Topic 9 - Rates of Reactions

Rate is inversely proportional to time

• change in mass • change in volume • change in pressure • time taken to turn opaque

• surface area:volume of reactant is increased • more of the solid is accessible to the other reactant • more collisions take place per unit time

Calcium carbonate + (dilute) hydrochloric acid -> calcium chloride + water + carbon dioxide CaCO3 (s) + 2HCl (aq) -> CaCl2 + H2O + CO2 Independent variable: surface area of CaCO3 ; whole, smashed up, powdered Dependant variable: volume of CO2 produced measured using a gas syringe (making sure its airtight) Take readings at regular intervals and record the results in a table Plot a graph of time (x) and volume (y)

Gradients differ because SA affects rate All level off at same point because same mass was used

Mass of a solute per unit volume

Rate is proportional to concentration When concentration increases, there are more particles per unit volume, so there are more collisions per unit time In a gas, increasing the pressure means that the particles are closer together, increasing the frequency of collisions

Magnesium ribbon + dilute hydrochloric acid in a conical flask bubbles through a delivery tube into an upside down test tube in a trough of water, producing magnesium chloride + hydrogen Mg (s) + 2HCl (aq) -> MgCl2 (aq) + H2 (g) Independent variable: concentration of HCl Dependent variable: time for all H2 to be produced

Rate = 1/time

• particles move faster- more collisions per unit time • particles have more energy, collide with more energy • a higher percentage of collisions have energy greater than the activation energy

Na2S2O3 (aq) + 2HCl (aq) in a beaker with an X underneath. Produces 2NaCl (aq) + S (s) + SO2 (g) + H2O (l) Independent variable: temperature of solution - 40°C-> 80°C using a waterbath Dependent variable: time taken for X to become invisible Control: volume Works because they are both clear solutions which produce a yellow precipitate of sulphur

• they provide an alternative reaction pathway with a lower activation energy • therefore a higher percentage of collisions have energy higher than the activation energy and are therefore successful

• reactants are being used up * concentrations are dropping

How fast reactants are turned into products

• graphs can be made with time on the x axis and either amount of product formed or amount of reactant used up on the y axis • over time, the line becomes less steep as the reactants are used up • quickest reaction have steepest gradient and flatten out in the shortest time

The rate of a chemical reaction depends on: 1) the collision frequency of recaring particles -> the more collisions there are, the faster the reaction is 2) the energy transferred during a collision -> particles have to collide with enough energy for a collision to be successful

1) activation energy is shown as the offered e between the nervy of reactants and the highest point on the curve 2) reaction profiles can be used to compare the reaction with and without a catalyst 3) the highest point on the curve for a reaction with a catalyst will be lower than without a catalyst

Amount of reactant used or amount of production/ time

1) works for when two clear solutions produce a precipitate, which clouds 2) mix the two reactant solutions and out the comical flask in a piece of paper marked with a large X 3) observe the mark through the mixture from above and measure how long it takes until you can no longer see the mark-> the faster it disappears, the faster the reaction 4) it is subjective

1) a production that produces a gas can be used to calculate rate using a mass balance 2) as the gas is released, the lost mass is measured on the balance- the quicker the reader drops, the faster the reaction 3) you know the reaction has finished when the reading stops changing 4) plot a graph of change in mass against time 5) do the experiment in a fine cupboard, stopper the conical flask with cotton wool to let the gases through but stop ant solid, liquid or aqueous reactants flying out

1) use a gas syringe to measure the volume of gas given off 2) the more has given off during a set time interval, the faster the reaction 3) you can tell the reaction has finished when no more gas is produced 4) plot a graph of gas volume against time elapsed 5) make sure you use the right size gas syringe- if the reaction is too vigourous, you can blow the plunger out of the end of the syringe

1) decomposition of hydrogen peroxide: 2H2O2(aq) -> 2H2O(l) + O2 (g) 2) use of manganese (IV) oxide (MnO2) or copper (II) oxide (CuO) or zinc oxide (ZnO) 3) measure oxygen given off using a gas syringe at regular time intervals 4) change catalyst, control mass of catalyst used