chemistry chapter 6 - rates of reaction

• the rate of a reaction is the speed in which reactants are turned into products

• slow - rusting of metal or chemical weathering of a limestone building

• moderate - magnesium and an acid (creates a slow stream of bubbles)

• fast - burning and explosions (over in half a second)

• you can use graphs to measure the rate of reaction: the steeper the graph the faster the rate of reaction

• on a graph the rate of the reaction is measured by the amount of product formed or the amount of reactant used over time

• when the line goes flat is when the reaction ends

• quicker reactions have steep lines and go flat in the least amount of time e.g explosions (fast reactions are over in half a second)

• collision theory - reactants contain particles that need to collide for a reaction to occur

• the speed of reactions can be affected by the amount of energy that particles have (as they need lots of energy for a succesfull collision) and the collision frequency (how often they collide

• particles with more energy collide more often

• activation energy - the minimum amount of energy needed for reaction

143 - factors affecting the rate of reaction

• the rate of a reaction is affected by 4 things - a catalyst, surface area, temperature and pressure of gasses + concentration of solutions

• catalysts - a substance that speeds up the rate of reaction by decreasing the amount of activation energy and creating an alternative reaction pathway (with lower activation energy) this speeds up a reaction as the reactants need less energy to begin reacting

• catalysts are also not used up in the reaction so are not part of the overall reaction equation

• enzymes are biological catalysts - they catalyse reactions in living things

• surface area - by using a powder or small chunks of a reactant instead of a large piece, you increase the surface area for reaction, meaning that collisions occur more frequently

• temperature - increasing temp transfers more energy to particles which allow them to move energetically and collide more frequently

• pressure and concentration - inc pressure means that reacting gas particles will be in a smaller space, so will collide more frequently and inc concentration of reacting solutions means there are more particles avaliable for collisions

• all of these methods increase the frequency of collisions

144 - measuring rates of reaction

• equation to find mean rate of reaction - rate = amount of product formed or amount of reactant used up / time

• to find the rate of reaction at an exact time - plot a graph and find the gradient at the given time

• can measure visual change

• if the solution is transparent and creates a participate it will go cloudy (turbidity = cloudiness) you can measure how long it takes for the solution to become cloudy by drawing a mark (a black x on white paper) and placing it under the beaker where the reaction occurs, then count until you can no longer see tghhe mark

• you can also measure how long it takes for a solution to become colourless if the reactants are coloured

• this is a subjective method, as not everyone will agree on the time the reaction becomes colourless or when the mark is no longer visible, you can also not plot a rate of reaction graph for this method, as you don’t know how much product has been formed

• if gas is given off, you can measure the volume using a gas syringe and how long it takes the reaction to stop giving off gas or the volume of gas produced at a certain time, the gas is the product in the reaction, this method is quite dangerous, as the bung could pop off if the reaction is too vigourous

• change in mass can also be measured, place your beaker on the mass balance and watch as the mass drops, this means gas has been given off, the quicker it drops the faster the reaction (as reactants are being used up and producing gas), measuring at regular time intervals means you can plot a rates of reaction graph and calculate the rate easily, plus its accurate due to the accuracy of the mass balance, however, it does allow gas to leak out which is bad if your reaction produces a poison

145 - two rates experiments

1) reacting HCL acid with Magnesium to produce H2 gas

• method used to measure rate of reaction - change in mass, as gas is given off as the product, using a stopwatch to measure the decrease in mass shown on the mass balance (could also be done by using a gas syringe to measure the volume of gas produced)

• method - add HCl to a conical flask and place on a mass balance, add the magnesium and plug with cotton wool ball, start stopwatch and measure at regular time intervals how much the mass decreases by, as this means H2 (the product) is being formed, the quicker the mass decreases by the faster the rate of reaction. Plot this on a rate of reaction graph, with loss of mass on the y axis and time on the x axis, repeat the experiment with more concentrated HCL to see how this effects rate of reaction

• amount of magnesium and volume of HCL acid should be kept constant, only the concentration of the acid should be changed.

• plot these reactions against each other and compare

2) sodium thiosulfate reacts with HCL to produce a cloudy precipitate

• method used to measure rate of reaction - recording visual change, in this case turbidity and the time it takes for a mark to no longer be visible

• method - pour sodium thiosulfate into conical flask and draw an x on a piece of paper in black pen (this is the mark), place the paper under the flask and start the stopwatch when you add the HCL, watch until the mark is no longer visible and stop the clock. You can’t draw a graph for this reaction as you can’t measure amount of product formed, but you can repeat the reaction with higher concentrations only one solution to see how it changes the time it takes for the mark to no longer be visible (the higher the conc, the quicker the mark won’t be visible)

• this reaction releases sulfur dioxide, so should be done in a well ventilated place

146 - finding rate of reaction from graphs

• you can find out the mean rate of reaction from a graph by finding the gradient

• gradient = change in y/ change in x

• the y axis has the values for amount of product formed or amount of reactant used and the x axis has the the time intervals

• these measurements are used in the equation rate = amount of product formed or amount of reactant used up / time

• if you have to find the rate of a reaction at a specific time, draw a tangent to the graph

• e.g if the desired time is 3 mins, find 3 mins on the graph and draw a straight line with a ruler, extending it so it’s equal on both sides, find 2 easy points to read off the line and calculate the gradient (should make a triangle shape)

147 - reversible reactions

• reactions can be reversible and not reversible

• a reversible reaction is one where if conditions are changed, the products can react to form the reactants

• e.g a reaction of ammonium chloride - ammonia + hydrogen chloride

• ammonium chloride is produced when the reactants are cooled but heating ammonium chloride can produce ammonia and hydrogen chloride, making this reaction reversible, the condition that gets changed is temperature

• as reactants react, their concentrations fall (as they’re used up to make products) so the reaction slows down

• but when the products react to produce reactant, their concentrations increase which means the backwards reaction speeds up

• at some point, both reactions will have the same rate, meaning they’re reached equilibrium - this can only occur in a closed system (when the reaction takes place in a sealed container) as nothing can escape or get in

• equilibrium = when both reactions are happening but there’s no big effect, as the concentrations of reactants and products have reached a balance and will not change

• equilibrium doesn’t mean the amount of products and reactant is equal

• left = reactants

• right = products

• if a reaction lies to the left, the concentration of reactants will be higher than the concentration of products

• if a reaction lies to the right, the concentration of products will be higher than the concentration of reactants

• endo and exothermic reactions can also be reversible

• for example, blue hydrated copper sulfate - white anhydrous copper sulfate and water

• this reaction is endothermic, as energy is needed to create the products, this reaction is called thermal decomposition

• the reverse (water + anhydrous copper sulfate - hydrated copper sulfate) is exothermic, as energy is given out when the product is formed

• the conditions that effect equilibrium position are temp, pressure (only in gasses) and concentration of reactants and products)

148 - le chatilier’s principle

• the idea that if you change the conditions of a reversible reaction at equilibrium (temp, conc and pressure), the system will counteract the change

• the system will try to reach equilibrium again e.g if u inc something the system will decrease it in the other direction

• how do different conditions counteract the change?:

• temperature : if a reaction is endothermic one way, it will be exothermic the other way, by increasing temperature of the system at equilibrium, the system moves in a endothermic direction (using energy) to try and decrease the temperature, creating more products from the endothermic reaction

• if you decrease the temperature, the system will produce more heat and move in an exothermic direction (as heat is being given off to counteract the change) meaning more products will be formed on the exothermic side

• pressure (only in gasses) : if you increase the pressure of the system, it will move in a direction where there are fewer gas particles to counteract the change

• if you decrease the pressure of a system, it will move in a direction where there are more gas particles to counteract the change

• you can use a balanced symbol equation to see which side has more molecules of gas

• concentration : if you change the conc of either the reactants or the products, the reaction will no longer be in equilibrium, so it will try to return to equilibrium, if u increasing the conc of the reactants the system will counteract the change by making more products

• if you decrease the concentration of the products, the system will counteract the change by reducing the amount of reactants