Unit 3- Chemical Equilibrium Flashcards
Explain our assumptions about chemical reactions vs the reality
we often assume:
- chemical reactions proceed only in one direction
- chemical reactions have a constant rate of reaction
in reality:
- chemical reactions can be reversible. Reactants combine to form products while the products turn back into reactants
What happens to the rate of reaction with time?
The rate of reaction tends to decrease with time. This happens because there are less atoms left to react as the time increases. This is true for both forwards and reverse rates of reaction.
What is chemical equilibrium?
- We often assume that chemical reactions always go to completion but many do not and instead attain chemical equilibrium
- Chemical equilibrium: A state in which the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant (do not have to be equal).
- At equilibrium there are no observable (macroscopic) changes
What is meant by “equilibrium is dynamic”?
Both forwards and reverse reactions take place
What conditions must be met to establish equilibrium?
- reversible reaction
- closed system (gas tube, sealed flask)
- temperature must remain the same
Note: For a closed chemical system in constant environmental conditions, the same equilibrium concentrations are reached regardless of the direction by which equilibrium was reached
Explain the equilibrium constant
- When equilibrium is reached, it is important to recognize that not all of the reactants will be converted into products (or products into reactants) -There is a mathematical RATIO, known as the equilibrium constant (Keq), which represents the proportion of products to reactants
- Keq is unitless
Explain what happens when Keq is greater and less than 1
- If Keq > 1: The forwards reactions or products and favoured; meaning the reaction essentially “goes to completion” and all or most of the reactants are used up to form the products
- If Keq < 1: The reverse reaction or reactants are favoured. The reaction does not occur to any great extent- most of the reactants remain unchanged, and there are few products produced.
Note: “Favoured” means that side of the equation has the higher number of moles and higher concentrations than the other.
What are the key points about Keq?
- Only gases and ions in solution (aq) are included in the equilibrium expression
- Solids or liquids are not included because while their amounts change during the reaction, their concentrations do not. They are instead assigned a value of 1.
What do the letters “ICE” stand for in ICE chart?
- I: The INITIAL concentrations
- C: the CHANGE in the concentrations as the system moves towards equilibrium
- E: the EQUILIBRIUM concentrations
Explain Le Chatelier’s Principle?
- Le Chatelier’s principle states that if a system at equilibrium is subjected to an external stress, the equilibrium will shift to minimize the effects of that stress
- External stresses in this situation are factors that will change the rate of either forward or reverse reaction
- In other words, an action that changes the temperature, pressure, or concentration of reactants, or presence of a catalyst in a system at equilibrium stimulates a response that counteracts said change while a new equilibrium condition is established.
Explain how changes in volume and pressure affect a reaction?
Changing the pressure or volume will only affect the reaction is gases are present
- Increasing the pressure (typically by reducing volume) results in the position of equilibrium moving towards the side with the fewest moles of gas molecules
- When pressure is decreased, the equilibrium will move to favour the side with the most moles of gas molecules
- If both sides have the same number of moles of gas, then there will be no change in the position of equilibrium.
How does addition of a catalyst affect the equilibrium of a reaction?
Adding a catalyst will not affect the position of an equilibrium. A catalyst speeds up both the forwards and reverse reactions, so there is no uneven change in reaction rates. Generally a catalyst will help a reaction reach the point of equilibrium sooner, but will not affect the equilibrium otherwise.
How does temperature affect equilibrium?
- If the temperature is increased the position of equilibrium will shift so that the temperature is reduced again
- If you decrease the temperature the reaction will shift to replace the heat that was removed
Affect on Keq: - If the forward reaction is favoured more products are produced and fewer reactants and Keq will increase.
- If the reverse reaction is favoured, then there are fewer products and more reactants and Keq will decrease.
How do changes in concentration affect equilibrium in a reaction?
If we increase one of the products or reactants, the reaction will shift so that its concentration decreases again. If we were to decrease one of them, the equilibrium of the reaction will shift so that they are increased again.
