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Le Chatelier's Principle

A sample interactive ICT resource

Jennie D

on 1 October 2012

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Transcript of Le Chatelier's Principle

Reversible Reactions Le Chatelier's Principle The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions. Le Chatelier "if a reaction in a state of chemical equilibrium is acted upon by an external influence which disturbs it, the reaction will proceed in such a direction as to counter the effect and return to a state of equilibrium" Le Chatelier's Principle states that: What does this mean? Lesson Objectives Le Chatelier's Principle is very important in providing information as to how the external conditions of temperature, pressure and concentration may be manipulated to enhance the extent of a chemical reaction. Chemical Equilibrium By understanding how these factors affect equilibrium, we can predict whether the forward or the reverse reaction is favoured in an equilibrium reaction. Factors that affect equilibrium reactions Check your understanding! Changing the concentration of an ingredient will shift the equilibrium to the side that would reduce that change in concentration. The chemical system will attempt to partially oppose the change affected to the original state of equilibrium. In turn, the rate of reaction, extent and yield of products will be altered corresponding to the impact on the system.

For example: CO + 2 H2 <=> CH3OH

Suppose we were to increase the concentration of CO in the system. Using Le Chatelier's principle, we can predict that the amount of methanol will increase, decreasing the total change in CO. If we are to add a species to the overall reaction, the reaction will favor the side opposing the addition of the species. Likewise, the subtraction of a species would cause the reaction to fill the “gap” and favor the side where the species was reduced.
Effect of Change in Concentration Effect of Change in Temperature Effect of Change in Pressure Using the following information, your task is to write up and perform an experiment to remove the carbon dioxide from a Experiment Know your definitions? Check against our crossword!

Click once on crossword to zoom in.

Or alternatively download the html file to play interactively. Crossword Quiz your knowledge and get instant feedback!

Download the Le Chatelier's Principle powerpoint available. Pop Quiz Click on Past exam questions once to zoom in! Define Le Chatelier's Principle Identify factors which can affect the equilibrium in a reversible reaction 2008 HSC Chemistry Exam HSC Practice Questions 2009 HSC Chemistry Exam 2011 HSC Chemistry Exam 2010 HSC Chemistry Exam Equilibrium Practical Activity identify data, plan and perform a first-hand investigation to decarbonate soft drink and gather data to measure the mass changes involved and calculate the volume of gas released at 25C and 100kPa The effect of changing the temperature in the equilibrium can be made clear by incorporating heat as either a reactant or a product. When the reaction is exothermic (H is negative), we include heat as a product, and, when the reaction is endothermic (H is positive), we include it as a reactant. An increase in pressure causes the reaction to shift to the side with fewer moles of gas and a decrease in pressure causes the reaction to shift to the side with more moles of gas. There is no effect on a reaction where the number of moles of gas is the same on each side of the chemical equation.

The number of moles of gas on the left and right side of a reaction should be noted.

For example: N2 + 3 H2 <=> 2 NH3 H = -92kJ mol-1
4 moles:2 moles
Equipment Method Control Measure and record the quantitative data required for the calculations to determine the volume of gas released at 25oC and 100kPa. Carry out repeat trials to ensure reliable data is collected. Calculations required:
- loss of mass due to release of CO2
- convert grams of CO2 lost to moles of CO2
- use knowledge that one mole of gas at 25oC and 100kPa occupies 24.8 L. Measurements To achieve a more accurate result include a control in your experimental design. For the salting-out method: 1g of table salt for each 50 mL of soda water. The addition of salt should be gradual so that the soda water does not foam or spray out of the container. Such loss of mass would require you to start all over again. scale balance that can at least measure to the nearest gram
unopened cans or bottles of soft drink
salt quantity of soft drink in a way that allows you to make accurate and reliable measurements. Some reactions are reversible.

This means that in a forward reaction, the reactants form products.

However the reverse reaction may occur causing the products to form the reactants again. Reversible Reactions Reversible reactions are shown with arrows going in both directions as shown above. Equilibrium Systems In an equilibrium system, two reactions take place at the same time in both directions.Equilibrium occurs when the rate of the forward reaction is equal to the reverse reaction. Equilibrium Disturbance If a system's equilibrium is disturbed, the equilibrium system will shift and the system will eventually return to a new equilibrium. Real Life Applications Is similar to / can also be known as homeostasis depending on the application. Any change in status quo prompts an opposing reaction in the responding system. Le Chatelier's Principle explained in other words Equilibrium If a chemical system at equilibrium experiences a change in concentration, temperature, volume, or partial pressure, then the equilibrium shifts to counteract the imposed change and a new equilibrium is established.
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