Lesmahagow High School Reactions and Speed

Lesmahagow High SchoolS3 Chemistry

Chemical Reactions and their Speed

Indicators of chemical reactions. Controlling the rate of reactions.SCN 3-19a

Reactions monitored and graphs interpreted.National 4

Page 1 of 10

Lesmahagow High School Reactions and SpeedLearning Outcomes – Rates Of Reaction

Circle a face to show how much understanding you have of each statement: if you fully understand enough to do what the outcome says, if you have some understanding of the statement, and if you do not yet understand enough to do what the statement says. Once you have completed this, you will be able to tell which parts of the topic that you need to revise, by either looking at your notes again or by asking for an explanation from your teacher or classmates.

Level 3 Learning Outcomes

By the end of this topic I will be able to:

1. State the difference between a chemical and aphysical change.

2. Change the speed of a chemical reaction by altering one of three variables: temperature,concentration or particle size of the reactants.

3. Explain the difference between deciding which is'faster' and accurately measuring 'rate' of a chemical reaction.

Level 4 Learning Outcomes

By the end of this topic I will be able to:

1. Plan and carry out an investigation into the effectof concentration on the reaction of acid with magnesium ribbon.

2. Give the definition of a “variable”.

3. State what must be done to control variables during investigations.

4. Select the appropriate fair test conditions to investigate the effect of a given variable on reaction rate.

Page 2 of 10

Lesmahagow High School Reactions and Speed5. Present experimental data in the form of an

appropriate table.

6. Select the appropriate type of graph to draw for any given data set.

7. Understand and apply the PLUS checklist to prepare a “good” graph.

8. Describe a range of ways in which the rate of reactions can be monitored.

10. Interpret and understand the meaning of data from graphs of reaction rate.

11. Compare the rates of different reactions using the steepness of graph slopes.

Page 3 of 10

Lesmahagow High School Reactions and SpeedRate of Reactions Notes

Level 3 Notes

Physical and Chemical Changes.

Physical changes occur when a material is boiled, condensed, melted or frozen. No new substance is made, but the material changes physical state. This also applies to bread being sliced or wood being chopped.

During a chemical reaction, there can be:1. A change in colour2. A detectable energy change - heat or light

emitted.3. A gas and/or solid formed - shown by bubbles or

a precipitate.

ALL chemical reactions involve the formation of one or more new substances.

Many chemical reactions happen in our everyday lives. Some examples are the digestion of food, the rusting of iron, and the burning of petrol in a car engine.

The Rate of Chemical Reactions

Chemical reactions do not all occur at the same rate. In the lab, the reaction of sodium sulphate solution and barium chloride solution to form a white solid (precipitate) is almost instantaneous but the reaction of magnesium with dilute hydrochloric acid to form bubbles of gas takes place over minutes. The reaction of aluminium with the oxygen of the air is a lot slower.

Everyday reactions are no different; some are over in a fraction of a second (fast: like a gas explosion) while others can take years (slow: like the rusting of iron). Most reactions occur at rates between these two extremes (medium: like a cake baking).

Page 4 of 10

Lesmahagow High School Reactions and SpeedFactors Affecting the Rate of a Reaction

There are three main factors affecting the rate of a chemical reaction:

a) Particle Size:The smaller the particles, the faster the reaction. This is because smaller particles provide more surface area for collision. Example – Marble powder reacts faster with acid than marble chips.

b) Concentration:The higher the concentration, the faster the reaction. The higher the concentration of solutions, the more particles you havecrowded into a small volume of liquid. Hence, the more likely they are to collide with each other.Example – 2 mol/l hydrochloric acid reacts faster with magnesium ribbon than 1 mol/l hydrochloric acid.

c) Temperature:Although a higher temperature will cause molecules to move faster, and there may be more collisions, this is not the main reason why higher temperature increases reaction rate. The main reason is that more of the collisions which occur will lead to a successful reaction. This is because at higher temperature, more particles have the activation energy required for a reaction to happen.As a rough guide, the rate of reaction doubles for every increase in temperature by 10OC.Example - Benedicts solution reacts faster with glucose solution at 50OC than at 25OC.

Experimental InvestigationsExperimental investigations should have the following key parts as a minimum:

Aim – What you are trying to do/ find out in the investigation.Hypothesis – What you think will happen in the investigation.Materials & Methods – What equipment you

Page 5 of 10

Lesmahagow High School Reactions and Speedneed to carry out the experiment and the exact instructions for what you will do in the investigation. Including any necessary safety precautions.

Results – What measurements or observations you made during the investigation. It is normal to have a table of data and the appropriate type of graph in this section.

Conclusion – What you found out expressed in terms of your hypothesis.

It is sometimes useful to have an Evaluation section in which you suggest future improvements you could make to the investigation (but this is usually only included when you design the experiment yourself).

Variables

A variable is something you can change in an experiment. When carrying out investigations on how different factors affect reaction rate it is very important that only one experimental variable is altered at a time.

e.g. When investigation how 3 different concentrations of hydrochloric acid affect the rate of reaction with magnesium only the concentration of acid should be different in each reaction tube. Everything else (i.e. the mass of magnesium, particle size of magnesium, type of

acid, volume of acid) must be identical.

Level 4/National 4 Notes

Drawing GraphsYou must be able to select the appropriate type of graph for your data.

Page 6 of 10

Lesmahagow High School Reactions and Speed If you have two sets of number information

in your data table you should draw a LINE GRAPH.

e.g. Concentration of acid (mol/l) vs Time to react (seconds).

If you have one set of number information and one set of word-based information in your data table you should draw a BAR GRAPH.

e.g. Type of acid vs Time to react (seconds)

Use the PLUSS checklist to help you remember the key features of a good graph.

Points Labels Units SizeScale

Following the Rate of a Chemical Reaction

As a chemical reaction proceeds, reactants are being used up while products are being formed. The rate at which this happens can be followed by measuring the change in a 'property' of a substance involved in the reaction over a period of time,

e.g. In the reaction of calcium carbonate with dilute hydrochloric acid:

CaCO3 (s) + HCl (aq) CaCl2 (aq) + H20 (l) + CO2 (g)

The change in mass (due to gaseous product being given off), volume of gas produced, concentration of acid or pH of acid can all be measured with time to follow the progress of the reaction.

Page 7 of 10

Lesmahagow High School Reactions and Speed

Special probes can be used to measure changes in concentration and also pH.

Interpreting Graph Information

The reaction rate is most rapid at the start of the reaction and in this can be seen in rate graphs when the slope of the graph is at its steepest.

The rate of the reaction then decreases as the reaction proceeds (since the concentration of reactants decreases as they are used up). This causes the slope of the graph to become less steep.

Eventually the graph levels off and this is when the reaction is fully complete.

Page 8 of 10

Time (s)

Am

ount

of p

rodu

ct (g

)

steep slope = fast reaction

shallow slope = slower reaction

level (no slope) = reaction complete

Time (s)

Am

ount

of p

rodu

ct (g

)

Time (s)

Am

ount

of p

rodu

ct (g

)

steep slope = fast reaction

shallow slope = slower reaction

level (no slope) = reaction complete

Lesmahagow High School Reactions and SpeedAltering Experimental Variables:

The Effect On Rate Graphs

We sometimes draw more than one curve onto a graph of reaction rate. This is usually when we are investigating the effect of changing an experimental variable such as concentration, particle size or temperature and want to directly compare the results.

As already mentioned, the steeper the slope the faster the rate of reaction. So in the following two examples Reaction 1 is faster than

Reaction 2 (as Reaction 1 has the steeper slope in both cases).

If we think back to what changes to experimental variables give rise to faster reactions we can see that the Reaction 1 curve could have come from using smaller particles or from using a higher concentration of reactant or from increasing the temperature of the reactants. The Reaction 2 curve could have come from using larger particles or from using a lower concentration of reactant or from decreasing the temperature of the reactants.

Note that on each of the graphs above both Reaction 1 and Reaction 2 eventually reach the same final level. This shows that a fair test was done and the same quantity of reactants was used in each case, since the same final quantity of products was made.

Page 9 of 10

Lesmahagow High School Rates of Reactions

Rate of Reactions - Glossary

Word Meaning

Chemical reaction

An interaction between substances (chemicals) in which their atoms re-arrange to form new substances.

Concentration The amount of particles in a given volume.

Fair testWhen only one experimental variable is altered at a time.

ProductsThe substances (chemicals) at the end of a chemical reaction.

Rate of reactionHow quickly a reactant is used up OR how quickly a product is created.

ReactantsThe substances (chemicals) at the start of a chemical reaction.

Surface areaTotal area of a substance which is exposed to the surroundings.

VariableSomething which can be changed in a chemical reaction.

Page 10 of 10