaction rate through temperature is what separates a great wine from a spoiled batch.
✍ Copy Into Your Books
▾Temperature
- Higher temp = faster particles
- More frequent AND more energetic collisions
- Rate approximately doubles per 10 degrees C rise
Catalysts
- Provide alternative pathway
- Lower energy needed for reaction
- Not used up, recovered unchanged
Enzymes
- Biological catalysts (proteins)
- Specific shape = specific reaction
- Denature at high temperature / extreme pH
Temperature Predictions
Catalyst Detective
At the start of this lesson, the hook told you that your body runs thousands of chemical reactions per second at just 37°C, reactions that would take hours in a lab, because of enzymes, biological catalysts that speed things up without being used up.
Now that you understand how catalysts work in both biological and industrial settings, can you explain what enzymes and industrial catalysts have in common? Think about what you understood about "speeding up" reactions before this lesson versus now, what's the key thing you've added to your thinking?
Q1. 1. Explain how increasing temperature affects both the frequency and energy of collisions between particles. Use these ideas to explain why a hot reaction is faster than a cold one. 4 MARKS
Q2. 2. Describe an experiment you could do to show that a catalyst is not used up in a reaction. Include the reactants, the catalyst and how you would prove the catalyst is unchanged. 4 MARKS
Q3. 3. Enzymes in the human body work best at about 37 degrees C. Explain why a fever of 42 degrees C is dangerous, using the terms "denature" and "active site" in your answer. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Can you now explain the difference between temperature effects on normal reactions and on enzyme-catalysed reactions?
- Can you give one example of a catalyst from industry and one from biology?
Model answers (click to reveal)
Answers
▾MCQ 1
BHigher temperature makes particles move faster, so they collide more frequently and with more energy. This increases the number of effective collisions.
MCQ 2
DCatalysts provide an alternative pathway for the reaction that requires less energy. They are not used up and do not change the products.
MCQ 3
CAt very high temperatures, enzymes denature. Their shape changes and the active site no longer fits the substrate, so they lose their catalytic activity.
MCQ 4
AYeast catalyses the decomposition of hydrogen peroxide into water and oxygen gas. The rapid bubbling is oxygen being released.
MCQ 5
BHeating increases particle speed and collision energy, while a catalyst lowers the energy needed for effective collisions. Both methods increase the rate without adding more reactants.
Short Answer 1
Model answer: Increasing temperature gives particles more kinetic energy, so they move faster. This means collisions occur more frequently. It also means each collision has more energy, so a greater proportion of collisions are effective (have enough energy and correct orientation). Because both the frequency and the proportion of effective collisions increase, the overall reaction rate increases significantly.
Short Answer 2
Model answer: Reactants: hydrogen peroxide solution. Catalyst: manganese(IV) oxide. Method: add a measured mass of manganese(IV) oxide to hydrogen peroxide and measure the volume of oxygen gas produced over time. After the reaction stops, filter the mixture to recover the solid. Dry and weigh the recovered solid, its mass should be the same as the starting mass. You could also test if the recovered solid still catalyses fresh hydrogen peroxide, which would prove it is unchanged.
Short Answer 3
Model answer: At 42 degrees C, enzymes in the body begin to denature. Denaturing means the enzyme's shape changes and its active site is destroyed or altered. The active site is the specific region where substrate molecules fit and react. If the active site no longer fits the substrate, the enzyme cannot catalyse its reaction. Since enzymes control vital processes like respiration and digestion, this can cause organ failure and is potentially life-threatening.