C7.4 Reversible Reactions and Equilibria

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  • Created by: Lili
  • Created on: 19-06-13 17:14
What is a Reversible Reaction?
Many chemical reactions are reversible, it means they can go forwards or backwards.
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What Does the Direction of the Reaction Depend On?
It depends on the conditions, such as temperature, pressure or the concentration of chemicals.
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Name an Example of a Reversible Reaction.
At room temperature, ammonia + hydrogen chloride ---> ammonium chloride. Gently Heating, Ammonium Chloride ---> Ammonia + Hydrogen Chloride.
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What is Equilibrium?
A reversible reaction will reach equilibrium, if it is in a closed container.
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What Happens During Equilibrium?
All products and reactants are present in the reaction mixture. Their amounts and concentration do not change.
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Describe What Happens During the Equilibrium Reaction of Ammonia and Hydrogen Chloride to Ammonium Chloride.
All the time ammonia and hydrogen chloride molecules are joining together to make ammonium chloride. At the same time ammonium chloride decomposes to make ammonia and hydrogen chloride.
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How Do Forward and Backwards Reactions Work in Equilibrium?
The happen at the same rate, so the amount of each substance in the equilibrium mixture does not change.
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What is Nitrogen Fixation?
This is when un-reactive nitrogen is converted into nitrogen compounds than plants can use, including ammonia, nitrogen dioxide and nitrates.
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How is Nitrogen Fixed?
Either at normal temperatures and pressures by bacteria that live in the roots of plants called legumes (peas, beans, clovers), or through the Haber process.
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What is the Haber Process?
It is an industrial process which produces huge amounts of ammonia every year.
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What are the Feedstocks for the Haber Process?
Nitrogen - Obtained from air & Hydrogen - Produced from a reaction between natural gas (mainly methane) and steam.
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What Type of Reaction is the Haber Process?
Nitrogen + Ammonia = Ammonia, is a reversible reaction, left in a closed container long enough it can reach equilibrium.
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What Does Le Chatelier's Principle Predict?
Predicts the effect on equilibrium mixture of changing reaction conditions: When conditions change, an equilibrium mixture responds as to counteract the effect of the change.
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How Does Changing the Pressure Affect the Haber Process?
Increasing pressure shifts equilibrium towards the right, increasing the yield of ammonia.
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How Does Changing the Temperature Affect the Haber Process?
Lowering the temperature increases the amount of ammonia in equilibrium mixture. Le Chatelier's Principle predicts that when temperature is lower, more heat energy is given out by the exothermic reaction can be absorbed.
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What Are the Conditions Chosen For the Haber Process?
Pressure = 200 times atmospheric pressure, temperature = 450oC and an Iron Catalyst is used.
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What is the Yield Under These Conditions?
The yield is about 15%, however unreacted hydrogen and nitrogen is recycled.
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Why Are the Conditions for the Haber Process a Compromise?
High Pressure increases the yield, but HP equipment is expensive to build and run, and risky to operate. Low temperature increase yield, but the reaction rate is slow, 450oC compromised the need to maximise yield and rate.
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Why Are Chemists Searching for Better Catalysts for the Haber Process?
They want to be able to make the process even more efficient. For example Ruthenium increases yield by 20%.
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Chemists Are Also Developing a new Catalyst That Mimics Nitrogen-Fixing Enzymes. Why is This?
This will increase efficiency and allow it to work well at lower temperatures and pressures. Therefore reducing the high energy costs of the process.
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What Impacts Do Nitrogen Fertilisers have on the Environment?
They run off land into lakes and rivers, causing eutrophication, damaging eco-systems.
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Other cards in this set

Card 2


What Does the Direction of the Reaction Depend On?


It depends on the conditions, such as temperature, pressure or the concentration of chemicals.

Card 3


Name an Example of a Reversible Reaction.


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Card 4


What is Equilibrium?


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Card 5


What Happens During Equilibrium?


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