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Investigating rates of reaction

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Explainer Video

Tutor: Lana

Summary

Investigating rates of reaction

In a nutshell

Many factors affect the rate of a reaction including changes in surface area, concentration and temperature. You can investigate the effect of these factors on the rates of chemical reactions through experiments which involve measuring gas production and observing changes in colour.

Variable definitions

QUANtity nAME	SYMBOL	UNIT NAME	UNIT
$mass$	$m$	$gram$	$g$
$volume$	$V$	$mililitre$	$ml$
$concentration$	$c_B$	$mole \, per \,cubic \, decimetre$	$mol \, dm^{-3}$
$time$	$t$	$second$	$s$
$temperature$	$T$	$degree \space celsius$	$^{\circ}C$

Equipment list

The following equipment list can be used to conduct experiments to measure the rate of reactions.

Equipment	Use
Conical flasks	To carry out a reaction in
Rubber bung	To cover a conical flask and prevent gas from escaping
Glass tubing	To connect the conical flask to the gas syringe
Glass syringe	To measure amount of gas formed
Hydrochloric acid $(0.1 \, M, \space 0.2 \, M, \space 0.5\, M)$	Reactant
Marble chips	Reactant
Powdered chalk	Reactant
Thermometer	To measure temperature
Water bath	To heat solutions
Sodium thiosulfate $(0.2 \,M)$	Reactant
Paper and marker	To create a mark for a precipitation reaction
Stopwatch	To record time taken for reaction

Experiment 1: Investigating the effect of surface area on rate of reaction

Marble chips/chalk contains calcium carbonate $(CaCO_3)$ and reacts with hydrochloric acid to produce carbon dioxide gas in a neutralisation reaction.

$calcium\, carbonate \, + \,hydrochloric \, acid \rightarrow calcium \, chloride\, + \, carbon \, dioxide + \, water$

$CaCO_3(s) \, + \, 2HCl(aq) \rightarrow CaCl_2(aq) \, + \, CO_2(g) \, +\, H_2O(l)$

Experimental variables

The independent variable is the one that you change to investigate how it alters the rate of reaction. The dependent variable is the one that you measure; it will be affected by changing the independent variable. A control variable is a variable that is unchanged through the experiment.

Independent variable	$Surface\ area$
Dependent variable	$Volume\ of\ gas\ formed$
Control variables	$Concentration\ and\ volume\ of\ HCl\ acid,\ mass\ of\ solid\ and\ temperature$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

Hazard	Risk	Safety measure
Hydrochloric acid	May burn skin and damage eyes if it comes into contact.	Wear lab coats, goggles and gloves throughout the experiment.
Glassware	May break and cause cuts.	Set up the experiment in the centre of the workbench and tuck away all stalls/belongings to avoid tripping up.

Chemistry; Chemistry practicals; KS4 Year 10; Investigating rates of reaction

1.	Marble chips $(CaCO_3)$
2.	$CO_2$ gas
3.	$HCl$

Method

To carry out the experiment follow the instructions below.

1.	Transfer $0.5 \,g$ of marble chips into a conical flask.
2.	Measure out $50 \,ml$ of $0.1 \,M \space HCl$ and add this to the conical flask. Quickly place the rubber bung on the conical flask. The rubber bung must be attached to the gas syringe using glass tubing as shown in the diagram above.
3.	Measure and record the volume of $CO_2$ produced every 20 seconds for five minutes using the gas syringe.
4.	Repeat the experiment again, using crushed marble chips. The volume of $HCl$ , concentration of $HCl$ and the mass of marble chips must be the same as the previous experiment.
5.	Repeat the experiment again, this time with powdered chalk.

Analysis

To observe the effects of surface area on rate of reaction the data is plotted on a graph.

1.	Plot the results of each experiment on a graph, with time on the $x$ -axis and volume of gas produced on the $y$ -axis.
2.	Connect the plots for each experiment to produce three lines.

Your graph should look something like this:

1.	Whole marble chips experiment
2.	Crushed marble chips experiment
3.	Powdered chalk experiment
4.	Double quantity of marble chips

Conclusion

When the line on the graph plateaus, it indicates that gas production stops and the reaction has ended. The powdered chalk experiment should have the steepest gradient and end first; indicating the fastest rate of reaction. This is because powdered chalk has the highest surface area. The experiment with whole marble chips should take the longest to end; indicating the slowest rate of reaction.

Evaluation

Once completing the experiment it is important to you check if there are any anomalies in the graph. If there are anomalous result(s), have a think about why this may be and whether they are down to external factors or how you carried out the experiment. If your trends do not match the graph above you may be required to repeat the experiment.

Experiment 2: Investigating the effect of concentration on rate of reaction

Changing the concentration of solutions can also affect the rate of reaction.

Experimental variables

Independent variable	$Concentration\ of\ HCl\ acid$
Dependent variable	$Volume\ of\ gas\ formed$
Control variables	$Volume\ of\ HCl\ acid,\ mass\ of\ solid\ and\ temperature$

Method

You should repeat experiment one, but this time you should not vary the size of the solid and should only use whole marble chips. You should then vary the concentration of $HCl$ acid in each experiment; use $0.1 \,M, 0.2 \,M$ and $0.5 \,M$ acid.

Analysis

Repeat the same analysis as experiment one. The lines now represent different concentrations of acid instead of different particle sizes.

1.	Lowest acid concentration
2.	Middle acid concentration
3.	Highest acid concentration

Conclusion

The experiment with the highest concentration of acid should have the steepest line and should come to an end first as it has the fastest rate of reaction.

Evaluation

Carry out the same evaluation to this set of data as you did to experiment one.

Experiment 3: Investigating the effect of temperature on rate of reaction

To investigate the rate of reaction you can carry out a precipitation reaction. Sodium thiosulfate and hydrochloric acid, both colourless solutions, react to form sulfur, a yellow precipitate. The amount of time it takes for the precipitate to form can be used to measure the rate of reaction.

$NaS_2O_3(aq) \, + \, 2HCl(aq) \longrightarrow SO_2(aq) \, + \, H_2O(l) \, +\, 2NaCl(aq)$

$sodium \, thiosulphate \, + \,hydrochloric \, acid \rightarrow sulphur \, dioxide \, + \, sulphur \, + \, water \, +\, sodium \, chloride$

$Na_2S_2O_3(aq) \, + \, 2HCl(aq) \rightarrow 2NaCl(aq) \, + \, SO_2(g) \, + \, H_2O(l)+S(s)$

Experimental variables

Independent variable	$Temperature$
Dependent variable	$Time\ taken\ for\ precipitate\ to\ form$
Control variables	$Volume, concentration\ and\ depth\ of\ solutions$

Safety precautions

When performing experiments, it is very important you consider safety precautions. This is so you and others do not get hurt.

HAZARD	RISK	SAFETY MEASURE
Hydrochloric acid	May burn skin and damage eyes if it comes into contact.	Wear lab coats, goggles and gloves throughout the experiment.
Sodium thiosulfate	May irritate eyes/skin if it comes into contact.	Wear lab coats, goggles and gloves throughout the experiment.
Sulfur dioxide	Toxic when inhaled.	Ensure that the lab is well ventilated.
Sulfur	May irritate nose and throat. Easily ignites.	Wear lab coats, goggles and gloves throughout the experiment. Ensure that the lab is well ventilated. Ensure there are no naked flames nearby.
Glassware	May break and cause cuts.	Set up the experiment in the centre of the workbench and tuck away all stalls/belongings to avoid tripping up.

Method

To carry out the experiment follow the instructions below.

1.	Transfer a fixed volume of sodium thiosulfate to a conical flask and record the temperature.
2.	Place the conical flask on a piece of paper marked with a black cross as shown by the diagram.
3.	Measure out an equal volume of hydrochloric acid.
4.	Transfer the hydrochloric acid to the conical flask containing the sodium thiosulfate and start the timer. The mixture will start to turn yellow and cloudy due to the formation of sulfur precipitate. Record the time it takes for the black mark to disappear.
5.	Repeat the experiment again at four other temperatures. Use a water bath to heat the two solutions separately to the desired temperature then mix them. It is important the the volume, concentration and depth of the liquids remain the same throughout the experiment.

Analysis

To observe the effects of surface area on rate of reaction the data has to be plotted on a graph.

1.	Plot a graph with temperature on the $x$ -axis and time taken for black cross to disappear on the $y$ -axis.
2.	Draw a line of best fit on the graph.

Your graph should look something like this:

1.	Lowest temperature
2.	Highest temperature

Conclusion

The experiment carried out at the lowest temperature should take the longest time to cover the cross mark, as it has the slowest rate of reaction. As temperature increases, the rate of reaction increases and the reaction time is shorter.

Evaluation

Look at the graph to see if all the results match the line of best fit. If there is an anomaly, repeat the experiment again at the given temperature. If you have more time it is worth carrying out the reaction at each temperature three times, and plotting the mean time taken for the cross to disappear on the graph.

Learn with Basics

Length:

Unit 1

Finding the gradient of a straight line

Unit 2

Calculating rates of reactions

Jump Ahead

Unit 3