# 7.1.2 Rate (Speed) of Reaction

### Rates of Reaction Factors

Effect of concentration

Diagram showing increase in concentration of solution

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

Diagram showing surface area to volume ratio of various sized cubes

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

Diagram showing the effect of temperature on particles

• 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

Graph showing the effect of the use of a catalyst on the rate of reaction

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

A single 2 cm length cube has a surface area of 2 x 2 x 6 = 24cm2. Cutting it into 8 x 1cm cubes means it now has a surface area of 1 x 1 x 6 x 8 = 48cm2

### Interpreting Data

Concentration

Graph showing the effect of the concentration of a solution on the 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

Particle size

Graph showing the effect of the surface area of a solid on the rate of reaction

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

Catalyst

Graph showing the effect of the use of a catalyst on the rate of reaction

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

Graph showing the effect of temperature on the 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
Extended Only

### Investigating the Rate of a Reaction

Effect of surface area of a solid on the rate of reaction:

Diagram showing the process of downwards displacement to investigate the effect of the surface area of a solid on the rate of reaction

Method:

• 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:

Diagram showing the apparatus needed to investigate the effect of concentration 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:

Diagram showing the apparatus needed to investigate the effect of temperature on the rate of reaction

Method:

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

Diagram showing the apparatus needed to investigate the effect of a catalyst on the rate of reaction

Method:

• 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
Extended Only

### Temperature & Concentration

Temperature

• Particles need to have 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

Diagram showing the increased kinetic energy that particles have at higher temperatures

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

Diagram showing the decreases in space between particles at higher concentrations

#### Exam Tip

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.

Extended Only

### Photochemistry

Photochemical reactions

• These reactions occur only when light is present
• The greater the intensity of ultraviolet light then the greater the rate of reaction
• Eg 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

### Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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