Rate of Reaction

How is rate of reaction calculated?

Rate of reaction = amount (e.g. grams, cm3 ) of reactant used or product formed / time

or

Rate of reaction (mol/s) = Moles of reactant used or product formed / time

What are the units for rate of reaction?

g/s, cm³/s or mol/s depending on what is being measured

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 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.

How can you find rate of reaction at a specific time from a graph?

• Pick a point corresponding to the time t, and find the tangent to the curve at this point.

• The tangent is the gradient of this graph - it tells you how fast the reaction proceeds at this point.

• The steeper the tangent line, the faster the rate. Gradient of tangent can be expressed in change in y values over change in x values (change in y ÷ change in x)

State five factors affecting the rate of a chemical reaction

Concentration of reactants, pressure of gases, surface area, temperature, and 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 rate of reaction

As temperature increases the rate of reaction increases. Higher temperature increases kinetic energy, so particles move faster and collide more frequently and energetically, increasing rate. However, they are not directly proportional to each other

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

Higher concentration increases the number of reactant particles in the same volume, causing more frequent collisions and a faster rate.

Describe and explain the effect of increasing gas pressure on 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. Volume and pressure are inversely proportional to each other. Increasing the volume retards the reaction.

Describe and explain the effect of increasing surface area on rate of reaction

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?

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.

How does a catalyst affect the reaction profile?

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.