Chemistry Chapter 17

How can we measure the rate of reaction in a given amount of time?

Observing how much product appears

Observing how much reactant disapears

How can we physically measure the appearance and disappearance of substances?

Measuring volume of gas produced in the reaction

Measuring the change in mass of a reactant or product

What is the formula for rate of reaction with volume of gas produced?

Volume of gas produced/ Time Taken

What is the formula for rate of reaction with mass of reactant used up?

Mass of reactant used up/ Time taken

How to find average rate of reaction?

Total mass of gas produced/ Total mass taken

Total mass of reactant used up/ Total mass taken

What are the steps to measure volume of gas produced?

Place a 5cm clean magnesium ribbon( sandpapered to remove oxide layer) in a conical flask

Add 50cm3 of 2mol/dm3 hydrochloric acid to conical flask. Start the stopwatch immediately

Collect gas produced in a gas syringe. Record the volume of gas produced in the gas syringe every 30s

Plot a graph of volume of gas against time

Lesser steep slope =

Slower reaction

Steepest slope =

Fastest reaction

Gradient of the slope is 0, which means …

reaction stops

Rate of reaction =

Gradient of graph

Why does the reaction decrease to a stop?

The reaction slows down because the mass of reactants remaining is decreasing. When the limiting reactant is used up, the reaction ends

What are the steps to measure the mass of the reactant?

Place 5g of marble chips and hydrochloric acid in a flask. Insert a cotton wool plug to prevent loss of mass due to acid spray

Stand the flask on an electronic balance. Record the mass of flask and its contents

Release the marble chips into the acid. Start the stopwatch immediately

Record the mass of the flask and its contents at one-minute intervals

Why is there a decrease in mass?

It is due to the carbon dioxide produced escaping into the air

For a reaction to occur,

the reactant particles must collide with each other

they must collide with energy that is equal to or greater than the activation energy

When a chemical reaction occurs between particles, …

the collision is said to be an effective collision

An increase in the frequency of effective collisions …

increases the rate of reactio

What are the factors affecting the rate of reaction?

Concentration of reaction

Particle size or surface area of the reactants( for solid reactants)

Pressure( for gaseous reactants)

Temperature

Catalysts

What are the steps to measure the effect of concentration on rate of reaction by measuring the time for the reaction to be completed?

Add a 5cm3 clean magnesium ribbon(sandpapered to remove oxide layer) to excess 50cm3 of 2mol/dm3 hydrochloric acid. Record the time taken for that piece of magnesium to completely dissolve

Add a similar piece of magnesium to excess 50cm3 of 1mol/dm3 hydrochloric acid( magnesium is limiting). Record the time taken for the magnesium to completely dissolve.

The reaction that used higher concentration of acid took a shorter time for magnesium to completely dissolve in acid, so rate of reaction is faster

What are the steps to measure the effect of concentration on rate of reaction by measuring the volume of gas produced?

Add 0.1g of clean magnesium ribbon( sandpapered to remove oxide layer) to excess 50cm3 of 1mol/dm3 hydrochloric acid

Record the volume of hydrogen gas produced at regular time intervals. This is expt 1

Add 0.1g of clean magnesium to excess 50cm3 of 2mol/dm3 hydrochloric acid

Record the volume of hydrogen gas produced at regular time intervals. This is expt 2

Plot a graph of volume of hydrogen gas against time for both experiments on the same axis

Why does expt 1 have a steeper gradient than expt 2?

At higher concentration, there are more particles of the reactant per unit volume. This increases the frequency of effective collisions between different reactants and so, increases the rate of reaction

What are the steps to measure the effect of particle size on rate of reaction?

Place 5g of marble chips CaCO3(larger particle size), into a conical flask. Add excess 100cm3 of 1mol/dm3 of hydrochloric acid. Start the stopwatch immediately.

Record the volume of carbon dioxide produced every 30s using a gas syringe. This is expt 1

Repeat the experiment but with 5g of powdered marble chips(smaller particle size)

Record the volume of carbon dioxide produced every 30s. This is expt 2

Plot the graph of volume of gas collected against time

Why does expt 2 have a steeper gradient than expt 1?

When the surface area of the solid reactant is increased( particle size is decreased), the rate of reaction increases

Pressure is the only factor for …

reacting gases ( not for solids or liquids)

The higher the pressure, …

the faster the rate of reaction

Why is there a faster reaction at higher pressure?

At higher pressure, the gas particles are closer together. The collisions between the particles of gaseous reactants become more frequent. So, the frequency of effective collisions increases and rate of reaction is higher.

What are examples of the effect on pressure on the rate of reaction?

Haber Process to make ammonia - high pressure at 200atm

poly(ethene) made from ethene

Pressure cookers rate up cooking

What are the steps to measure the effect of temperature on rate of reaction?

5cm3 of 2mol/dm3 of hydrochloric acid is quickly poured into sodium thiosulfate solution and the stopwatch is started immediately.

The mixture is swirled once and the stopwatch is stopped when the cross disappears from view

Repeat the experiment 3 times using fresh solutions of the same reactants, but with sodium thiosulfate solution heated to higher temperatures

Why does the rate of reaction increase with temperature?

When heated, particles gain energy, move faster, and collide more often. There are also more particles with energies greatly than the activation energy, resulting in a higher frequency of effective collisions. So, rate of reaction is faster

Why do we plot 1/time vs temperature?

To see how rate of reaction varies with temperature more clearly

What is a catalyst?

A catalyst is a substance which increases the rate of reaction and remains chemically unchanged at the end of the reaction

What are the characteristics of catalysts?

A catalyst increases the reaction rate but not the yield of a chemical reaction. The same amount of products is formed whether a catalyst is used or not

Only a small amount of a catalyst is needed to increase the rate of reaction

A catalyst is not used up during a reaction. The amount of catalyst before and after a reaction will remain the same

A catalyst is specific in actions. Different catalysts rate up different reactions

The physical appearance of the catalyst may change but its chemical properties remain the same

Catalysts can be poisoned or inactivated by impurities

A catalyst provides an alternative pathway of lower activation energy ( For reversible reactions, the catalyst provides an alternative pathway for both the forward and backward reactions)

What are the steps to measure the effect of catalysts on rate of reaction?

Place 2 test tubes, A and B, containing 10cm3 of 1mol/dm3 of hydrogen peroxide solution

Add half a spatula of manganese(IV) oxide (acts as a catalyst) to test tube B

Place a glowing splint at the mouth of each test tube and record the observations

What will be observed in the effect of catalyst experiment? And why?

The glowing splint at test tube B rekindles but not test tube A, because manganese(IV) oxide rates up the decomposition of hydrogen peroxide

What are the characteristics of biological catalysts?

They are made up of proteins

They are specific in action

They are sensitive to temperature changes’

They are sensitive to pH changes

Why are transition metals used as industrial catalysts?

It is due to their ability to have variable oxidation states.

Where is iron used in industrial processes?

Haber process to manufacture ammonia

Where is platinum, palladium or rhodium used in industrial processes?

Catalytic converters

Where is aluminium oxide or silicon dioxide used in industrial processes?

Cracking process of hydrocarbons

Where is nickel used in industrial processes?

It is used to manufacture margarine from vegetable oils