Hey hey, anything that changes the conditions of the system or experiment will either cause that "sway" to the forwards or backwards reaction. The main changes you'll come across are temperature, pressure and a change to the product. Those are the only things that will change the reaction direction, unless I'm forgetting one.... Oops.
Temperature - If the forward reaction is exothermic and temperature is increased, then this will cause a "sway" to the backwards reaction as this will be endothermic. Likewise, if the forward reaction was endothermic then increasing temperature will cause a stronger "sway" towards the forwards direction. You know that exothermic reactions produce energy, let's just assume thermal, heat energy for now; and endothermic reactions always consume energy. Basically, exothermic gets hotter and endothermic gets colder. So if you use this thinking on a problem then you should be able to work out what will happen because the reaction will try to keep the temperature the same. If you increase temperature, the reaction will try to cool the experiment and thus favour an endothermic reaction. Does that make sense???
Pressure - This is the same as volume (basically). If you think about PV=nRT then you can realise that if pressure is increased, then volume will have to have decreased. Like when you squash a balloon or something, you decrease the volume and so the pressure builds until BOOM, it pops. Anyway, most reactions will have an uneven amount of stuff on either side the -->. Example, in 2H2 + O2 --> 2H2O there are 3 mols of stuff on the left and 2 on the right. If this was an equilibria reaction and we decreased volume (increased pressure) then the system will want to resist this change by doing the opposite of what you did. Therefore, you increasing pressure will mean the system will want to decrease the pressure by favouring the side with the least amount of particles. The opposite would occur if you decrease pressure, the system will want to increase the pressure.
Adding/Removing Chemicals - If you add something, the system will try to remove it. If you remove something, the system will try to add it back. If I had 2HI -> H2 + I2 and I added a lot more HI, then the system will try to remove the amount of it by favouring the forward reaction. Similarly, if I removed HI the system will try to add more of it by going backwards
Long story short, equilibria reactions are like little children. If you tell them to do something, they will always do the opposite. If you tell them to sit down, they're going to run around. If you tell them to eat their lunch, they'll do everything but eat it. Using this analogy, if we heat, increase pressure or add stuff then the system will try to cool, decrease pressure or remove stuff respectively to maintain a balance. This is the basis of Le Chatelier's principle.
Let me know if anything is confusing or I haven't explain something well! Don't stress if you don't get it, all you have to do in chem is try and it come to you! Good luck!
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