2. Reaction rate

REACTION RATE

Working with pictures

Imagine that one side of the cube is 1 cm. Calculate the available surface in all three cases. Work out a general rule as a function of the number of divisions. The reaction rate of a chemical reaction is defined in terms of the time it takes to make a reactant disappear or for a product to appear.

Sometimes, it is possible to determine the reaction rate from a change in another property where the chemical reaction occurs.

ä 2.1 Collision theory

As we have seen, chemical reactions break and form bonds as a result of collisions between elemental units of the reactants. Not all collisions cause the breaking or forming of bonds; not all collisions are effective. Only collisions with sufficient energy and whose molecules have the right orientation may break or form bonds.

ä 2.2 Factors that affect reaction rates

Many factors may change the rate of a reaction. Their importance depends on the state of matter of the reactants. They are:

Temperature In reactions involving liquid or gas, raising the temperature accelerates the velocity of the elemental units in the reactant. This causes more collisions which have sufficient energy to contribute to the reaction. Concentration of reactants

In reactions that are in aqueous solutions, increasing the concentration raises the probability of collisions, which accelerate the reaction rate. State of sub-division of reactants In cases where one of the reactants is in a solid state, the size of the reactant determines the reaction rate. You can see an example of this in the figure below.

Effect of sub-divisions

The figure explains why the reaction rate is a function of the size of the solid reactant. Note that as the number of subdivisions grows, the surface area in contact with the other reactant (which may be a gas or liquid) grows. The number of effective collisions between molecules of both reactants will rise and so will the reaction rate.

ä 2.3 Catalysts

In many cases, when modifying a reaction rate, we must change certain factors to a point that it is no longer worth it. In that case, we tend to use catalysts.

Catalysts are chemical substances that modify the reaction rate without changing the nature of the reactants or the products.

This property, of not changing the reactants or products in a reaction, is the most important since it determines whether or not a catalyst may be used. Catalysts have two other properties: • They are extremely specific. Generally, for each chemical reaction there is a specific catalyst. While many reactions might be possible with the same reactants, the presence of a catalyst initiates a specific chemical reaction. • They take part in the chemical reaction without undergoing permanent changes. Most catalysts reappear after a reaction, so they are not included in the chemical equation as reactants or products. Enzymes as catalysts Focus on English

Tend - to often behave or act in a certain way or to have certain characteristic.

E N Z Y M E S Formed by

Properties

Applications

Amino acids (combinations of H, C, O, N, S)

Which determine their

Structure and function

• Change their reaction rate • Are very specific • Act at specific pH and T

Biological

Industrial • Digestive system • Degradation of molecules • Celular respiration • Transport of energy and signals

• Pharmaceuticals • Food production • Development of biofuels Enzymes are molecules that act as catalysts inside of organisms. Their action is very specific; there is practically one enzyme for each chemical reaction that occurs inside each living thing. In the case of humans, their presence is vital since if their catalytic effect does not occur, thermoregulation, arterial pressure, and DNA formation may be harmed. The figure shows the threedimensional structure of an enzyme. Enzymes can come in many sizes and shapes.

Understand, think, search…

4 If we mix a solution of sulfuric acid with water, we must always add the acid drop by drop to the water container and not the other way round.

Given that these two substances react violently, explain why we take this precaution. With which physical factor related to reaction rates would you associate it? 5 Explain why ground meat spoils faster than a similar-sized cut of unground meat. With which physical factor would you associate this? 6 Write around. What do you think would happen if the chemical reactions in our bodies took place without catalytic enzymes? 7 Search for pictures of different enzymes in our bodies. Draw them in your notebook and explain the differences and similarities in their shapes and sizes. What do you think explains these differences and similarities? 8 Search for information about lactase and the reason why some people cannot digest milk. Do you know any other type of enzyme whose absence may also prevent digestion? How might we fix this?