What do catalysts change in a reaction
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NOTIFICATIONSA chemical reaction involves a chemical change, which happens when two or more particles (which can be molecules, atoms or ions) interact. For example, when iron and oxygen react, they change to a new substance, iron oxide (rust). Iron oxide has different chemical properties to iron and oxygen. This is different to a physical change. For example, water can turn to ice, but ice is still water in another physical state – ice and water have the same chemical properties. When chemicals react, particles need to collide with each with enough energy for a reaction to take place. The more often they collide, the more likely they are to react. Not all collisions result in reactions – often there is not enough energy for this to happen. Some reactions happen faster than others. The rate depends on the likelihood of collision between particles. A number of things affect the rate of a reaction.
CatalystsCatalysts play an important part in many chemical processes. They increase the rate of reaction, are not consumed by the reaction and are only needed in very small amounts. There are two main ways that catalysts work. AdsorptionParticles stick onto the surface of the catalyst (called adsorption) and then move around, so they are more likely to collide and react. A good example is the way the platinum catalyst in a car’s catalytic converter works to change toxic carbon monoxide into less-toxic carbon dioxide. Intermediate compoundsIn this process, a catalyst first combines with a chemical to make a new compound. This new compound is unstable, so it breaks down, releasing another new compound and leaving the catalyst in its original form. Many enzymes (special biological catalysts) work in this way. Many industrial chemical processes rely on such catalysts. One example of a catalyst that involves an intermediate compound can be found high in the Earth’s atmosphere. Up there, the chemical ozone (with molecules containing three oxygen atoms) helps protect the Earth from harmful UV radiation. But also up there is chlorine, which gets into the atmosphere from chemicals (chlorofluorocarbons, CFCs) used in some refrigerators, air conditioners and aerosol cans. Chlorine is a catalyst, which steals an oxygen atom from ozone (O3) leaving stable oxygen (O2). At the same time, it forms an unstable intermediate chlorine-oxygen compound, which breaks down to release its oxygen. This leaves the chlorine free to repeat the process. One chlorine atom can destroy about a million ozone molecules every second. This can have a drastic effect on the atmosphere’s ability to protect us from UV radiation. Useful linkWatch a demonstration of how platinum acts as a catalyst with an explanation of the use of platinum in catalytic converters in this video from the BBC. Would you like to take a short survey?This survey will open in a new tab and you can fill it out after your visit to the site. What is a catalyst?Let’s talk about what a catalyst is. A catalyst is a compound or element that increases the rate of a chemical reaction, e.g. the speed at which it occurs, without itself being part of the reaction. Generally speaking, a catalyst is not destroyed or changed in the reaction. A catalyst does this by lowering the activation energy, which we explain in the next section What is activation energy?In simple terms, the definition of activation energy is the energy needed to start a reaction between two or more elements or compounds. A reaction with high activation energy may proceed slowly, or not at all. To speed up or start a reaction, you can either add the necessary activation energy or use a catalyst which lowers this requirement, effectively speeding up the reaction. Activation energy units are in terms of energy units. The most common units used are KJ/mol or J/mol. How does a catalyst work?A catalyst increases the rate of reaction by decreasing the activation energy. Decreased activation energy means less energy required to start the reaction. The graph below shows the energy of a reaction both with and without a catalyst present. The x-axis is the reaction coordinate or progression of the reaction from reactant (left side) to product (right side). The y-axis is the energy. With the catalyst present, the activation energy (Ea) is smaller. Visually, the hill the reaction has to climb before going downhill to the products is smaller. Just as riding a bike over a small uphill is easier than a larger uphill, a reaction proceeds faster when the activation energy hill is smaller. A catalyst lowers the activation energy by changing the transition state of the reaction. The reaction then goes through a different pathway/mechanism than the uncatalyzed reaction. The catalyst does not change the net energy difference between reactant and product. The reaction’s net equation will be the same in a catalyzed and uncatalyzed reaction even though the transition state changes. Overall reaction: A + B + catalyst –> AB + catalyst Net Reaction: A + B –> AB Main Categories of CatalystHeterogeneous CatalystA heterogeneous catalyst is in a different phase than the reactants. Usually, that means the catalyst is in the solid phase and reactants are in the liquid or gas phase. Another name for a heterogeneous catalyst is a surface catalyst. Heterogeneous catalysts work by attaching the catalyst to a solid support structure and the reactants flow over and past the catalyst, reacting along the way. A benefit of this type of catalyst is that the catalyst is easily separated from the product when the reaction is complete. The catalyst can then easily be reused. In manufacturing, this is an important cost-cutting measure. A drawback of the heterogeneous catalyst is that the amount of interaction between reactant and catalyst can be limited by surface area and diffusion of the product away from the surface. A common heterogeneous catalyst is a catalytic converter for gasoline in cars. Another important heterogeneous catalyst is the Haber-Bosch process which forms NH3. Homogeneous CatalystIn a homogeneous catalyst, both the reactants and the catalyst are in the same phase. Normally they are both in either the liquid or gas phase. The main benefit of a homogeneous catalyst is the increased interaction between reactant and catalyst. Both can move freely and are therefore more likely to interact and lead to a reaction. Common homogeneous catalysts are transition metals and acids. One homogeneous catalyzed reaction is the conversion of oxygen to ozone in the atmosphere. Nitric oxide (NO) catalyzes the reaction. All of the participants in the reaction reside in the gas phase. Therefore we know it is a homogeneous catalytic reaction. EnzymesEnzymes are large proteins that are biological catalysts. They are powerful forces in the body. Often they catalyze only one very specific reaction (compared to inorganic catalysts that often catalyze a much more broad set of reactions). The specificity is due to the active site in the catalyst—a pocket of specific chemical composition formed by amino acids where only one very specific reactant model will fit. This is also referred to as the lock-and-key model. Enzymes play a lot of important roles in the body. They catalyze the breakdown of starch to create glucose. They also convert carbon dioxide (CO2) to other molecules the body needs such as HCO3–. Enzymes assist and sped up almost all processes in the body. Calculating Activation EnergyActivation energy calculations use the Arrhenius equation. The basics will be covered here, but for more examples and in-depth analysis be sure to check out the Arrhenius equation article. The Arrhenius equation is: Where  We can find the activation energy if we know the rate constant ( For example problems on the activation energy formula, see the Arrhenius equation page! List of common catalysts:
Catalyst & Activation Energy Vocabulary Definitions
We hope you now know what is a catalyst, how they effect a reaction, and how the activation energy comes into play. These concepts will become more important if you delve more deeply into kinetics of chemical reactions. Thanks for visiting ChemTalk! Does a catalyst change the products?Catalysts do not alter the energy change between the products and reactants. Catalysts increase the forward rate, while reducing the reverse rate. Catalysts will not alter the equilibrium of the reaction.
Does a catalyst change the energy?A catalyst lowers the activation energy by changing the transition state of the reaction. The reaction then goes through a different pathway/mechanism than the uncatalyzed reaction. The catalyst does not change the net energy difference between reactant and product.
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