chemical reactions Flashcards
Physical change
Physical changes (such as melting or evaporating) do not produce any new chemical substances.
These changes are often easy to reverse and mixtures produced are usually relatively easy to separate.
Chemical change
In chemical reactions, new chemical products are formed that have very different properties to the reactants.
Most chemical reactions are impossible to reverse.
Energy changes also accompany chemical changes and energy can be given out (exothermic) or taken in (endothermic).
The majority of chemical reactions are exothermic with only a small number being endothermic.
Effect of Concentration on the rate of the reaction
Explanation:
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 per second, increasing the rate of reaction
Effect of Surface Area on the rate of the reaction
Explanation:
Increase in the surface area of the solid, the rate of reaction will increase
This is because more surface area particles will be exposed to the other reactant so there will be more frequent and successful collisions per second, increasing the rate of reaction
Effect of Temperature
Explanation:
Increase in the temperature, the rate of reaction will increase
This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more frequent and successful collisions per second, increasing the rate of reaction.
Effect of Using a Catalyst
Explanation:
Catalysts reduce the activation energy as they create alternative pathways requiring lower activation energy, allowing more successful and frequent collisions.
This shows that when a catalyst is used, the rate of reaction will increase.
Explosive Combustion
Explosive combustion occurs when there are many fine particles in the air.
Many industrial processes such as metal working, coal mining or flour milling produce very fine and tiny particles.
These particles have a very large surface area and are combustible in air.
Even a small spark may cause them to ignite and since the surface area is so large, the rate of reaction can be incredibly fast, hence they are explosive.
Methane gas mixed with air in coal mines can also form an explosive mixture.
Interpret data obtained from experiments concerned with rate of reaction
Explanation:
Compared to a reaction with a reactant at a low concentration, the graph line for the same reaction but at a higher concentration has a steeper gradient at the start and becomes horizontal sooner.
This shows that with increased concentration of a solution, the rate of reaction will increase.
Interpret data obtained from experiments concerned with rate of reaction Particle Size
Explanation:
Compared to a reaction with lumps of reactant, the graph line for the same reaction but with powdered reactant has a steeper gradient at the start and becomes horizontal sooner.
This shows that with increased surface area of the solid, the rate of reaction will increase.
Interpret data obtained from experiments concerned with rate of reaction Catalyst
Explanation:
The diagram shows that when a catalyst is used, the activation energy is reduced as it creates an alternative pathway requiring lower activation energy, allowing more successful and frequent collisions.
This shows that when a catalyst is used, the rate of reaction will increase.
Temperature Interpret data obtained from experiments concerned with rate of reaction
Explanation:
Compared to a reaction at a low temperature, the graph line for the same reaction but at a higher temperature has a steeper gradient at the start and becomes horizontal sooner.
This shows that with increased temperature, the rate of reaction will increase.
Effect of Surface Area of a Solid on the Rate of Reaction
METHOD AND RESULT
Method:
Add dilute hydrochloric acid into a conical flask.
Use a capillary tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement).
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 (solid, crushed and powdered).
Result:
Smaller sizes of chips causes an increase in the surface area of the solid, so the rate of reaction will increase.
This is because more surface area of the particles will be exposed to the other reactant so there will be more frequent and successful collisions, increasing the rate of reaction.
Effect of Concentration of a Solution on the Rate of Reaction:
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.
Effect of Temperature on the Rate of Reaction:
INVESTIGATION
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.
Result:
With an increase in the temperature, the rate of reaction will increase.
This is because the particles will have more kinetic energy than the required activation energy, therefore more frequent and successful collisions will occur, increasing the rate of reaction.
Effect of a Catalyst on the Rate of Reaction:
INVESTIGATION
Method:
Add Hydrogen Peroxide into a conical flask.
Use a capillary tube to connect this flask to a measuring cylinder upside down in a bucket of water (downwards displacement).
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.
Result:
Using a catalyst will increase the rate of reaction.
The catalyst will provide an alternative pathway requiring lower activation energy so more colliding particles will have the necessary activation energy to react.
This will allow more frequent and successful collisions, increasing the rate of reaction.
Describe and explain the effects of temperature and concentration in terms of collisions between reacting particles.
Particles need to have at least a minimum amount of energy to react when they collide.
This is called the activation energy.
At low temperatures only a small number of particles will have enough activation energy so the reaction will be slow.
At higher temperatures the particles have more kinetic energy so they move faster and with more energy.
The collisions are thus more energetic and there is a greater number of particles with sufficient energy to react, so the rate of reaction increases.
Concentration
Increasing the concentration means there are more particles per cm3, so there is less space between the particles.
Since there are more particles then it follows that there are more collisions, hence the rate of reaction increases.
When answering questions on the effect of concentration on the rate of reaction, you should mention that
there are more particles per unit volume (usually cm3) and this causes an increase in the rate of collisions.
Photochemical reactions
These reactions occur only when light is present.
The greater the intensity of ultraviolet light then the greater the rate of reaction.
E.g. the substitution of hydrogen atoms in methane by chlorine:
CH4 + Cl2 → CH3Cl + HCl
Silver salts in photography
Black and white photography film surfaces contain crystals of silver bromide.
When exposed to light they decompose to silver:
2AgBr → 2Ag + Br2
AgBr is colourless at low concentrations but the Ag appears grey-black.
Parts of the film appear black, grey or white depending on the exposure:
Stronger light = black or dark grey
Weaker light = light grey
Not exposed = white
Photosynthesis
This is the process in which plants produce food for reproduction and growth.
The equation is:
6CO2 + 6H2O → C6H12O6 + 6O2
The process requires sunlight and chlorophyll.
Chlorophyll is the green pigment in plants which absorbs sunlight and acts as the catalyst for photosynthesis.
Example Chemical equations for reversible reactions
The reaction for the Haber Process which is the production of ammonia from hydrogen and nitrogen:
Dehydration of Hydrated Copper (II) Sulfate:
Hydrated Copper (II) Sulfate ⇌ Anhydrous Copper (II) Sulfate + Water
Explanation:
When anhydrous copper (II) sulfate crystals are added to water they turn blue and heat is given off, this reaction is reversible.
When Copper (II) Sulfate crystals are heated in a test tube, the blue crystals turn into a white powder and a clear, colourless liquid (water) collects at the top of the test tube.
The form of Copper (II) Sulfate in the crystals is known as Hydrated Copper (II) Sulfate because it contains water of crystallisation.
When Hydrated Copper (II) Sulfate is heated, it loses its water of crystallisation and turns into anhydrous Copper (II) Sulfate:
Hydration of Cobalt(II) Chloride
When anhydrous blue cobalt(II) chloride crystals are added to water they turn pink and the reaction is reversible.
When the cobalt(II) chloride crystals are heated in a test tube, the pink crystals turn back to the blue colour again as the water of crystallisation is lost.
The form of cobalt(II) chloride in the crystals that are pink is known as hydrated cobalt (II) chloride because it contains water of crystallisation.
When hydrated cobalt(II) chloride is heated, it loses its water of crystallisation and turns into anhydrous cobalt(II) chloride:
The reaction between H2 and N2 in the Haber process
When only nitrogen and hydrogen are present at the beginning of the reaction, the rate of the forward reaction is at its highest, since the concentrations of hydrogen and nitrogen are at their highest.
As the reaction proceeds, the concentrations of hydrogen and nitrogen gradually decrease, so the rate of the forward reaction will decrease.
However, the concentration of ammonia is gradually increasing and so the rate of the backward reaction will increase (ammonia will decompose to reform hydrogen and nitrogen).
Since the two reactions are interlinked and none of the gas can escape, the rate of the forward reaction and the rate of the backward reaction will eventually become equal and equilibrium is reached:
Effect of catalyst on equilibrium position
The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached.
This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy).
As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.
Le Chatelier’s Principle Effects of temperature
AND EXAMPLE
Effects of pressure:
Le Chatelier’s Principle
Effects of concentration:: Le Chatelier’s Principle
