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Maltase, amylase, protease, lipase - these are all examples of digestive enzymes. They are responsible for turning that juicy burger you ate for lunch, topped with cheese and lettuce and sandwiched between a bun, into small molecules that can be used by the body. Theoretically, we could digest our food without enzymes. However, this would take a very long time. You'd die of starvation before your body could get the nutrients it needed. Enzymes are therefore great examples of how we can increase the rate of a reaction.
Rate of reaction is a measure of how quickly either reactants are used up, or products are formed, in a chemical reaction. In other words, it is the change in concentration of reactants or products over time. Factors affecting rate of reaction are variables we can manipulate in order to speed up or slow down reactions.
As we defined at the start, rate of reaction is a measure of how quickly either reactants are used up, or products are formed, in a chemical reaction. In other words, it is a change of concentration of reactants or products compared to time. The units of rate of reaction vary, but they are usually , or .
There are a few different ways of measuring the rate of a reaction. They depend on the products and reactants involved in the reaction. For example:
For each of these methods, take measurements at regular time intervals until the reaction is complete. You can then move on to graphing the rate of your reaction.
Once you've made your measurements, you can draw a graph and use it to find out the rate of the reaction at any specific time period. The graph will typically take the form of a curve. Here's an example that measures the volume of gas given off in a reaction:
If we measure the change in mass, the graph looks slightly different. The curve starts off high and then gets lower. This is because mass is decreasing as some of the reactants turn into gaseous products and leave the system.
To measure the overall rate of reaction, you divide the change in whatever you were measuring, be it mass or volume, by the time taken for the reaction. To find the rate of reaction at a specific point, you need to draw a tangent to the curve and calculate its gradient. You can find out more about this in Chemical Kinetics.
If you've read Collision Theory, you'll know that in order to react, particles need to collide with the correct orientation and sufficient energy. This energy is known as the activation energy.
Activation energy is the minimum amount of energy needed to start a chemical reaction. It takes the symbol .
The reaction between two particles is like a three-step process. Firstly, do they collide? Secondly, are they orientated in the correct way? Thirdly, do they have enough energy? If the answer is "no" at any stage of the process, then a reaction won't happen - it is as simple as that.
So, in order to react, particles need to collide with the correct orientation and sufficient energy. Because the particles are constantly moving, we can't really control their orientation, but we can influence two other things: the rate of collisions and the energy of the particles. Any factors that affect these two variables will affect the rate of reaction. These include:
When we heat a system, we supply it with energy. This energy is transferred to the particles inside of the system. Some of it is transferred as kinetic energy. This means that the particles move faster. Because they are moving faster, they collide more frequently, and so the rate of collisions increases. This increases the rate of reaction.
However, heating a mixture has another effect that is even more significant than increasing the rate of collisions. Because the particles have more energy, on average, more of them meet or exceed the activation energy needed for a particular reaction. This means that there is an increased chance of the particles reacting when they collide - the chance of a successful collision increases.
Here's a Maxwell-Boltzmann distribution graph. It shows the energy distribution for particles at two temperatures. So, it is a useful way of showing the effect caused by heating a reaction. Here, temperature Y is higher than temperature X.
The area under the graph to the right of the activation energy line tells us the number of particles that meet or exceed the activation energy. You can clearly see that the area under Y is greater than the area under X. This means that a greater number of particles meet or exceed the activation energy, and thus there is a greater chance of a successful reaction when they collide.
In summary, increasing the temperature of a system not only increases the number of collisions per second but increases the proportion of successful collisions.
When looking at the effect of concentration on the rate of reaction, it can help to define what concentration actually is.
Concentration is the amount of a substance in a particular volume.
If we increase the concentration of a solution, we increase the number of solute particles in a particular volume. This means that there is an increased chance of collision between a solute molecule and another reactant - the frequency of collisions increases. Typically, we do this by adding more of the solute and taking away some of the solvent, keeping the overall volume the same.
Increasing the concentration of a solution also increases the rate of reaction if one of the reactants is a solid. There is still an increased chance of a solute particle colliding and reacting with the solid, as shown below:
Actually, increasing the concentration of some reactants doesn't always increase the rate of reaction. It all depends on the order of reactants for each particular species. For some species, when you double their concentration, you double the rate of reaction. For some species, doubling their concentration quadruples the rate of reaction. But for some species, doubling their concentration has no effect on the rate whatsoever. You can find out more about this in Rate Equations.
Increasing the pressure of a gas has much the same effect as increasing the concentration of a solution. In gases, pressure, volume, and number of particles are directly related. So, if you want to increase the pressure of a gas but keep its number of particles the same, you must decrease the volume. This results in a higher concentration of gaseous particles and increases the rate of reaction.
The pressure, volume, number of moles and temperature of a gas are all related by something called the gas constant, R. You can read more about it in Ideal Gas Law.
Dissolving a solid tablet in a beaker of water can take a long time. But if you crush it up into a fine powder, it dissolves much more quickly. This is because it has a larger surface area and there are more molecules exposed on its surface. Only molecules on the surface of a solid can collide and react with other particles, so increasing its surface area increases the rate of reaction.
Increasing the surface area of a solid only impacts the rate of reaction if the solid reacts with a liquid, gas, or aqueous solution.
However, this doesn't just work for reactants - increasing the surface area of a solid catalyst can increase the rate of reaction too. We'll look at catalysts next.
The final factor we'll look at today is the presence of a catalyst.
Catalysts are substances that increase the rate of a reaction without being chemically changed themselves in the process.
Catalysts don't affect the individual energies of particles themselves, nor how often they collide. Instead, they work by decreasing the activation energy requirements of a reaction. Thus, on average, more of the particles meet or exceed the activation energy, therefore there is a greater chance of a successful collision. The rate of reaction increases.
There are several theories about how catalysts work. The first looks at transition states and the second focuses on adsorption.
All reactions have a transition state. This is the point in the middle of the reaction with the highest energy level, where some of the bonds have been broken but not all of the new bonds have been formed. The transition state often contains intermediates, which are molecules that are created from the reactants that themselves react further to give the products. Intermediates only exist for a split second; this is what the activation energy is used for - to make these intermediate molecules.
The most common catalytic theory is that catalysts react with some of the reactants to form more stable intermediates than those formed in the original reaction. This requires less energy. The intermediates then react to form the products of the reaction, regenerating the catalyst in the process. This creation of more stable intermediates most often occurs when you use homogenous catalysts.
For example, reactant AB may react with catalyst X to form the intermediates AX and B. AX then reacts with reactant C to form AC and X. X cancels out on each side of the equation. Overall, you've produced AC and B and regenerated the catalyst in the process. The equations are shown below:
Another idea is that reactant particles form weak bonds with the surface of the catalyst, which hold them in place with just the right orientation. This means that there is an increased chance of particles reacting when they collide with each other. The process of binding to the catalyst is called adsorption.
Adsorption may also weaken the bonds found within the reactants, making them easier to break. The new products then detach from the catalyst, which is known as desorption. Adsorption and desorption most often occur when you use heterogenous catalysts.
Enzymes are biological catalysts. They work inside of living organisms, speeding up chemical reactions without being used up in the process. Again, they do this by lowering the activation energy of a reaction. Common examples of enzymes include:
For more on these biological catalysts, check out Enzymes.
Let's now take a look at the action of catalysts on energy profiles and Maxwell-Boltzmann distributions.
Here's an energy profile for an exothermic reaction, shown both with and without a catalyst. The overall energy change for both reactions is the same. However, the activation energy is lower for the reaction involving a catalyst:
Now let's look at a Maxwell-Boltzmann distribution for a reaction with and without a catalyst. The activation energy for the reaction without a catalyst is marked . The activation energy for the reaction with a catalyst is marked . Note that the overall energies of the particles don't change. Instead, is simply lower than and so a greater number of particles meet or exceed this energy.
Finally, let's discuss some applications of factors that increase the rate of a reaction.
Keeping food in a refrigerator helps stop it going bad. This is because the low temperature slows down the activity of microorganisms by lowering the rate of all of their reactions.
An example of using surface area to increase the rate of reaction is the Haber process, used to make ammonia. In this reaction, iron is used as a catalyst. However, the iron is powdered to increase its surface area and increase the rate of reaction.
Catalysts are frequently used by those working in industry. Because they aren't used up in the reaction, they offer a cheap way to increase the rate of reaction and hence increase the reaction's yield. Even if the catalyst is expensive to purchase, you only need to buy it once - you can then reuse it many times!
One example of using catalysts in industry is the production of margarine. To make margarine, unsaturated oils with C=C double bonds are hydrogenated, turning them into saturated molecules. This requires bubbling hydrogen gas through the oil in the presence of a nickel catalyst.
You might have heard of trans fats. These are also made in margarine production. They are produced when the oils used aren't fully hydrogenated. Instead, the high temperatures used cause some of the C=C double bonds to flip to the trans-isomeric state, forming a trans fat. Trans fats are getting an increasingly bad reputation due to their link to cardiovascular disease.
Nickel is a transition metal. In fact, many transition metals make good catalysts. This is because they easily adopt many different oxidation states. If you want to learn more about their properties, head over to Transition Metals.
You can increase the rate of a reaction in the following ways:
To control the rate of a reaction, you need to control several factors such as temperature, pressure, and concentration. You can do this, for example, by heating the reaction to a constant temperature or continuously distilling off the products of the reaction, in order to keep the concentrations of the reactants the same.
Enzymes are biological catalysts. This means that they increase the rate of reactions. They are affected by variables like temperature, pH, and the concentration of the molecules they act on.
Factors that affect reactant rate include the concentration of the reactants, the surface area of solids, the pressure of gases, temperature, and the presence of a catalyst.
Increasing the rate of reactions is useful because it can speed up chemical processes. In industry, this saves time and money. For example, many industrial reactions use catalysts to increase the rate of reaction in order to increase their yield.
What is necessary for a chemical reaction to occur?
In order for a chemical reaction to occur, the reactant molecules must collide. The more often they collide, the quicker the reaction will be. This is also known as collision theory.
What is an effective collision?
An effective collision is a collision which results in a chemical reaction. This means that not all collisions participate in the reaction. This is because a collision is only effective if the particles collide with sufficient energy.
What is the activation energy?
The activation energy is the minimum kinetic energy required for a reaction to occur. It’s also the energy required to overcome the repulsive forces caused by the outer electrons of the reactant molecules as they collide, enabling the pre-existing bonds between atoms to be broken, and new ones formed, resulting in the products of the reaction.
How can you increase the rate of a reaction ?
There are many ways of increasing the rate of a chemical reaction. First of all, lowering the activation energy means more reactants are capable of colliding effectively. Second, by altering experimental conditions, we can increase the frequency or the number of collisions, meaning that we can increase the amount of successful collisions and thus speed up the reaction. Both of these methods are achieved thanks to kinetic factors.
What is a kinetic factor?
A kinetic factor is a parameter which can impact the rate of a reaction.
Name a kinetic factor that increases the proportion or frequency of collisions. Explain how it works.
Temperature is a kinetic factor that increases the number of collisions, and thus successful collisions. This is because when the temperature of the reaction medium increases:
Concentration is a kinetic factor that increases the frequency of collisions and thus successful collisions. This is because increasing the concentration of the reactant brings about more collisions, since there are more particles in a given volume to collide with, and hence more successful collisions.
Pressure is a kinetic factor that increases the number of collisions and thus successful collisions. This is because if the pressure of gaseous reactants is increased, there are more reactant particles for a given volume. There will be more collisions and so the reaction rate is increased.
Agitation is a kinetic factor that increases the frequency of collisions and thus successful collisions. This is because agitation helps improve contact between reactant particles by mixing them together better, thus increasing the frequency of collisions.
The surface area to volume ratio is a kinetic factor that increases the frequency of collisions and thus successful collisions. This is because when a solid reagent is broken up, its contact surface with the surrounding medium is increased. Collisions are therefore more likely to occur and the duration of the reaction is reduced.
What is the kinetic factor which lowers the activation energy?
The kinetic factor which enables chemists to lower the activation energy is catalysis.
What is chemical kinetics?
Chemical kinetics is a branch of physical chemistry that is all about the rate of chemical reactions.
How does a catalyst affect the rate of reaction?
A catalyst increases reaction rates by lowering the activation energy so that a greater proportion of the particles have enough energy to react. A catalyst can lower the activation energy for a reaction by:
What are enzymes?
Enzymes are proteins that act as biological catalysts.
What is the 'lock and key' model?
The 'lock and key' model describes the specificity of enzymes, whose active site and substrate are complementary to each other in shape. Enzymes are in fact substrate-specific.
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