Workshop 11th.docx

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Time: 5 minutes Worksheet (Energy changes)

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Worksheet (Haber and Contact process) 1.

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Worksheet (Redox Reactions) 1.

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Time: 5 minutes Worksheet (Speed of Reaction)

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Name: ___________________ Worksheet (Chemical bonding and atomic structure) 1.

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Name: ___________________ Worksheet (Methods of purification) 1.

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Name: ___________________ Worksheet (Atmosphere and environment) 1.

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Name: ___________________ Worksheet (Metals and metal extraction) 1.

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Time: 5 minutes Worksheet (Salts)

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Time: 5 minutes Worksheet (Stoichiometry)

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Time: 5 minutes Worksheet (Periodic table)

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Iron

Metals

Organic chemistry

Salts

Reversible reactions and Equilibrium REMINDERS about reversible reactions before learning about CHEMICAL EQUILIBRIUM 

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A reversible reaction is a chemical change in which the products can be converted back to the original reactants under suitable conditions.  This means the reaction can go in either direction i.e. A + B ==> C + D or C + D ==> A + B  In a reversible reaction, changing the reaction conditions e.g. concentration, pressure or temperature will change the net direction the reaction goes i.e. more to the right (forward) or more to left (backward). .So, WHAT IS A CHEMICAL EQUILIBRIUM?

 When a reversible reaction occurs in a closed system an equilibrium is formed, in which the original reactants and products formed coexist in the same reaction mixture AND the concentrations of all components in the mixture remain constant.  

The 'closed system' might be a beaker of a solution containing a reaction mixture in the school laboratory or gaseous reactants in an enclosed reactor chamber in the chemical industry. A closed system quite simply means nothing can escape from the reaction mixture.

 In an equilibrium there is a state of balance between the concentrations of the reactants and products and once a state of chemical equilibrium is reached there is no further change in concentrations BUT the reactions don't stop! 

Because neither the forward reaction or backward reaction stops, and the concentrations do not change, the situations is referred to as a dynamic equilibrium.

What are the RULES GOVERNING THE POSITION OF A CHEMICAL EQUILIBRIUM? The three rules outlined below are known as Le Chatelier's Principle. This essentially states that if a change is imposed on a system, the system will change to minimise the enforced change to reestablish equilibrium.

Rule 1 The effect of temperature change on the position of an equilibrium Reminder: If a forward reaction is exothermic, the reverse backward reaction is endothermic and vice versa. Rule 1a: If the forward reaction forming the products is endothermic, raising the temperature favours its formation increasing the yield of product (lowering the temperature decreases the yield). So increasing temperature favours the endothermic direction reaction. The system attempts to absorb the heat and minimise the increase in temperature. Rule 1b: If the forward reaction forming the products is exothermic, decreasing the temperature favours its formation (increasing temperature decreases the yield).

So decreasing temperature favours the exothermic direction reaction The system attempts to release heat to minimise the temperature decrease. Rule 1 examples The equilibrium between hydrogen gas, gaseous iodine and gaseous hydrogen iodide. H2(g) + I2(g)

2HI(g) (plus 10 kJ of heat energy, exothermic L to R)

Increasing temperature favours the endothermic direction, backward reaction, some hydrogen iodide will decompose. Decreasing temperature favours the exothermic reaction, so more hydrogen and iodine react to form hydrogen iodide.

Rule 2 The effect of changing pressure on the position of an equilibrium You can increase/decrease the pressure by decreasing/increasing the volume of the gases OR increasing/decreasing the concentration of gases in the same volume. Rule 2a: Increasing the pressure favours the side of the equilibrium with the least number of gaseous molecules as shown by the balanced symbol equation. So increasing pressure favours the reaction direction to reduce the number of gaseous molecules. The system is changing to minimise the impact of the increase in pressure by removing some gas molecules. Rule 2b: Decreasing the pressure favours the side of the equilibrium with the most number of gaseous molecules as shown by the balanced symbol equation. So decreasing pressure favours the reaction direction to produce the most gaseous molecules. The system is changing to minimise the impact of the decrease in pressure by increasing the number of gas molecules. i) N2(g) + 3H2(g)

2NH3(g)

4 gas molecules ==> 2 gas molecules, so to re-establish a dynamic equilibrium ... (ii) N2O4(g)

2NO2(g)

1 gas molecule ==> 2 gas molecules, so to re-establish a dynamic equilibrium ... (iii) N2(g) + O2(g)

2NO(g)

2 gas molecules ==> 2 gas molecules Change in pressure has no effect on equilibrium position. Rule 3 The effect of changing concentration on the position of an equilibrium Rule 3a: If the concentration of a reactant (on the left) is increased, then some of it must change to the products (on the right) to maintain a balanced equilibrium position. Rule 3b: If the concentration of a reactant (on the left) is decreased, then some of the products (on the right) must change back to reactants to maintain a balanced equilibrium position. Rule 4 The effect of using a catalyst on the position of an equilibrium Rule 4: A catalyst does NOT affect the position of an equilibrium. You just get to the equilibrium position here faster! A catalyst usually speeds up both the forward and reverse reaction but there is no way it can influence the final 'balanced' concentrations. However, the importance of a catalyst lies with economics e.g. (i) bringing about reactions with high activation energies at lower temperatures and so saving the cost on energy, (ii) and saving time is saving money, i.e. a catalyst increases the efficiency of the chemical process e.g. the Haber synthesis of ammonia.