6: Rate of Reaction

Physical changes

alter the form or appearance of a substance without changing its chemical composition. Physical changes usually can be reversed because they don't involve any change in the chemical composition of the substances involved.
Examples include:
melting ice
dissolving sugar in water and
mixing two or more substances, like sand and salt.’

Chemical changes

involve the formation of new substances with different chemical compositions. new chemical substances are produced, and they have characteristics different from the original reactants. most chemical reactions cannot be reversed to return to the original reactants because they involve the formation of new substances with different chemical compositions.

Examples include:
burning wood
rusting iron

Signs that a chemical reaction has taken place include

Formation of a solid (precipitate)

Formation of gas (bubbling or fizzing)

Formation of a new odor

Change in color

Release or absorption of heat (temperature change)

Rate of reaction

Can be determined by measuring change in mass of a reactant of a product, volume of gas produced. Number of successful collisions per unit time.

Catalyst

Increases the rate of a reaction and is unchanged at the end of a reaction’

Graph

1. highest concentration of reactants, fastest rate of reaction, steepest gradient. 2. concentration of reactants decreases, rate of reaction decreases, gradient decreases. 3. at least one reactant is used up, rate of reaction is zero, horizontal gradient’

Collision theory

frequency of collisions between particles, number of particles per unit volume, kinetic energy of particles, activation energy’

frequency of collisions between particles

more collisions = more succesful collisions = increased ROR’

no. of particles

more particles per unit volume= more collisions = more succesful collisions = increased ROR’

Kinetic energy

more kinetic energy = more collisiosn = more succesful collisions = increased ROR’

high Ea

less succesful collisions = decreases ROR’

Increasing concentration

more reactant particles in a given volume = increased frequency of collisions = increased frequency of successful collisions = increased rate of reaction. increasing concnetration of limiting reactant can also lead to more gas produced

increasing pressure of gases

same number of reactant particles in a smaller volume = increased frequency of collisions = increased frequency of successful collisions = increased rate of reaction’

changing the surface area of solids

more particles exposed to other reactant = increased frequency of collisions = increased frequency of successful collisions = increased rate of reaction’

changing temp

particles have more kinetic energy = increased frequency of collisions = more particles have energy greater than Ea = increased frequency of successful collisions = increased rate of reaction’

Catalyst

lowers the activation energy = more particles have energy greater than Ea = increased frequency of successful collisions = increased rate of reaction’

Measuring ROR with syrigne

Flask, bung, delivery tube, glass syringe, stopwatch, reaction mixture. Measure volume of gas at regular time intervals

Mesuring ROR with cotton

Balance/Scale, flask, cotton wool allows gas to escape, not liquid, stopwatch. Messure change in mass over time’

A&D of syringe

A: works for all reactions that produce a gas

easy to set up

all the gas produced from the reaction is collected

clear markings make it easy to record gas volume

precise scale allows for accurate measurements. 

D: gas may be lost while connecting the bung to the flask

seal can deteriorate over time, resulting in gas leaks

gas syringes are fragile and expensive

gas syringes can stick, affecting reliability of results

cannot collect large volumes of gas

A&D of cotton

A:quick and easy to set up uses inexpensive and common lab equipment. D: not suitable for reactions that produce gases with low relative molecular mass, Mr, because the loss in mass may be too small to accurately measure

Reversible Reaction

A reversible reaction is one where the products of the reaction can react with each other to form the original reactants.’

Closed System

Nothing can enter or leave

Equilibrium

a state where the rate of the forward reaction is equal to the rate of the reverse reaction (b) the concentrations of reactants and products are constant/no longer changing

Reversible reaction in a closed system is at equilibrium when*

the rate of the forward reaction is equal to the rate of the reverse reaction

the concentrations of reactants and products are no longer changing’

Hydrated Compounds

substance that is chemically combined with water

Anhydrous Compounds

substance containing no water

Hydrated Copper(Il) sulfate

CuSO4 . 5H20. blue crystal salt. When heat is added to it, it loses its water molecules and changes color from blue to white. When water is added to anhydrous Copper(Il) sulfate, it can revert back to its blue hydrated form. COPPER IS BLUE

Cobalt(II) chloride

CoCl2 . 6H20. is a salt that is pink in its hydrated form.

Heating it makes it lose its water molecules and changes colour from pink to blue. When water is added to anhydrous Cobalt (Il) chloride, it can revert back to its pink hydrated form.’

The equilibrium position

shows how much of each substance is present when a reaction is balanced.

These factors affect the position of equilibrium:

changing temperature

changing pressure

changing concentration’

Temperature

Raising the temperature shifts the equilibrium toward the endothermic reaction (to absorb heat), while lowering the temperature shifts it toward the exothermic reaction (to release heat).’

Pressure

This factor applies only in reactions with gases. Increasing the pressure causes the equilibrium position to shift towards the side that has fewer gas molecules.’

Concentration

When you increase the concentration of reactants in a system at equilibrium, the position of equilibrium shifts to the right (toward the products), When you decrease the concentration of reactants, the position of equilibrium shifts to the left (toward the reactant side).’

Catalyst

Using a catalyst will not affect the position of equilibrium.They only increase the rate of reaction.’

Haber Process

industrial method for making ammonia with iron catalyst. N2 + 3H2 reversible 2NH3. Hydrogen from Methane, Nitrogen from air. a temperature of 450°C, a pressure of 200 atm (20,000 kPa), and an iron catalyst.’

Contact Process

industrial method for producing sulfuric acid. 2SO2 + O2 reversible 2SO3. sulfur dioxide from burning sulfur or roasting sulfide ores. oxygen sourced from the air. Over a Vanadium(V) Oxide. a temperature of 450°C, a pressure of 2 atm (200kPa), vanadium(V) oxide catalyst
After sulfur trioxide is created, it goes through additional steps to make sulfuric acid.’

Oxidation, Reduction

oxidation as gain of oxygen, loss of electrons, increase in oxidation number. Reduction as loss of oxygen, gain of electrons, decrease in oxidation number’

Redox

reactions involving gain and loss of oxygen, electrons’

To identify oxidation/reduction

write equation as ionic equation. Cut the spectator ions. Qrite down oxidation number. Will tell you what got oxidised, reduced’

Oxidation number

the oxidation number of elements in their uncombined state is zero

the oxidation number of a monatomic ion is the same as the charge on the ion

the sum of the oxidation numbers in a compound is zero

the sum of the oxidation numbers in an ion is equal to the charge on the ion’

Oxidising, reducing agents

oxidisng agents cause oxidation and are themselves reduce. Reducing agents cause oxidation and are themselves oxidised.’

Acidified aqueous potassium manganate

solution containing unknown substance -> 

add aqueous potassium manganate(VII), KMnO ->

if solution turns colourless, it must contain a reducing agent’

Aqueous potassium iodide

 solution containing unknown substance ->

add aqueous potassium

iodide, KI -> if solution turns brown, it must contain an oxidising agent’