chemistry - unit 6 - the rate and extent of chemical change

What is the rate of a chemical reaction in terms of reactants?

How quickly the reactants in a reaction are used up

What is the rate of a chemical reaction in terms of products?

How quickly the products in a reaction are formed

What is the formula for a mean rate of reaction in terms of reactants?

quantity of reactant used/time taken

What is the formula for a mean rate of reaction in terms of products?

quantity of reactant product formed/time taken

How can you measure the quantity of a reactant or product?

In grams or in cm³

What are the two possible units for rate of reaction?

g/s or cm³/s (where s is seconds)

How could you measure the rate of a reaction from a graph?

Draw a tangent to the curve and calculate the gradient.

What is "collision theory"?

The theory that chemical reactions only occur when particles collide with sufficient energy

What factors can affect the rate of a reaction?

Temperature, surface area of a solid, concentration or reactants in solution, pressure of gases

State the effect of increasing the surface area on the rate of a reaction

Increases the rate

Explain why increasing the surface area increases the rate of a reaction

More particles are available to collide, there are therefore more frequent collisions between reactants.

State the effect of increasing the concentration on the rate of reaction

Increases

Explain why increasing the concentration increases the rate of reaction

More concentrated means more particles in solution, therefore more frequent collisions between reactants.

State the effect on increasing the pressure of a gas on the rate of reaction

Increases

Explain why increasing the pressure of a gas increases the rate of a reaction

Less space for the particles to move around in, therefore more frequent collisions

State the effect of increasing the temperature on the rate of reaction

Increases

What is the activation energy?

The amount of energy a particle needs before it will be able to react when it collides with another particle

Explain why increasing the temperature increases the rate of reaction

Increases the speed at which particles move therefore more frequent collisions. Increases the number of particles which have the activation energy therefore more collisions result in a reaction.

What is a catalyst?

Something which changes the rate of a reaction but is not used up in that reaction

How do catalysts speed up reactions?

They provide another route for the reaction to take place which has a lower activation energy.

reaction rate

How quickly a reaction takes place. Measured by the change in amount of reactant or product divided by the time taken.

excess

Any reagent that has had enough added so that there is still some left after the reaction has ended. This ensures that all o the other reagent will be used up.

limiting reagent

A reagent not in excess that determines the final amount of product that will be produced.

temperature

A measure of the average kinetic energy of the particles in a substance. Increasing this will increase the rate of reaction.

concentration

A measure of the amount of solute dissolved in a solvent. Increasing this will increase the rate of reaction.

pressure

A measure of the force exerted by gas particles colliding with the walls of a container. Increasing this will increase the rate of reaction.

surface area

A measure of the total amount of area available on the surface of a solid mass. Increasing this will increase the rate of reaction.

catalyst

A substance that increases the rate of a reaction by providing an *alternative pathway* with a lower activation energy.

collision theory

A scientific theory that suggests that reactions occur because reactant particles collide with the correct orientation and enough energy to react.

collision frequency

A measure of the number of collisions that occur in a given time. Higher frequency means a faster rate.

activation energy

The minimum amount of energy needed for a collision to be successful and a reaction to take place.

reversible reaction

A reaction that can go forwards and backwards.

forward reaction

A reaction read from left to right in which the reactants become the products.

backward reaction

The opposite of a forward reaction. Products become the reactants again.

equilibrium

A dynamic state reached in a closed system when the rates of the forward and backward reactions are equal.

Le Chatelier's principle

Can be used to predict the effect of changing conditions on the position of equilibrium: a system will respond to counteract any change in conditions.