What does it mean to shift equilibrium?

What does it mean to shift a chemical equilibrium? For example,

the equilibrium shifts to the left …

I don't understand that.

Probably you are having problems with Le Chatelier's Principle.

Suppose you have an equilibrium established between four substances $$\ce{A}$$, $$\ce{B}$$, $$\ce{C}$$ and $$\ce{D}$$, such that

$$\ce{A + B <=> C + D}$$

What would happen if you changed the conditions by increasing the concentration of $$\ce{A}$$?

According to Le Chatelier, the position of equilibrium will move in such a way as to counteract the change. That means that the position of equilibrium will move so that the concentration of $$\ce{A}$$ decreases again - by reacting it with $$\ce{B}$$ and turning it into $$\ce{C + D}$$. The position of equilibrium moves to the right, making more of the products. Of course, this assumes that there is still some amount of $$\ce{B}$$ left in the reaction vessel.

For better understanding, refer this.

Simply it means that the equilibrium will force the conversion of certain substances to reduce the disturbance, to maintain the equilibrium itself. So, for example, $\ce{A + B <=> C + D}$ (both directions).

If you increase the concentration of the reactants — $\ce{A}$ or $\ce{B}$, then you’re increasing the conversion of reactants into products, because the rate of reaction and overall number of product formed increases. Therefore, we need to form more $\ce{C + D}$, to decrease the concentration of $\ce{A + B}$ and balance out the rate of change.

Therefore, even though the reaction occurs simultaneously as a forward and backward reaction, the movement of equilibrium to the right (to the products) means the conversion of reactants to products is further emphasised, and generally — there is more of it occurring than in a usual equilibrium, in order to balance the change. Same thing would’ve happened if you increase the concentration of products, you want the equilibrium to shift to the left (reactants) so concentrations of both reactants and products balances out, reducing the disturbance (Le Chatelier’s Principle — specifically referred to by Ashu). Simply shifting equilibrium means increased rate of conversion of substances, predicating on the change in the reaction in the first place.

Sorry, it’s quite long winded and irrelevant at times, I hope you’ve benefitted from this though. I’ve also used the idea of changing concentrations in particular from Ashu, if that’s alright.

Let’s do it simply. You start from some equilibrium. For example you have simple reaction $\ce{A <=> B}$, and in the equilibrium state there is $60~\%$ of $\ce{A}$ and $40~\%$ of $\ce{B}$. If it said that equilibrium is shifted to the right it means that you have more product $\ce{B}$ (i.e. more than $40~\%$). If you shifting to the left you gaining more substrate. Now how it's done — look on the previous answers.

“Equilibrium shift” is jargon. The equilibrium constant stays the same when you disturb an equilibrium by adding or removing species. However, the system might not be at equilibrium anymore.

When the system was at equilibrium, there was no net reaction. Now that the equilibrium is disturbed, there will be a net reaction in the direction of reactants or products (depends on the specifics of how the equilibrium was disturbed). The jargon for this net reaction is to say the equilibrium “shifted to the left” (or to the right).

It would be more formal to say equilibrium was attained again by a net reverse (or forward) reaction. This would also be easier for someone learning these concepts because it better describes what is going on.

1. The reaction is at equilibrium
2. The equilibrium is disturbed
3. There is a net reaction (in one or the other direction) until we are at equilibrium again.

Typically, the concentrations present at the original equilibrium state will be different from those for the second equilibrium state.

When a change in temperature causes a change in the equilibrium constant, the phrase “equilibrium shift” might be a little bit more intuitive. There will be a net reaction to adjust to the new equilibrium constant.