C6 - Rates of Reaction

• the rate of chemical reactions is how fast the reactions are changed into products
• the collision frequency of reaction particles : the more collisions the faster the reaction
• the energy transferred during a collision : particles have to collide with enough energy for the collision to be successful
• activation energy : the minimum amount of energy that particles need to react
• rate of reaction depends on concentration of a solution or the pressure of the gas, temperature, surface area and catalyst
• increasing the temperature increases the rate : when the temperature is increased the particles all move faster. if they are moving faster they are going to collide more frequently. also the faster they move the more energy they have so more of the collisions will have enough energy to make the reaction happen
• increasing concentration or pressure : if a solution is made more concentrated it means more particles knocking about in the same volume of water. similarly when the pressure of a gas is increased it means that the same number of particles occupies a smaller space. this makes collisions between the reactant particles more frequent
• increasing surface area increases the rate : if one of the reactants is a solid then breaking it up into smaller pieces will increase its surface area to volume ratio. this means that for the same volume of the solid the particles around it will have more area to work on so there will be collisions more frequently
• using a catalyst increases the rate : a catalyst is a substance that speeds up a reaction without being used up in the reaction itself. this means it is not part of them overall reaction equation. different catalyst are needed for different reactions but they all work by decreasing the activation energy needed for the reaction to occur, they do this by providing an alternative reaction pathway with lower activation energy.
• rate of reaction = amount of reactant used or amount of product formed / time
• precipitation and colour change : you can record the visual change in a reaction if the initial solution is transparent and the product is a precipitate which clouds the solution. you can observe a mark through the solution and measure how long it takes for it to disappear the faster it disappears the faster the reaction. if the reactants are coloured and the products are colourless you can time how long it takes for the solution to gain or lose its colour
• change in mass : measuring the speed of a reaction that produces a gas can be carried out using a mass balance. as the gas is released the mass disappearing is measured on the balance. the quicker the reading on the balance drops the faster the reaction. if you take measurements at regular intervals you can plot a rate of reaction graph and find the rate quite easily.
• volume of gas given off : this involves a gas syringe to measure the volume of gas given off. the more gas given off during a given time interval the faster the reaction. gas syringes usually give volumes accurate to the nearest cm3 so they are quite accurate. you can take measurements at regular intervals and plot a rate of reaction graph using this method too
• magnesium and HCI react to produce H2 gas RP: start by adding a set volume of dilute hydrochloric acid to a conical flask and carefully place on a mass balance. now add some magnesium ribbon to the acid and quickly plug the flask with cotton wool. start the stopwatch and record the mass on the balance take readings of the mass intervals. plot the results in a table and work out the mass lost for each reading . repeat with more concentrated acid solutions
• sodium thiosulfate and HCI produce a cloudy precipitate RP : these two chemicals are both clear they react together to form a yellow precipitate of sulfur. start by adding dilute sodium thiosulfate to a conical flask. place the flask over the black cross. add some dilute HCI and start the stop watch. now watch the cross disappear through the cloudy sulfur and time how long it takes
• calculate mean reaction rate from a graph : it shows the product formed on the y axis and the time on the x axis. so to find the mean rate for the whole reaction you just work out the overall change in the y value then divide by total time taken
• reversible reactions will reach equilibrium
• Le chateliers principle is the idea that if you change the conditions of a reversible reaction at equilibrium the system will be counteract that change
• temperature : if you decrease the temperature the equilibrium will move in the exothermic direction to produce more heat. this means you will get more products for the exothermic reactions and fewer products for the endothermic reaction. if you raise the temperature the equilibrium will move to endothermic direction to try and decreases it. you will now get more products for the endothermic reaction and fewer products for the endothermic reaction
• pressure : changing the pressure the equilibrium involves gases. if you increase the pressure the equilibrium tries to reduce it by moving to the direction with fewer molecules. if you decrease pressure the equilibrium tries to increase it by moving to the side where there are the most molecules.
• concentration : if you change the concentration of either the reactants or products the system will no longer be at equilibrium. so the system responds to bring itself back to equilibrium again. if you increase the concentration of the reactant the system tires to decreases it by making more products. if you decrease the concentration of products the system tries to increase it again by reducing the amount of reactants