Chemical reactions Flashcards
Physical and chemical changes
● Physical changes
No new substances are made, but there is a change to the appearance of a substance, made into a mixture, since you can separate mixtures – since they consist of elements/compounds that are not chemically combined
▪ E.g. fractional distillation of crude oil
● Chemical changes
o A process of re-arrangement of the atoms present in the reactants to form one or more products, which have the same total number of each type of atoms as the reactants E.g. neutralisation reactions to produce salts or rusting
effect of concentration, particle size, catalysts (including enzymes) and temperature on the rate of reactions
- When temperature increase, particles gain more energy, thus move faster, collide more frequently, thus higher rate of successful collisions and faster rate of reaction.
- Concentration: (Per unit Volume) When increase, more particles per unit of volume, collisions more frequent, increases the rate of reaction
- Surface area (ATP): MEASURE VOLUME OF GAS USING: GAS SYRINGE, INVERTED CYLINDER. Higher surface area, faster the rate of reaction.
- Catalyst (ATP): They Provide alternate pathway by decreasing activational energy of the reaction. Therfore higher proportion of succesful collisions.
Suitable method for investigating the effect of a given variable on the rate of a reaction
-Rate of reaction = amount of reactant used / Time
-Rate of reaction = amount of product formed / Time
application of the above factors to the danger of explosive combustion with fine powders
●Flour mills
-Particle size is very small
-Therefore, surface area is very large
-Could easily combust causing an explosion due to these flammable substances that have a large surface area
●Methane in Mines
-Increase in pressure
-Same as increasing the concentration of the reactants – because now the
volume has decreased, therefore there are more particles per unit
volume
-Increases chance of successful collisions
method for investigating the rate of a reaction involving gas evolution
- Measure the volume of a gas (if the gas is a product) using a gas syringe or an upside down measuring cylinder or burette
- Record the total volume of gas collected at regular intervals and plot a graph Use the rate of reaction equation above: volume of gas / time taken to find the rate at the specific time
Effects of temperature and concentration in terms of collisions between reacting particles.
- An increase in temperature causes an increase in collision rate more colliding molecules have sufficient energy (activation energy) to react, so rate of reaction increases
- Increasing concentration: causes an increase in collision rate, increasing frequency of successful collisions and so rate of reaction increases
role of light in photochemical reactions and the effect of light on the rate of these reactions.
- Photochemical reactions = reactions that are initiated by light
- The brighter the light or the greater the light intensity, the faster the rate of reaction
use of silver salts in photography as a process of reduction of silver ions to silver; and photosynthesis as the reaction between carbon dioxide and water in the presence of chlorophyll and sunlight (energy) to produce glucose and oxygen
-Use of silver salts in photography:
o Silver halide salts are used in black and white photography
o AgCl is sensitive to light & breaks down to form metallic silver Ag+ -> Ag
▪ Appears black
o Brighter the light on the film, the faster the reaction & the darker that part of the photograph appears i.e. improves efficiency & accuracy of photos
-Photosynthesis:
o Chemical change that occurs in the leaves of green plants
o Chlorophyll, a green pigment in the plants, absorbs light energy ▪ CO2 reacts with H2O to produce C6 H 12O 6 (glucose) and O2
some chemical reactions can be reversed by changing the reaction conditions
In some chemical reactions, the products of the reaction can react to produce the original reactants
o These are called reversible reactions
o The direction of the reaction can be changed by changing the conditions
Dehydration of hydrated copper(Il) sulfate
o Anhydrous copper(II) sulfate+ water hydrated copper(Il)sulfate
o White solid turns blue in presence of water
o Forward reaction add water
o Reverse reaction heat the hydrated copper(ll) sulfate (water evaporates)
effect of changing the conditions (concentration, temperature and pressure) on other reversible reactions
● The relative amounts of all the reacting substances at equilibrium depend on the conditions of the reaction.
● If a system is at equilibrium and a change is made to any of the conditions, then the system responds to counteract the change.
o Effects of changing conditions on a system at equilibrium can be predicted using Le Chatelier’s Principle.
● Effect of changing concentration:
● If the concentration of one of the reactants or products is changed, the
system is no longer at equilibrium and the concentrations of all the
substances will change until equilibrium is reached again.
● If concentration of reactants is increased: position of equilibrium shifts
towards products (right) so more product is produced until equilibrium is
reached again
● if concentration of products is increased: position of equilibrium shifts
towards reactants (left) so more reactant is produced until equilibrium is reached again
● Effect of changing pressure:
● In gaseous reactions, an increase in pressure will favour the reaction that
produces the least number of molecules as shown by the symbol equation for that reaction.
Equilibrium
When a reversible reaction occurs in a closed system, equilibrium is reached when the reactions occur at exactly the same rate in each direction.
Temperature:
- Increase: Equilibrium moves to endothermic side
- Decrease: Equilibrium shifts to Exothermic direction
Pressure:
- Increase: Equilibrium shifts to direction which produces small number of molecules
- Decrease: Equilibrium shifts to direction which produces large number of molecules.
Concentration:
- Increase: Equilibrium shift to right
- Decrease: equilibrium shifts to left
oxidation and reduction
-Oxidation is gain of oxygen
-Reduction is loss of oxygen
iron(II) is Fe2+, iron(III) is Fe3+, copper(II) is Cu2+,manganate (VII) is Mn7+
Redox
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
Identify redox reactions by changes in oxidation state and by the colour changes involved when using acidified potassium manganate(VII), and potassium iodide.
-Potassium manganate(VII):
o Deep purple and when reduced, it becomes colourless
o E.g.react with iron(II) chloride and colourless Mn2+ ionsare formed
o E.g. react with sulphur dioxide and the same thing happens
o This is because, potassium manganate(VII) is an oxidising agent and
therefore is reduced itself
-Potassium iodide:
o Colourless solution is oxidised by an oxidising agent to form brown iodine solution
o React with hydrogen peroxide (which oxidises the iodide ions to iodine, which is brown in colour)
Oxidising agent
substance which oxidises another substance during a redox reaction.
