Chemistry: Measuring rates of reactions

What is the method of mass loss to measure rate of reaction and when do we use it?

Change in mass/time taken

Used when gas is produced (grams per second g/s)

Measures decrease in mass due to loss of gas

Why do we use a cotton wool plug in mass loss method?

To prevent loss of liquid

What is the gas syringe system?

Gas pushing syringe

measures volume of gas released

Catches gas to measure volume

Centimeters cubes per second (cm³/s)

What is the displacement method?

Fill cylinder with water, gas pushes water

Measure volume of water displacement

Measures volume of gas released

Which method would you use for the effect of surface area of a solid on the rate of reaction?

Displacement:

• Add dilute hydrochloric acid to the conical flask

• Use a delivery tube to connect this flask to an inverted measuring cylinder upside down in a water trough

• Add calcium carbonate chips into the conical flask and close the bung

• Measure the volume of gas produced in a fixed time using the measuring cylinder

• Repeat with different sizes of calcium carbonate chips

Which method would you use for the effect of concentration of a solution on the rate of reaction?

Disappearing cross (precipitation) method:

• Measure 50 cm³ of sodium thiosulfate solution into a flask

• Measure 5 cm³ of dilute hydrochloric acid into a measuring cylinder

• Draw a cross on a piece of paper and put it underneath the flask

• Add the acid into the flask and immediately start the stopwatch

• Look down at the cross from above and stop the stopwatch when the cross can no longer be seen

• Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it)

Result:

• With an increase in the concentration of a solution, the rate of reaction will increase

• This is because there will be more reactant particles in a given volume, allowing more frequent and successful collisions, increasing the rate of reaction

Which method would you use for the temperature on the rate of reaction?

Method:

• Dilute hydrochloric acid is heated to a set temperature using a water bath

• Add the dilute hydrochloric acid into a conical flask

• Add a strip of magnesium and start the stopwatch

• Stop the time when the magnesium fully dissolves

• Repeat at different temperatures and compare results

Which method would you use for the effect of catalyst on the rate of reaction?

Method:

• Add hydrogen peroxide into a conical flask

• Use a delivery tube to connect this flask to a measuring cylinder upside down in water trough

• Add the catalyst manganese(IV) oxide into the conical flask and close the bung

• Measure the volume of gas produced in a fixed time using the measuring cylinder

• Repeat experiment without the catalyst of manganese(IV) oxide and compare results

What factors affect the rate of reactions?

• The concentration of the reactants in solution or the pressure of reacting gases

• The temperature of the reaction 

• Surface area of solid reactants

• The presence of a catalyst

What would the line on a graph look like when the rate of reaction increases in comparison?

Steeper, Becomes horizontal sooner, Forms the same amount of product

Why does increasing the concentration increase rate of reaction?

• Increasing the concentration means that there are more reactant particles in a given volume

  • This causes more collisions per second

  • Leading to more frequent and successful collisions per second

  • Therefore, the rate of reaction increases 

number of collisions is proportional to the number of particles present

Why does increasing the surface area increase rate of reaction?

Increasing the surface area means that a greater surface area of particles will be exposed to the other reactant

• This causes more collisions per second

• Leading to more frequent and successful collisions per second

• Therefore, the rate of reaction increases

Why does increasing the pressure increase rate of reaction?

Increasing the pressure means that there are the same number of reactant particles in a smaller volume

• This causes more collisions per second

• Leading to more frequent and successful collisions per second

• Therefore, the rate of reaction increases

Why does increasing the temperature increase rate of reaction?

• Increasing the temperature means that the particles have more kinetic energy

  • This causes more collisions per second

  • Leading to more frequent and successful collisions per second

  • Therefore, the rate of reaction increases

What are catalysts?

substances that speed up rate of reaction but remain unchanged by the end of the reaction.

Why do catalysts speed up rate of reaction?

They provide an alternative pathway for the reaction to occur with a lower activation energy, meaning particle collisions need less energy in order for reaction to occur.