Speed of Reaction Summary

2012 Edited by Mr Chong, Singapore

Speed of Reaction Key Concepts: 1.

Speed of a reaction can be measured by : (i) change in mass of reactant or product over time (ii) volume of gas liberated over time (iii) temperature / pressure changes (iv) colour / precipitate or pH change

2.

Speed of a reaction can be followed through by analyzing a reaction rate curve. Plateau indicates the end of reaction – determined by limiting reagent

Volume of gas (cm3)

Gradient of slope indicates the rate of reaction – determined by temperature, particle size or concentration, pressure and catalyst.

Time Taken (minutes)

3.

Collision Theory: For a reaction to occur,  the reacting particles must collide with one another;  the particles must possess sufficient activation energy Any factor that increases the rate of effective collision will increase the speed of reaction.

4.

The main factors which can affect the speed of a chemical reaction are:  temperature  particle size  concentration  pressure  catalyst (To be discussed in WS 2) Temperature of reactants:  The higher the temperature, the faster the speed of reaction because:

5.

1. At a higher temperature, reacting particles have more energy. They move faster and collide more frequently. 2. Due to the increase in the energy of reacting particles, there are more reacting particles having the activation energy required for the reaction to occur. This leads to an increase in the number of effective collisions per unit time.  6.

For every 10oC rise in temperature, the speed of reaction approximately doubles.

Particle size ( surface area ) of reactants:  A decrease in the size of the particles of the reactants (larger surface area) 1

2012 Edited by Mr Chong, Singapore

increases the speed of reaction because: 

7.

The area of contact between the reacting particles increases, the number of effective collisions per unit time also increases.

Concentration of reactant:  An increase in concentration of the reactants increases the speed of reaction because:  When the concentration of a reactant is increased, there are more reacting particles per unit volume.  The frequency of collisions between the reacting particles thus increases.  This leads to an increase in the number of effective collisions per unit time.

8.

Pressure of reactant (for gases only):  An increase in pressure of the reactants increases the speed of the reaction because:  When the pressure of a gas is increased, there are more gas particles per unit volume.  The frequency of collisions between the gas particles thus increases.  This leads to an increase in the number of effective collisions per unit time.

Key Concepts: 1.

A catalyst is a substance that changes the speed of the reaction but it remains chemically unchanged at the end of the reaction.

2.

Catalysts provide an alternative pathway for reactions to occur. In the catalyzed pathway, less energy is required for the reaction to occur, so the activation energy is lowered.

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2012 Edited by Mr Chong, Singapore

3.

Catalysts only changes the speed of reaction, they do not increase the yield of products.

4.

Some common industrial uses of catalysts: Catalysts iron vanadium(v) oxide Nickel Enzymes in yeast

Uses Manufacture of ammonia Manufacture of sulphuric acid Manufacture of margarine Manufacture of ethanol

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