Rates of Reaction
What is a rate of reaction?
The amount of change of a product or reactant in a given time.
What affects the rate?
a) Concentration of solution (more likely to collide as more in same vol.) b) Temperature (increases speed of molecules, more exceed EA)
c) Surface area (same as concentration)
d) Pressure of gasses (same as conc.)
e) Catalyst (guide particles to correct orientation + lowers EA)
f) Others e.g. light(light = increase in speed of particles)
How are rates altered?
Collision theory:
Particles must collide with an energy exceeding the EA and they must have the correct orientation.
A reaction rate increased by,
- Increasing collision frequency
- Increasing average kinetic energy of particles
- Lowering activation energy
- Giving molecules correct orientation
Effect of concentration on rate
rA = rate of reaction with respect to A
E.g. RH+ = rate of reaction with respect to H+(aq)
Rate of reaction = K [A]a [B]b [C]c
Units of rates of reaction = mol dm-3 s-1
Units of concentration = mol dm-3
Indices a, b, c etc: order of reaction - what is the effect on the rate of doubling the concentration
E.g. [H+]1 = 1st order = if conc. is doubled, rate is doubled
[H+]2 = 2nd order = if con. is double, rate is quadrupled
[H+]0 = zero order = if conc. is doubled, rate is unaffected
The constant, K
K = rate constant. This is a constant of proportionality
Units may vary but must include s-1
rA = k[A]1[B]1
mol dm-3 s-1‑ = k (mol dm-3)(mol dm-3)
for mol dm-3 s-1 to = mol dm-3 s-1
because k must = dm3 mol-1 s-1
because (mol dm-3)(mol dm-3) = mol2 dm-6, so to equal this out,
k = dm3 mol-1 s-1
Expt. 11.2a the kinetics of the reaction between calcium carbonate and hydrochloric acid
CaCO3(s) + 2HCl(aq) ® CaCl2(aq) + CO2(g) + H2O(l)
Collect gas given off in a syringe.
Create a table:
Time (s)
Volume of CO2 Vt (cm3)
(Vfinal - Vt) cm3
Then to find the order, find the half - life if time periods are roughly the same between each half - life then it is first order. If it is second order the graph would increase and not be constant.
Methods of following a reaction
By titration
E.g. reaction between iodine and propane in acid solution.
Then on a graph time can be plotted against titre values, in this case a straight line is seen, suggesting the reaction to be zero order
By colorimetry
Can be used when one of the reactants or products have a colour, the intensity changing during the reaction. The intensity can be followed using a special colorimeter.
By Dilatometry
Where during the reaction the total volume of the solution changes.
4. The reason why the position of the equilibrium was unaffected in this reaction because of reaction:
Investigating Rates of Reactions My aim is to investigate what factors affect the rate of reaction between magnesium and hydrochloric acid. The factor that I will be focusing on is the concentration of Hydrochloric acid. [IMAGE]Reaction Equation: Mg (s) +2HCl (aq) = MgCl2 (aq) +H2 (g)
What does any kind of order have to do with math? It’s just left to right isn’t it? Wrong, the order of operations is a specific method to figuring out the correct answer to certain problems. For those of you who do not realize what I am yammering on about, this procedure piece is about the order of operations.
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The rate of reaction is how quickly or slowly reactants in chemical reactants turn into products. A low reaction rate is when the reaction takes a long time to take place; hence, a reaction that occurs quickly has a high reaction rate. A rate refers to how slow or quick the product is produced. It is possible to control the rate of chemical reactions and speed up or slow down the rate of chemical reactions by altering three main factors which are temperature, concentration and the surface area. When the temperature of the reactants increases, the molecules vibrate at a more intense speed therefore colliding with each other more frequently and with increased energy resulting in a greater rate of reaction. Accordingly, as the temperature decreases the molecules will move slower, colliding less frequently and with decreased energy resulting in the rate of reaction decreasing. Concentration is how much solute is dissolved into a solution and is also a factor that affects the rate of reaction. When the concentration is greater this means there is an increased amount of reactant atoms and molecules resulting in a higher chance that collisions between molecules will occur. A higher collision rate means a higher reaction rate. Consequently at lower concentrations there are reduced chances of the molecules colliding resulting in a lower reaction rate. The measurement of how much an area of a solid is exposed is called the surface area. The quicker a reaction will occur the more finely divided the solid is. For example, a powdered solid will usually have a greater rate of reaction in comparison to a solid lump that contains the same mass for it has a lower surface area than the powdered solid.