C5.2 Controlling Reactions

Measure of how quickly reactants are used or products are formed Rate of reaction = amount of reactant used / time taken Rate of reaction = amount of product formed / time taken

Example: Magnesium reacts with dilute hydrochloric acid to form magnesium chloride solution and hydrogen: Mg(s) + 2HCI(aq) -> MgCl2(aq) + H2(g) Measure how fast hydrogen is produced by carrying out the following steps: Place dilute hydrochloric acid in a conical flask connected to a gas syringe Add a piece of magnesium ribbon into the acid, stopper the flask, and start a stop clock immediately Draw a results table and record the time and volume of hydrogen at regular intervals Analyse your results by drawing a line graph (curved) Before adding the stopper to the conical flask in this investigation, you should push the plunger all the way in, to make sure the reading starts at 0 cm³ You also need to make sure that you stopper the flask as soon as the reaction starts, to make sure all the hydrogen is collected

Example: Calculate the mean rate of reaction between 60s and 90s 1) Calculate the changes in volume and time Change in volume = (50 cm³ - 38 cm³) = 12 cm³ Change in time = (90s - 60s) = 30s 2) Calculate the mean gradient (equal to the mean rate of reaction) Change in volume / change in time = 12cm³ / 30s = 0.4cm³/s

Example: Find the rate of reaction at 75 seconds In this example, a tangent to the curve has been drawn at 75s Instantaneous rate of reaction at this time = gradient of the tangent

A reaction can only happen if: Reactant particles collide with each other, and colliding particles have enough energy to react Collision that leads to a reaction = successful collision Collision will not be successful if particles have less energy than the activation energy As temperature of a reaction mixture increases: Particles move more quickly, so they collide more often Greater proportion of the colliding particles have the activation energy or more The greater the rate of successful collisions, the greater the rate of reaction

Example: Reaction between sodium thiosulfate solution and hydrochloric acid is often used to investigate the factors affecting rates of reaction: Na2S2O3(aq) + 2HCI(aq) -+ 2NaCI(aq) + H2O(l) + SO2(g) + S(s) The equation above looks complicated, but you can easily measure how quickly a pale yellow precipitate of sulfur appears: 1 In a beaker, mix the reactants 2 Look down through the mixture to a cross drawn on a piece of paper: The longer it takes for the cross to disappear, the lower the rate of reaction 3 You can vary the temperature of the reaction mixture by warming up one of the solutions before mixing Reaction will start as soon as the sodium thiosulfate solution and dilute hydrochloric acid are mixed together This means that you should start the stop clock as soon as you mix the solutions You should stop the clock as soon as the cross disappears, so that the timing will stop at the same point in all the reactions

Rate of reaction is inversely proportional to the reaction time This means that 1 / time is directly proportional to the rate: 1 / reaction time ∝ rate of reaction You can plot a graph with rates of reaction instead of reaction times, if you calculate 1 / time for each result: For example, a reaction time of 10s gives a rate of 1 / 10s = 0.1/s Sometimes 1000/time is used instead of 1/time, as this can make the vertical axis scale easier to work out

Concentration of a solution is measure of how much solute is dissolved in the solvent The more concentrated a solution is, the more solute is dissolved in the solvent If a reaction involves one or more reactants in solution, rate of reaction increases as the concentration increases: The particles become more crowded, so they collide more often The energy stored in the particles does not change, but because the rate of collisions increases, rate of successful collisions increases

Reaction between magnesium ribbon and hydrochloric acid is often used to investigate rates of reaction Mg(s) + 2HCI(aq) - MgCI2(aq) + H2(g) You could use a gas syringe to measure the volume of hydrogen gas produced However, it is easier to measure the time taken for a piece of magnesium ribbon to be used up in the reaction You can add water to the hydrochloric acid to reduce its concentration When you have recorded the reaction times at different concentrations of acid, you can calculate 1/time to obtain the reaction rates When carrying out this investigation, the length of the magnesium ribbon needs to be controlled because the reaction time is affected by the mass (and therefore size) of the piece of magnesium, so this must be kept the same to make it a fair test

In a reaction involving 2 reactants, one of the reactants will be limiting and the other in excess Amount of product will be proportional to amount of limiting reactant

If a reaction involves 1 or more reactants in gas state: Rate of reaction increases as the pressure of the gas increases: Particles in the gas state become more crowded, so they collide more often Energy stored in particles does not change but, because the rate of collisions increases, the rate of successful collisions increases Industrial chemical processes often use high pressures to achieve high rates of reaction

Particles in a substance in the solid state can only vibrate about fixed positions: they cannot move from place to place This means that only the particles at the surface can take part in collisions Rate of reaction increases as surface area increases because: More reactant particles are available for collisions Collisions are more likely, so particles collide more often Energy stored in the particles does not change but, because the rate of collisions increases, rate of successful collisions increases Powders have an even larger surface area than lumps, so their reactions are very fast What matters is surface area : volume ratio of the lumps: As the size of the lumps decreases, surface area : volume ratio increases

Reaction between calcium carbonate and dilute hydrochloric acid is often used to investigate rates of reaction: CaCO3 (s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) You could use a gas syringe to measure the volume of carbon dioxide gas produced, but another method involves weighing the reaction mixture: Add dilute hydrochloric acid to a conical flask and stopper it with cotton wool Place flask on a balance Tare (counterweight) the balance, remove the cotton wool, drop the calcium carbonate into the acid, start a stop clock, and replace the cotton wool The mass goes down as carbon dioxide escapes, so the balance reading must be recorded at regular intervals You can repeat the experiment with different sized lumps of calcium carbonate or with calcium carbonate powder Diagram: . |*| — cotton wool . / \ — conical flask . /_____ \ — hydrochloric acid . / ** \ —calcium carbonate ‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾ | |-0.449| | — balance ______________

Substance which increases rate of reaction but is unchanged at the end of the reaction Catalysts provide an alternate reaction pathway with a decreased activation energy If you add 1g of catalyst to a reaction mixture, there will still be 1g left when the reaction has finished Catalysts are specific to particular reactions: a substance that acts as a catalyst for one reaction may not catalyse a different reaction Small amount of catalyst can catalyse reaction between large amounts of reactants Helpful because catalytic converters use platinum, rhodium, and palladium These metals very expensive so are coated onto an inert ceramic ‘honeycomb’ This uses only a few grams of catalyst, provides a large surface area for the reactions, and allows exhaust gases out

Example: Powdered manganese(IV) oxide, MnO,, catalyses the decomposition of hydrogen peroxide: 2H2O2 (aq) -> 2H2O(l) + O2(g) Place hydrogen peroxide solution in a conical flask connected to a gas syringe Put a little manganese(IV) oxide into the flask, stopper it, and start a stop clock Only small amounts should be added, as gas can be produced very quickly and violently Record the time and volume of oxygen at regular intervals Draw a graph to analyse your results You could vary the mass of catalyst or try other catalysts When investigating a catalyst, you should keep volume, temperature, and concentration of hydrogen peroxide the same This due to volume of oxygen produced is affected by these variables, so they must be kept the same to make it fair test

Enzymes Proteins that act as catalysts in biological systems Different enzymes catalyse different reactions Example: liver cells contain an enzyme called catalase - catalyses the decomposition of hydrogen peroxide

Catalyst = substance that ‘catalyses’ or speeds up a reaction If a substance acts as a catalyst, it shows catalytic activity

Provide alternative reaction pathway with a lower activation energy Amount of energy stored in reactant particles does not change but in presence of a catalyst: Greater proportion of colliding particles have the activation energy or above Rate of successful collisions increases compared to the rate in an uncatalysed reaction Graph: Energy | __ | / ↑ \ — activation energy without catalyst |Hydrogen peroxide /‾‾‾‾‾‾‾ \ — activation energy with catalyst |_____________________| ↑ | — activation energy with catalyst | |water + oxygen | |_________________ |_______________________________________________ . Progress of reaction