Chemistry 2b- Reaction rates, Salts and Electrolysis

The Rate of a Reaction Depends on Four Things:

• Temperature
• Concentration ----> (or pressure for gases)
• Catalyst 
• Surface area of solids ----> (or size of solid pieces)

The plot below shows how the rate of a particular reaction varies under different conditions.           The quickest reaction is shown by the line with the steepest slope.                                                 Also, the faster a reaction goes, the sooner it finishes, which means that the line becomes flat earlier.

 1) Graph 1 represents the original fairly slow                                                                                                                      reaction. The graph is not too steep.

 2) Graphs 2 and 3 represent the reaction taking                                                                                                                  place quicker but with the same initial amounts.                                                                                                              The slope of the graphs gets steeper.

3) Grap 4 produces more product as well as going                                                                                                               faster. This can only happen if more reactants                                                                                                                  are added at the start. Graphs 1, 2 and 3 all                                                                                                                   converge at the same level, showing that they                                                                                                                     all produce the same amount of product, although                                                                                                          they take different times to get there.

• The rate of a reaction can be observed either by measuring how quickly the reactants are used up or how quickly the products are formed. It's usually a lot easier to measure products forming. The rate of reaction can be calculated using the following formula: 

Rate of Reaction = Amount of reactant used or amount of product formed

                          Time

• There are different ways that the rate of a reaction can be measured.

• This is when the product of the reaction is a precipitate which clouds the solution.
• Observe a mark through the solution and measure how long it takes for it to disappear.
• The quicker the mark disappears, the quicker the reaction. 
• This only works for reactions where the initial solution is rather see-through.
• The result is very subjective- different people might not agree over the exact point when the 'mark' disappears.

• Measuring the speed of a reaction that produces a gas can be carried out on a mass balance.
• As the gas is released the mass disappearing is easily measured on the balance.
• The quicker the reading on the balance drops, the faster thr reaction. 
• Rate of reaction graphs are particularly easy to plot using the results from this method. 
• This is the most accurate of the three methods because the mass balance is very accurate. But it has the disadvantage of releasing the gas straight into the room.
  

• This involves the use of 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. 
• A graph of gas volume against time elapsed could be plotted to give a rate of reaction graph.
• Gas syringes usually give volumes accurate to the nearest millimetre, so they're quite accurate. 

• 1) REACTION OF HYDROCHLORIC ACID AND MARBLE CHIPS

• Measure the volume of gas evolved with a gas syringe and take readings at regular intervals.
• Make a table of readings and plot them as a graph. You choose regular time intervals, and time goes on the x-axis and volume goes on the y-axis. 
• Repeat the experiment with exactly the same volume of acid, and exactly the same mass of marble chips, but with the marble more crunched up. 
• Then repeat with the same mass of powdered chalk instead of marble chips.

• Using finer particles means that the marble has a larger surface area.
• A larger surface area causes more frequent collisions so the rate of reaction is faster. 
• The extra surface area gives a quicker reaction and there is also more gas evolved.

• 2) REACTION OF MAGNESIUM METAL WITH DILUTE HCl

• This reaction is good for measuring the effects of increased concentration ( as the marble/acid reaction).
• This reaction gives of hydrogen gas, which we can measure with a mass balance. 
• In this experiment, time also goes on the x-axis and volume goes on the y-axis.
• Take readings of mass at regular intervals.
• Put the results in a table and work out the loss in mass for each reading. Plot a graph.
• Repeat with more concentrated acid solutions, but always with the same amount of magnesium. 
• The volume of the acid must always be kept the same too- only the concentration is increased. 

3) Sodium Thiosulfate and HCl produce a cloudy precipitate

• These two chemicals are both clear solutions.
• They react together to form a yellow precipitate of sulfur.
• The experiment involves watching a black mark disappear through the cloudy sulfur and timing how long it takes to go.
• The reaction can be repeated for solutions at different temperatures. In practise, that's quite hard to do accurately and safely. The best way to do it is to use a water bath to heat both solutions to the right temperature before you mix them. 
• The depth of the liuid must be kept the same each time.
• The results will show that the higher the temperature the quicker the reaction  and therefore the less tme it takes for the mark to disappear. 

4) The Decomposition of Hydrogen Peroxide

• This is normally quite slow but maganese oxide catalyst speeds it up.
• Oxygen gas is given off, which provides an ideal way to measure the rate of reaction using the gas syringe method.
• Better catalysts give a quicker reaction, which is shown by a steeper graoh which levels off quickly.
• This reaction can also be used to measure the effects of temperature, or of concentration of the H2O2 solution. 

Temperature 

• More kinetic energy --> More frequent, successful collisions.

Surface Area

• Increase of surface area--> More frequent, successful collisions
• More surface area- chopped up- more particles availiable to collide.

Concentration

• More particles in the same volume--> More frequent, successful collisions

• Higher temperature also increases the energy of the collisions, because it makes all the particles move faster. 
• Reactions only happen if the particles collide with enough energy.
• The minimum amount of energy needed by particles to react is known as the activation energy. 
• At a higher temperature there will be more particles colliding with enough energy to make the reaction  happen.

• Many reactions can be speeded up adding a catalyst. 

A catalyst is a substance which speeds up a reaction without being changed or used up in the reaction.

• A solid catalyst works by giving the reacting particles a surface to stick to.
• This increases the number of successful collisions.

• Catalysts are very important for commercial reasons.
• Catalysts increase the rate of the reaction, which saves a lot of money because the plant does not need to operate for as long to produce the same amount of stuff.
• A catalyst will allow the reaction to work at a much lower temperature. That reduces the energy used up in the reaction, which is good for sustainable development.
• They can be very expensive to buy, and often need to be removed from the product and cleaned. They never get used up in the reaction, so once you've got them you can use them over and over again. 
• Different reactions use different catalysts, so if you make more than one product at your plant, you'll probably need to buy different catalysts for them. 
• Catalysts can be poisoned by impurities, so they stop working, e.g. sufur impurities can poison the iron catalyst used in the Haber process. That means you have to keep your reaction mixture very clean.

An Exothermic reaction is one which transfers energy to the suroundings, usually in the form of heat and usually shown by a rise in temperature. 

• An example of an exothermic reaction is burning fuels- COMBUSTION.
• Neutralisation reactions are also exothermic.
• Many oxidation reactions are exothermic. For example, adding sodium to water produces heat, so it ust be exothermic. The sodium emits heat and moves about on the surface if the water as it is oxidised. 
• Exothermic reactions have lots of everyday uses. For example, some hand warmers use the exothermic oxidation of iron in air to generate heat. 

An Endothermic reaction is one which takes in energy from the surroundings, usually in the form of heat and is usually shown by a fall in temperature.

• Endothermic reactions are less common. Thermal decomposition is a good example. 
• Sports injury packs use endothermic reactions- they take in heat and the pack becomes very cold.

Reversible Reactions can be endothermic and exothermic

• In reversible reactions, if the reaction is endothermic in one direction, it will be endothermic in the other direction. The energy absorbed by the endothermic reaction is equal to the energy released during the exothermic reaction. 
• A good example is the thermal decomposition of hydrated copper sulfate. 
• If you heat blue hydrated copper sulfate crystals it drives the water off and leaves white anhydrous copper sulfate powder. This is endothermic.
• If you then add a couple of drops of water to the white powder you get the blue crystals back again - this is exothermic.