revision chemistry notes for Rate of reaction/Speed of reaction

Rate of reaction/Speed of reaction: It is the speed for a reactant to be used up or product to be formed.

2 ways to measure speed of reaction

1. Measuring time for reaction to complete

• Speed of reaction is inversely proportional to time taken; the shorter the time needed for reaction to complete, the faster the speed of reaction is.

Speed of reaction =1/time taken

Speed of reaction A = = 0.333/s

Speed of reaction B = = 0.667/s

Therefore reaction B is faster than reaction A as time taken for B is shorter

Number of times B faster than A = = 2 times

2. Measuring the amount of product produced in a period of time or measuring the amount of reactant remain in a period of time.

• Can be measured by plotting change in volume of gas evolved, mass of reaction mixture as reaction proceeds and change of pressure of gas formed.

A. Measuring the amount of gas evolved.

• Consider reaction of limestone with acid to produce carbon dioxide. 
• A syringe is used to help in measurement of gas produced in volume every time interval. 
• A graph of volume of gas against time is plotted.
• Gradient largest at start indicating speed at its greatest.
• Gradient decreases with time – speed decreases with time.
• Gradient becomes zero, speed is zero. The reaction has finished.

B. Measuring change in mass of reaction mixture.

• Marble is reacted with acid in a flask with cotton wool stucked at top to prevent splashing during reaction but it allows gas to be free. 
• The reading on balance is plotted on a graph on every time interval.

Factors Affecting Speed of Reaction

1. Particle Size of Reactant

• When large marble is reacted with acid and compared to reaction of fine marble solids being reacted with acid and the graph of volume of gas against time is plotted, it is found that the reaction involving finer marble chips produces gas faster than the one with larger marble chunk as the graph of finer chips is steeper. 
• The volume of gas at the end is the same for both reactions. 
• Therefore, reactions of solids with liquid/gas is faster when the solids are of smaller pieces
• Reactions occur when particles collide. 
• Small particles creates larger surface area for more collisions between reacting particles which increases speed of reaction.
• Explosions: chemical reactions occuring extremely rapid rate producing heat+gas
• Examples
• Coal dust burn faster than large pieces as it has larger surface area. In coal mines, when air contains too much coal dust, explosion can occur from a single spark or match. Water is sprayed into the air to remove coal dust.
• Flour in mills can ignite easily due to large surface area.

2. Concentration of Reactant

• In the increase of concentration means there are more solute particles per unit volume of the solution which favours for more effective collision resulting in an increase in speed of reaction.

3. Pressure of Reactant

• Only gaseous reactions are affected as gas is compressible. 
• At higher pressure, molecules are forced to move closely together, hence increasing the particles per unit volume of gas and effectively increases the collision between reacting molecules so the speed of reaction increases.
• High pressure is used in industrial processes (e.g. Haber Process Plant) so that the reaction goes faster.

4. Temperature of Reaction

• Speed of reaction increases when temperature increases. 
• Particles don’t always react upon collision but just bounce as they don’t have enough activation energy to react. 
• With increase in temperature, particles absorb the energy and having enough activation energy, they move faster and collide more effectively per second. 
• Therefore, speed of reaction is increased.
• Usually, speed of reaction doubles for every 10oC rise in temperature.

5. Effect of Catalyst

• Catalysts are chemical substances which alters speed of reaction without itself being used at the end of a reaction. 
• It can be reused and only small amount of catalyst is needed to affect a reaction.
• transition metals (e.g. Titanium, Nickel, Iron, Copper) are good catalysts
• most catalyst catalyse one kind of reaction (except titanium)
• Catalysts lower the need of energy to break bonds so activation energy is lower. 
• Consequently, bond breaking occurs easily and more often when particles collide

Factors Affecting Speed of Catalysed Reactions:

Speed of catalysed reactions can be increased by:

• increasing temperature
• increasing concentration of solutions
• increasing pressure of gas reactions

Catalyst provide “alternative path” which results in lower activation energy.

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