Core Practical: Investigating Rate of Reaction
Specification Point 7.1:
Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by:
a) measuring the production of a gas (in the reaction between hydrochloric acid and marble chips)
b) observing a colour change (in the reaction between sodium thiosulfate and hydrochloric acid)
Effect of Surface Area
- To Investigate the effect of changing surface area of the reactant on the rate of reaction.
- Dilute hydrochloric acid, calcium carbonate chips of varying sizes
- Conical flask, balance, bung with hole, gas tubing, gas jar, beehive shelf, water trough, stopwatch
Diagram showing the process of downwards displacement to investigate reaction rate
- Add around 100cm3 of HCl into the conical flask.
- Use a gas tube to connect the flask to a measuring cylinder upside down in a bucket of water (downwards displacement).
- Record a mass of calcium carbonate chips and add to the conical flask and close the bung quickly.
- Carbon dioxide gas is produced:
2HCl + CaCO3 → CaCl2 + H2O + CO2
- Measure the volume of gas produced in a fixed time using the measuring cylinder.
- Neatly record the time and rough size of the carbonate chips.
- Repeat with same volume of acid but different sizes of calcium carbonate chips (solid, crushed and powdered) but using the same mass.
Analysis of Results:
- Plot a bar graph with the sizes of the chips on the x-axis and the volume of gas produced on the y-axis.
- The smaller the chips used (but with the same mass) the larger the volume of gas produced in the same time.
As there is a larger surface area of the solid reactant then more of it is exposed to the other reactant so there will be collisions more frequently, increasing the rate of reaction.
Effect of Concentration of Reactant
- To Investigate the effect of changing concentration on the rate of reaction.
- Concentrated sodium thiosulfate solution, dilute hydrochloric acid
- Conical flask, stopwatch
Diagram showing the apparatus needed to investigate reaction rate in a precipitation reaction
- Measure 50 cm3 of Na2S2O3 solution into a flask.
- Measure 5 cm3 of dilute HCl into a measuring cylinder.
- Draw a dark cross on a piece of clean white paper and put it underneath the flask.
- Add the acid into the flask and immediately start the stopwatch.
- Solid sulfur is formed which precipitates in solution, turning cloudy:
Na2S2O3 + 2HCl → 2NaCl + S + SO2 + H2O
- Look down at the cross from above and stop the stopwatch when the cross can no longer be seen
- Repeat using different concentrations of sodium thiosulfate solution (mix different volumes of sodium thiosulfate solution with water to dilute it).
Analysis of Results:
- Plot a graph of the results with time on the x-axis and the concentration of Na2S2O3 on the y-axis.
- With an increase in the concentration of a solution, the rate of reaction will increase so the time for the cross to disappear decreases.
- As there are more reactant particles in a given volume collisions occur more frequently, increasing the rate of reaction.
Determining the Rate of Reaction
Specification Point 7.2:
Suggest practical methods for determining the rate of a given reaction.
- Rates of reaction can be measured either by how fast a reactant is used up or by how fast the product is made.
- Rate is concerned with amounts of substances and time and can be calculated using the formula:
- Several measurements need to be made during the reaction which makes determining the reaction rate a technically difficult procedure.
- The product is usually the one that is chosen as practically it is easier to measure a product forming than a reactant disappearing.
- The quantity to be measured depends on the reaction and may be in grams for mass or cm3 or dm3 for volume if the product is a gas.
- The units of the rate of reaction would therefore be g s-1 or cm3 / dm3 s-1.
- Time is usually in seconds as most reactions naturally occur fast.
- If one of the products is a gas which is given off, then the reaction can be performed in an open flask on a balance to measure the loss in mass of reactant.
- Cotton wool is usually placed in the mouth of the flask which allows gas out but prevents any from being ejected from the flask (if the reaction is vigorous).
- Alternatively the gas could be captured in a gas syringe which measures its volume.
Diagram of a gas syringe used to determine the rate of reaction
- Precipitation reactions form a solid precipitate when two clear solutions are mixed together.
- The precipitate clouds the reaction mixture so if the flask is placed over a piece of paper with a cross on it, the time it takes for the cross to disappear from view (due to the formation of the precipitate) can be measured.
- This method is susceptible to error though, as different people may not agree on the exact moment that the cross disappears.
Specification Point 7.3:
Explain how reactions occur when particles collide and that rates of reaction are increased when the frequency and/or energy of collisions is increased.
- Collision theory states that chemical reactions occur only when the reactant particles collide with sufficient energy to react.
- The minimum amount of energy needed is called the activation energy, which is different for each reaction.
- Particles that collide with insufficient energy have unsuccessful collisions and just bounce off each other.
- The rate of a reaction is therefore also dependent on the energy of collisions as well as the number of collisions.
- To increase the rate of a reaction then the number of successful collisions needs to be increased.
- This can be done by:
- Increasing temperature
- Increasing pressure
- Increasing concentration of reactants
- Increase the surface area of the reactants
- Use of a catalyst
The particles in the top collision do not collide with sufficient energy to produce a product whereas the bottom ones do
Edexcel GCSE Chemistry Notes
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