how quickly a reaction happens is called the rate of reaction any rate is the change in a quantity divided by time in this case that something can be the quantity of reactant used or product formed this quantity could be mass or volume of gas that's usually made anyway they like to stipulate that this technically gives you the mean rate as the rate could be changing over the time you measure but that's true for any measurement over time ever so that's a bit redundant but we'll go with it for now an experiment on this could be reacting hydrochloric acid and sodium thos sulfate in a conical flask sitting over a piece of paper with a cross on as the reaction continues the product form turns the solution cloudy we say that's increased turbidity we stop the timer when we can no longer see the cross from above the flask repeat this at different temperatures and you should see that the hotter the temperature the less time it takes another potential experiment is measuring the volume of gas produced by using a gas syringe that fills up when connected to the reaction vessel a graph to show this would have the quantity on the y- AIS and time on the x-axis it's usually a curve that starts off steeply but then levels out or plateau which shows that the reaction has completed or we can say reached its end point to find the rate at any time you draw a tangent at that point Pro tip Turn the page so you're drawing the tangent horizontally that will help you draw it accurately then like the equation says you can take the changing quantity and divide by time up divided by Across the rate of a reaction can be increased by the following increasing the concentration of reactants that are in solution increasing the pressure of gas reactants and increasing the surface area of solid reactants that is crushing into a powder now these three all have this effect because the reacting particles Collide more frequently they come across each other more often increasing temperature does that too due to the particles moving more quickly but there's the added bonus that they also collide with more energy meaning they're more likely to react when they Collide due to activation energy remember your energy diagrams from paper 1 finally adding a catalyst also increases rate as it reduces the activation energy needed so particles are more likely to collide successfully and react of course any catalyst is not used up in a reaction it is not a reactant or product itself reversible reactions are pretty self-explanatory once the products are made they're able to return to their original reactants the prime example here is the harbor process hydrogen and nitrogen react to make ammonia which can also break down back into the separate gases again more on what ammonia is used for later in a closed system that is no particles or energy going in or out both reactions will continually take place eventually the quantity of particles on both sides will reach a point at which the rates of both the forward and reverse reaction will be the same so that means there will be no more overall change in the quantities on both sides remember that's not saying that the reaction is stopped per se it's just that there's no more overall change that is until a condition is changed which will affect these rates later's principle states if a system at equilibrium is subjected to a change the system will adjust to counteract that change sounds aw aul vague so let's see what that means in practice there are a greater number of moles on the left than the right of this reaction which means that the reactants take up more space therefore if you increase the pressure of all of these gases we say this favors the forward reaction that is the rate of the forward reaction will increase until equilibrium is once again reached but that will happen when there's a greater proportion of ammonia than there was before we could also say that the position of equilibrium is shifted to the right reducing the pressure would of course do the opposite by shifting it to the left instead concentration follows the same principle when it comes to solutions by the way naturally if you remove molecules from one side of the reaction the position of equilibrium shifts in that direction so more is produced increasing the temperature in essence means it's harder for a reaction to produce heat that means that a hotter temperature favors the endothermic reaction in this case that's the reverse reaction you could also think of it like this an endothermic reaction requires energy being put in so a higher temperature supplies that a colder temperature will favor the exothermic reaction in this case that's the forward reaction as a rule of thumb any reaction that involves the breaking down of one reactant ammonia in this case that's going to be endothermic in any reversible reaction if the forward reaction is exothermic the reverse reaction must be endothermic and vice versa so I hope you found that helpful leave a like if you did and pop any questions or comments below I'll see you in the next video