Rates of Reaction

Kalam Kudus Christian School International Program

IGCSE

By Markus HP, ST

Collision Theory

• The amount of time required for a chemical reaction to come to completion can vary tremendously
• When you strike a match it seems flame up instantly
• Coal is made over millions of years from very slow chemical reactions

Collision Theory

• The word rate can be used as a synonym of speed
• Rates measure the speed of any change that occurs within an interval of time
• The interval of time may range from fractions of a second to centuries
• Rates of chemical change usually are expressed as the amount of reactant forming products per unit time.

Rate of reaction graph

Amount of product formed

Time

Collision Theory

• Rates of reactions are related to the properties of atoms, ions, and molecules through a model called collision theory
• According to collision theory, atoms, ions, and molecules can react to form products when they collide
• provided that the particles have enough kinetic energy

Activation Energy

• The minimum amount of energy that the particles or reactants must have in order to react is called the activation energy.
• In a sense the activation energy is a barrier that reactants must get over to be converted to products
• The higher the barrier the larger the investment of energy in order to get the reactant to proceed

Activation energy is the minimum energy required before a reaction can occur. You can show this on an energy profile for the reaction. For a simple over-all exothermic reaction, the energy profile looks like this:

Factors affecting the rate of reaction

• There are four things that we can change to make the reaction go faster. They are
• Increase the concentration
• Increase the temperature
• Decrease the particle size (increase surface area)
• Employ a catalyst

Rates of Reaction

THE EFFECT OF CONCENTRATION ON REACTION RATES

THE EFFECT OF CONCENTRATION ON REACTION RATES

• The more reacting particles you have in a given volume, the higher the rate of reaction.
• Cramming more particles into a fixed volume increases the concentration of reactants,
• Increasing the concentration, increases the frequency of the collisions, and therefore increasing the reaction rate.

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THE EFFECT OF CONCENTRATION ON REACTION RATES

• For many reactions involving liquids or gases, increasing the concentration of the reactants increases the rate of reaction. In a few cases, increasing the concentration of one of the reactants may have little noticeable effect of the rate
• Don't assume that if you double the concentration of one of the reactants that you will double the rate of the reaction. It may happen like that, but the relationship may well be more complicated.

Example

• Zinc and hydrochloric acid

In the lab, zinc granules react fairly slowly with dilute hydrochloric acid, but much faster if the acid is concentrated.

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• The catalytic decomposition of hydrogen peroxide

Solid manganese(IV) oxide is often used as a catalyst in this reaction. Oxygen is given off much faster if the hydrogen peroxide is concentrated than if it is dilute.

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• The reaction between sodium thiosulphate solution and hydrochloric acid

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When a dilute acid is added to sodium thiosulphate solution, a pale yellow precipitate of sulphur is formed.

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As the sodium thiosulphate solution is diluted more and more, the precipitate takes longer and longer to form.

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Cases where changing the concentration affects the rate of the reaction

• This is the common case, and is easily explained.
• Collisions involving two particles
• The same argument applies whether the reaction involves collision between two different particles or two of the same particle.
• In order for any reaction to happen, those particles must first collide. This is true whether both particles are in solution, or whether one is in solution and the other a solid. If the concentration is higher, the chances of collision are greater.

Cases where changing the concentration doesn't affect the rate of the reaction

At first glance this seems very surprising!

• Where a catalyst is already working as fast as it can

Suppose you are using a small amount of a solid catalyst in a reaction, and a high enough concentration of reactant in solution so that the catalyst surface was totally cluttered up with reacting particles.

Increasing the concentration of the solution even more can't have any effect because the catalyst is already working at its maximum capacity.

Cases where changing the concentration doesn't affect the rate of the reaction

• In certain multi-step reactions
• Suppose you have a reaction which happens in a series of small steps. These steps are likely to have widely different rates - some fast, some slow.
• For example, suppose two reactants A and B react together in these two stages:

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• The overall rate of the reaction is going to be governed by how fast A splits up to make X and Y. This is described as the rate determining step of the reaction.
• If you increase the concentration of A, you will increase the chances of this step happening for reasons we've looked at above.
• If you increase the concentration of B, that will undoubtedly speed up the second step, but that makes hardly any difference to the overall rate.

Rates of Reaction

THE EFFECT OF SURFACE AREA ON REACTION RATES

THE EFFECT OF SURFACE AREA ON REACTION RATES

• The more finely divided the solid is, the faster the reaction happens. A powdered solid will normally produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump.

Examples

• Calcium carbonate and hydrochloric acid

In the lab, powdered calcium carbonate reacts much faster with dilute hydrochloric acid than if the same mass was present as lumps of marble or limestone.

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• The catalytic decomposition of hydrogen peroxide

This is another familiar lab reaction. Solid manganese(IV) oxide is often used as the catalyst. Oxygen is given off much faster if the catalyst is present as a powder than as the same mass of granules.

• Catalytic converters

Catalytic converters use metals like platinum, palladium and rhodium to convert poisonous compounds in vehicle exhausts into less harmful things. For example, a reaction which removes both carbon monoxide and an oxide of nitrogen is:

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Because the exhaust gases are only in contact with the catalyst for a very short time, the reactions have to be very fast.

Explanation

• You are only going to get a reaction if the particles in the gas or liquid collide with the particles in the solid. Increasing the surface area of the solid increases the chances of collision taking place.
• Imagine a reaction between magnesium metal and a dilute acid like hydrochloric acid. The reaction involves collision between magnesium atoms and hydrogen ions.

Increasing the number of collisions per second increases the rate of reaction.

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Rates of Reaction

THE EFFECT OF TEMPERATURE ON REACTION RATES

THE EFFECT OF TEMPERATURE ON REACTION RATES

• Increasing the temperature speeds up the rate, while lowering the temp slows down the rate
• Increasing the temp increases the frequency of the collisions
• Collisions taking place more often more likely they are to stick
• And the extra energy increases the power of the collisions

Rates of Reaction

THE EFFECT OF CATALYSTS ON REACTION RATES

What are catalysts?

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• A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning.

�Catalysts and activation energy

• To increase the rate of a reaction we need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy.

Adding a catalyst has exactly this effect on activation energy. A catalyst provides an alternative route for the reaction. That alternative route has a lower activation energy. Showing this on an energy profile: