The Rate and Extent of Chemical Change

Name three common ways of measuring rate of reaction

- Loss in mass of reactants

- Volume of gas produced

- Time for a solution to become opaque

Describe measuring the rate by monitoring mass loss

Place the reaction flask on a balance. In these reactions (e.g. metal carbonate +

acid) a gas is given off, so record the decrease in mass in time intervals (note

hydrogen is too light). Plot a graph of mass vs time.

Describe measuring the rate by monitoring the volume of a gas

Connect a gas syringe to a reaction flask and measure the volume of a gas

formed in time intervals. Plot a graph of volume vs time.

Describe measuring the rate by monitoring the disappearance of a cross

Take a piece of paper and mark a cross (X) on it. Put the reaction flask on this

cross. Mix the reagents, and measure how long it takes for a cloudy mixture to

conceal a cross.

State five factors affecting the rate of a chemical reaction

- Concentration of reactants

- Pressure of gases (volume)

- Surface area

- Temperature

- Catalysts

What is the collision theory?

Chemical reactions can occur only when reacting particles collide with each other

with sufficient energy (more than or equal to activation energy)

Describe and explain the effect of increasing temperature on the rate of

reaction

T increases = faster reaction

As T increases, kinetic energy of particles increases, i.e. more energetic

collisions

Also, they move faster, so they collide more frequently

However, there is no straight line relationship between rate and temperature, i.e. they are not directly proportional to each other

Describe and explain the effect of increasing concentration on the rate of reaction

Conc. increases = faster reaction,

More reactants = more frequent collisions

Describe and explain the effect of increasing pressure of a gas on the rate of reaction

Increasing the pressure of reacting gases, is the same as increasing

concentration. It increases the number of gas molecules in the same volume and

so increases the frequency of collisions and therefore increases the rate of

reaction.

Note that volume and pressure are inversely proportional to each other.

Increasing the volume retards the reaction.

Describe and explain the effect of increasing surface area

If solid reactants are in smaller pieces, they have a greater surface area. Increasing the surface area of solid reactants increases the frequency of collisions and so increases the rate of reaction, e.g. block of magnesium reacts slower with acid then magnesium powder.

What is a catalyst and how does it work? How does it affect the reaction profile?

A catalyst changes the rate of reaction but is not used up. It increases rate of reaction by providing a different pathway for the reaction that has a lower activation energy. The reaction profile for a catalysed reaction will have a lower

maximum of the curve (lower activation energy).

What is an enzyme?

An enzyme is a molecule that acts as a catalyst in a biological system.

What is a reversible reaction?

A reversible reaction occurs when the products of a reaction can react backwards

to produce the original reactants

When is dynamic equilibrium reached?

In a closed system, when the forward and reverse reactions occur at the same rate

and the concentrations of reactants and products remain constan

Describe Le Chatelier’s Principle

If a system is at equilibrium and a change is made to any of the conditions, then

the system responds to counteract change and restore the equilibrium.