RATES OF REACTION [dchemlc]

the speed at which the amount of reactants decreases or the amount of products increases is known as the ____ of the reaction.

Answer explanation

The rate of a reaction is the speed at which the amount of reactants decreases or the amount of products increases.

It is measured as a change in the concentration (or amount) of reactants or products per unit time (per second, per minute etc.).

substances that increase the rate of a reaction, but is chemically unchanged at the end of the reaction is known as _________

Answer explanation

Catalysts are substances that speed up chemical reactions but aren't used up in the process. Because they aren't consumed, small amounts of catalyst can be used to process lots and lots of reactant particles. Different reactions need different catalysts.

Enzymes are __________ ________

Answer explanation

Enzymes are biological catalysts.

How do catalysts work?

• They provide an alternative route with a lower activation energy

• They lower the activation energy of the reaction

• They speed up the reaction and get used up in the reaction

• They provide an alternative route with a higher activation energy

Answer explanation

Catalysts provide an alternative route with a lower activation energy. They do not 'lower the activation energy' any more than building a tunnel lowers the mountain. The original route is still there, and if particles collide with enough energy, they will still use it, just as very energetic people will still choose to climb over the top of the mountain.

Which of the following can be used to catalyze the following reaction?

hydrogen peroxide ------> water + oxygen

• manganese dioxide

• potassium permanganate

• potassium dichromate

• nitrogen dioxide

Answer explanation

Manganese(IV} oxide (MnO₂) is what is normally used in the lab to speed up the decomposition of hydrogen peroxide.

Notice that we don't write catalysts into the equation because they are chemically unchanged at the end of the reaction. If you like, you can write their name or formula over the top of the arrow.

In a reaction that involves gases as the reactants, increasing the pressure would

• increase the rate of the reaction

• decrease the rate of the reaction

• have no effect on the rate of the reaction

• stop the reaction

Answer explanation

Changing the pressure of a reaction in which the reactants are only solids or liquids makes almost no difference to the rate of reaction. But increasing the pressure of a reaction where the reactants are gases does speed up the reaction.

Increasing the temperature of a reaction would,

• increase the rate of the reaction

• decrease the rate of the reaction

• have no effect on the rate of the reaction

• stop the reaction

Answer explanation

Reactions get faster as the temperature is increased.

decreasing the concentration of the reactants would

• increase the rate of the reaction

• decrease the rate of the reaction

• have no effect on the rate of the reaction

• stop the reaction

Answer explanation

We find that reducing the concentration of the acid makes the reaction slower.

when the surface area of solid reactants is increased,

• the reaction would be faster

• the reaction would be slower

• there will be no change

• the reaction would stop

Answer explanation

The frequency of successful collisions increases as the surface area of the solid increases. This is because the surface area in contact with the gas or liquid is much greater and there are more particles of the solid exposed on the surface.

 The collisions with energy greater than or equal to the activation energy are called _________ __________

Answer explanation

Reactions can happen only when particles collide. In order for a reaction to actually happen, the particles have to collide with a minimum amount of energy, called the activation energy. The collisions with energy greater than or equal to the activation energy are usually called successful collisions.