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1.5Chemical Substances, Reactions and Essential Resources

Rate Of Chemical Change — WJEC GCSE Chemistry Revision Notes & Topical Questions

10 min read 19 parts Lesson 5 of 6

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Complete PDF with figures and original page layoutPage 1 WJEC GCSE Chemistry Your notes 1.5 Rate Of Chemical Change Contents Determining the Rate of Reaction Factors Affecting Rates Hydrochloric Acid & Sodium Thiosulfate Particle Theory Catal…

Section 1 of 19Measuring rate of reaction

  • Range of rates
    • reactions vary from very slow, such as rusting, to very fast, such as explosives
  • Two ways
    • measure how fast a reactant is used up or how fast a product is made
  • Equations
    • rate = amount of reactant used ÷ time taken, or rate = amount of product made ÷ time taken
  • Time
    • it is usually measured in seconds
  • Product measurement
    • measuring a product forming is usually easier than measuring a reactant used up
  • Units
    • g/s for mass, and cm3/s or dm3/s for volume
  • Repeats
    • several measurements are taken to give enough data for a conclusion

Section 2 of 19Mass loss method

  • Principle
    • a gas that escapes from the vessel reduces the mass of the mixture
  • Example
    • calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
  1. Place the reaction in a conical flask on a balance.
  2. Put cotton wool in the neck to let gas escape but stop materials being ejected.
  3. Start the timer when the reactants are combined.
  4. Record the mass every few seconds.
  • Limitation
    • the gas must be dense enough to give a measurable change on a 2 or 3 decimal place balance,
    • so carbon dioxide (Mr 44) suits but hydrogen (Mr 2) does not

Section 3 of 19Gas collection method

  • Principle
    • the gas produced is trapped and its volume measured over time
  • Use
    • it works for any gas and is best for hydrogen or other gases of small Mr
  • Example
    • magnesium + hydrochloric acid → magnesium chloride + hydrogen
  • Downward displacement of water
    • The gas bubbles into an inverted measuring cylinder filled with water.
    • The gas displaces the water and the volume is recorded over time.
  • Gas syringe
    • the gas fills a syringe that measures its volume directly

Section 4 of 19Precipitation method

  • Principle
    • mixing two clear solutions forms a solid that clouds the mixture
  • Example
    • sodium thiosulfate + hydrochloric acid → sodium chloride + sulfur dioxide + water + sulfur
  1. Place the flask over paper with a cross on it.
  2. Mix the reactants and start the stopwatch.
  3. View from above and stop the timer when the cross disappears.
  • Disadvantages
    • It is subjective, because people may disagree on when the cross disappears.
    • It gives only one data point per experiment, so no rate graph can be drawn.
  • Alternative
    • data-logging apparatus records how much light passes through the mixture, which falls as the precipitate forms

Section 5 of 19Effect of concentration or pressure

  • Result
    • increasing the concentration of a solution or the pressure of a gas increases the rate
  • Compared with a lower concentration, the graph of product against time
    • Has a steeper gradient at the start.
    • Becomes horizontal sooner.
    • Forms the same amount of product.

Section 6 of 19Effect of temperature

  • Result
    • increasing the temperature increases the rate of reaction
  • Compared with a lower temperature, the graph of product against time
    • Has a steeper gradient at the start.
    • Becomes horizontal sooner.
    • Forms the same amount of product.

Section 7 of 19Effect of surface area

  • Result
    • increasing the surface area of a solid increases the rate
  • Compared with lumps, powder gives a graph that
    • Has a steeper gradient at the start.
    • Becomes horizontal sooner.
    • Forms the same amount of product.
  • Particle size
    • surface area increases as particle size decreases, so a 2 cm cube with area 24 cm2 cut into 8 cubes has area 48 cm2
  • Exam approach
    • the effects of decreasing these factors must also be applied,
      • and answers on concentration or pressure must say there are more particles per unit volume

Section 8 of 19Practical: sodium thiosulfate and hydrochloric acid

  • Reaction
    • Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + H2O(l) + SO2(g) + S(s)
  • Principle
    • the solid sulfur makes the solution cloudy, so the rate of cloud formation measures the reaction rate
  • Aim
    • to investigate how temperature affects the rate
  • Apparatus
    • 10 cm3 and 25 cm3 measuring cylinders, 250 cm3 conical flask, white paper with a cross, stopwatch, 1 mol/dm3 hydrochloric acid, thermometer,
      • and 40 g/dm3 sodium thiosulfate at 5 °C and at 60 °C

Section 9 of 19Practical: method

  1. Measure 25 cm3 of hot sodium thiosulfate in the 25 cm3 measuring cylinder and transfer it to the conical flask.
  2. Record the temperature of the solution.
  3. Measure 5 cm3 of hydrochloric acid in the 10 cm3 measuring cylinder.
  4. Place the flask on the cross.
  5. Add the acid, swirl, and start the stopwatch at the same time.
  6. Look from above and stop the stopwatch as soon as the cross disappears.
  7. Record the time.
  8. Repeat with other temperatures.
  • Making different temperatures
    • mix hot (60 °C) and cold (5 °C) solutions in volumes 25/0, 20/5, 15/10, 10/15, 5/20 and 0/25 cm3
  • Avoiding premature reaction
    • Use the 25 cm3 cylinder only for thiosulfate and the 10 cm3 cylinder only for acid.
    • Rinse the flask thoroughly between experiments.

Section 10 of 19Particle theory of rates

  • Principle
    • reactions happen only when reactant particles collide
  • Successful collision
    • the particles rearrange to form products
  • Unsuccessful collision
    • the particles bounce off each other and stay unchanged
  • Rate
    • more frequent successful collisions mean a faster reaction, so a greater proportion of reactant particles form products

Section 11 of 19Explaining the effect of each factor

1Concentration2 points
  • there are more reactant particles in a given volume
  • More collisions occur per second, and the number of collisions is proportional to the number of particles.
2Pressure2 points
  • the same number of particles occupy a smaller volume
  • Collisions per second increase.
3Temperature3 points
  • particles gain kinetic energy
  • Collisions are more frequent and more are successful, and a small rise gives a large rate increase.
  • As a rough rule, every 10 °C rise approximately doubles the rate in aqueous and gaseous systems.
4Surface area2 points
  • more of the solid is exposed to the other reactant
  • Collisions per second increase, and doubling the area doubles the collisions.

Section 12 of 19Activation energy and collision energy (Higher tier)

  • Activation energy
    • the minimum energy needed for a reaction, different for each reaction
  • Successful collision
    • particles collide with energy greater than or equal to the activation energy
  • Unsuccessful collision
    • insufficient energy means the particles bounce apart
  • Factors affecting successful collisions per unit time
    • Number of particles per unit volume.
    • Frequency of collisions.
    • Kinetic energy of the particles.
    • Activation energy, since a high value means fewer successful collisions.
  • Temperature
    • it is the only factor that directly raises collision energy, by turning thermal energy into kinetic energy
  • Other factors
    • concentration, pressure and surface area change only the number of collisions
  • Importance
    • temperature raises both the number and the energy of collisions, and the increase in energy matters more

Section 13 of 19Catalysts

  • Catalyst = a substance that speeds up a reaction while remaining chemically unchanged
  • Mass
    • its mass is the same at the start and end
  • Equations
    • it is not part of the equation but may be written above or below the arrow, such as an iron catalyst at 450 °C and 200 atm for N2 + 3H2 ⇌ 2NH3
  • Industry
    • different reactions need different catalysts, they are not used up, but impurities make them less active so they must be replaced regularly
  • Graph
    • a catalyst gives a steeper start and a sooner plateau, with the same amount of product
  • Syllabus note
    • names of specific catalysts need not be recalled

Section 14 of 19How catalysts work (Higher tier)

  • Principle
    • they provide an alternative reaction pathway with a lower activation energy
  • Effect
    • a higher proportion of particles have energy at or above the lower activation energy
  • Result
    • more successful collisions per second, so a faster reaction
  • Analogy
    • a catalysed journey needs less energy and is faster

Section 15 of 19Enzymes

  • Enzymes = biological catalysts made in living cells that speed up reactions without being changed or used up
  • Specificity
    • they catalyse specific reactions under specific conditions
  • In cells
    • they allow reactions to happen much faster at relatively low temperatures such as body temperature
  • In industry
    • they allow reactions at lower temperatures and pressures than usual, saving money and energy
  • Important reactions
    • fermentation, respiration and photosynthesis

Section 16 of 19Enzyme structure and temperature

  • Shape
    • an enzyme has a specific shape held in place by chemical bonds, so each enzyme acts on a specific reaction
  • Low temperature
    • the enzyme works but there are few successful collisions
  • Optimum temperature
    • the temperature at which the enzyme is the most effective, giving the maximum product
  • High temperature
    • above about 60 °C the thermal energy breaks the bonds, so the enzyme loses its shape and stops catalysing, which is denaturation
  • Graph
    • the rate rises with temperature up to the optimum and then falls as enzymes denature

Section 17 of 19Uses of enzymes

  • Food processing
    • enzymes improve taste and texture, breaking starch into sugar in bread making and converting milk into cheese
  • Fermentation
    • enzymes in yeast break glucose down into ethanol and carbon dioxide
  • Medicine
    • enzymes are used in making medicines such as antibiotics and insulin
  • Detergents
    • enzymes break down stains such as blood and grass
  • Bioethanol
    • enzymes break plant material into simple sugars that are fermented to bioethanol
  • Syllabus note
    • a variety of uses should be known, but no specific details need be recalled

Section 18 of 19Practical: gas collection method

  • Reaction
    • Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
  • Aim
    • to study how the concentration of hydrochloric acid affects rate
  • Apparatus
    • 3 cm lengths of magnesium ribbon, 1 mol/dm3 hydrochloric acid, 250 cm3 conical flask with a single-holed bung and delivery tube, trough, 100 cm3 and 250 cm3 measuring cylinders, clamp stand and stopwatch
  1. Set up the apparatus.
  2. Measure 20 cm3 of 1 mol/dm3 acid and transfer it to the flask.
  3. Fill the 100 cm3 measuring cylinder with water, invert it and clamp it above the trough.
  4. Add a 3 cm strip of magnesium, fit the bung and delivery tube, and start the stopwatch.
  5. Record the volume of hydrogen every 10 seconds.
  6. Continue until no more gas appears.
  7. Repeat with 10 cm3 of acid and 10 cm3 of water, which gives 20 cm3 at 0.5 mol/dm3.
  • Gas syringe alternative
    • connect the flask to the syringe immediately after adding the magnesium so no gas escapes

Section 19 of 19Practical: results and conclusion

  • Results table
    • time every 10 s with hydrogen volume for each concentration, extending beyond 60 s if needed
  • Graph
    • a line graph with time on the x-axis and volume of hydrogen on the y-axis, with both lines labelled
  • Description
    • the higher concentration gives the same final volume but in a shorter time
  • Explanation
    • more particles per unit volume give more frequent, successful collisions
  • Example data
    • 1 mol/dm3 reaches 50 cm3 by 50 s, whereas 0.5 mol/dm3 reaches 50 cm3 by 90 s
  • Exam switching
    • questions may ask about concentration in the thiosulfate practical, or temperature or surface area in the magnesium practical

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