Rate of reaction — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

Section 1 of 28Factors affecting rate

Factors

• Factors concentration of solutions, pressure of gases, surface area of solids, temperature and a catalyst.

Economic importance

• Economic importance a higher rate means faster production and a more efficient process.

Section 2 of 28Effect on rate-of-reaction graphs

Increased factor

• Increased factor a steeper gradient at the start and a line that becomes horizontal sooner.

Final amount

• Final amount the same amount of product forms because the amount of reactants has not changed.

Section 3 of 28Concentration and pressure

Rule

• Rule a higher concentration of solution or higher pressure of gas increases the rate.

Explanation

• Explanation more particles per unit volume give more frequent collisions and so more successful collisions per second.
• Doubling the number of particles doubles the collisions per second.

Section 4 of 28Surface area

Surface area

Rule

• Rule a larger surface area, such as a powder instead of lumps, increases the rate.

Particle size

• Particle size smaller pieces have a larger total surface area.
• A 2 cm cube has 24 cm² but cut into 8 small cubes it has 48 cm².

Explanation

• Explanation more particles are exposed to the other reactant, so collisions are more frequent.

Section 5 of 28Temperature

Rule

• Rule a higher temperature increases the rate.
• As a rule of thumb the rate roughly doubles for each 10 °C rise.

Explanation

• Explanation particles have more kinetic energy, so they collide more often and a greater proportion of collisions have at least the activation energy.
• The increase in energy is the more important effect.

Section 6 of 28Catalysts

Catalyst

• = substance that speeds up a reaction without being altered or consumed.
• Its mass is the same at the start and end and it is not in the equation.

Action

• Action provides an alternative pathway with a lower activation energy.
• More particles have energy above the activation energy, so more collisions succeed.

Examples

• Examples iron in the Haber process and enzymes, which are biological catalysts working best at specific temperature and pH.

Section 7 of 28Collision theory (supplement)

Conditions for reaction

• Conditions for reaction particles must collide with energy at least equal to the activation energy and the correct orientation.

Successful collision

• Successful collision reactant particles rearrange to form products.

Unsuccessful collision

• Unsuccessful collision particles bounce apart unchanged.

Rate

• Rate depends on the number of successful collisions per unit time.

Section 8 of 28What changes the collisions

Factor	Frequency of collisions	Energy of collisions
Concentration / pressure	Increases	No change
Surface area	Increases	No change
Temperature	Increases	Increases
Catalyst	No change	More particles reach the lower activation energy

Section 9 of 28More from the specification

1Rate of reaction measurement

• Determining reaction speed by measuring the volume of gas collected or mass loss on a balance over time.
  • (e.g., hydrogen in an inverted burette or syringe)

2Rate curves and gradients

• Reaction graphs showing tangent gradients that are steepest at t=0 and plateau as limiting reactants are fully consumed.

3Disappearing cross method

• Investigating rates in precipitation reactions by recording the time taken for a flask solution to turn opaque and obscure a marked cross.

4Gas collection over water

• Collecting insoluble gases in an inverted graduated tube submerged in a water trough.

5Thermal energy

• Total kinetic energy of the particles in a substance.

6Hydrogen

• A gas that gives a squeaky pop with a lighted splint.

Section 10 of 28Collision theory

Collision theory

Collision theory a reaction occurs only when particles collide with at least the activation energy.

Activation energy

Activation energy the minimum energy colliding particles must have to react.

Successful collision

• Successful collision particles have energy greater than the activation energy and rearrange to form products.

Unsuccessful collision

• Unsuccessful collision particles bounce off unchanged because they lack energy or collide in the wrong orientation.

Rate

• Rate depends on the number of successful collisions per unit of time.

Section 11 of 28Explaining the factors with collision theory

Concentration

• Concentration more particles in the same volume, so more collisions per second;
  • doubling the particles doubles the collisions.

Pressure of gases

• Pressure of gases the same particles in a smaller volume, so more collisions per second.

Surface area

• Surface area a larger area of the solid is exposed to the other reactant, so more collisions per second.
• Smaller particle size means larger surface area: a 2 cm cube has area 24 cm², but cut into 8 cubes the area is 48 cm².

Temperature

• Temperature particles have more kinetic energy, so they collide more often and more collisions exceed the activation energy.
• The increase in energy is the more important effect, and a rough rule is that the rate doubles for each 10 °C rise.

Catalyst

Catalyst a substance that speeds up a reaction without being altered or consumed.

• It provides an alternative pathway with a lower activation energy, so more collisions are successful.

Section 12 of 28Rate graphs for the factors

Rate graphs for the factors

Higher concentration, surface area, temperature or a catalyst

• Higher concentration, surface area, temperature or a catalyst the curve has a steeper gradient at the start and becomes horizontal sooner.

Amount of product

• Amount of product the same total amount is formed, because it depends on the limiting reactant.

Rate during a reaction

• Rate during a reaction it is fastest at the start and slows as reactants are used up;
  • the reaction stops when one reactant is used up and the line is horizontal.

Section 13 of 28Catalysts in detail

Catalysts in detail

Mass

• Mass the catalyst has the same mass at the start and end and is not in the equation.

Industrial example

• Industrial example iron catalyses the Haber process, and iron beads increase the surface area available.

Enzymes

• Enzymes biological catalysts that work best at specific temperatures and pH values.

Amount

• Amount normally only a small amount of catalyst is needed.

Temperature and concentration

• Temperature and concentration concentration, pressure and surface area change only the number of collisions,
  • whereas temperature also changes their energy.

Section 14 of 28Measuring the rate

Principle

• Principle measure how quickly reactants are used up or products are formed.

Changing properties

• Changing properties colour, mass or volume, taken as proportional to concentration.

Techniques

• Techniques mass loss on a balance, volume of gas produced, and timing a colour change.

Mass-loss method

• Mass-loss method use cotton wool in the flask mouth;
  • not suitable for hydrogen or other gases with small M_r because the mass loss is too small.

Section 15 of 28Investigating concentration (disappearing cross)

1

Measure 50 cm³ of sodium thiosulfate solution into a flask.

2

Measure 5 cm³ of dilute hydrochloric acid in a measuring cylinder.

3

Draw a cross on paper and put it under the flask.

4

Add the acid and start the stopwatch immediately.

5

Look from above and stop the watch when the cross can no longer be seen as sulfur precipitates.

6

Repeat with thiosulfate diluted with water to different concentrations.

Section 16 of 28Investigating surface area, temperature and a catalyst

Surface area

• Surface area add magnesium ribbon to dilute hydrochloric acid in a conical flask connected by a delivery tube to an inverted measuring cylinder in water.
• Measure the gas volume in a fixed time, repeating with different sizes of magnesium of the same total mass.

Temperature

• Temperature heat the dilute hydrochloric acid in a water bath to a set temperature, add a strip of magnesium and time until it disappears.
• Repeat at different temperatures.

Catalyst

• Catalyst add hydrogen peroxide to a flask, add manganese(IV) oxide and collect the gas by downward displacement.
• Repeat without the catalyst and compare the gas volumes.

Section 17 of 28Evaluating methods

Method	Advantage	Disadvantage
Disappearing cross	Simple, no specialist equipment	Hard to judge when the cross is obscured; easy to contaminate equipment
Gas syringe	Works for any gas; all the gas is collected; easy to set up	Fragile, expensive, can stick, limited volume; gas can be lost before the bung is secure
Inverted measuring cylinder	Common equipment	Delivery tube can pop out; scale is hard to read upside down; gas lost when connecting the bung
Mass loss on a balance	Easy to set up with common equipment	Not suitable for gases with low Mr

Section 18 of 28Fair tests and conclusions

Control variables

• Control variables change only one variable and keep the others constant: volume, concentration, mass or size of solid, and temperature.

Repeats

• Repeats repeat the experiment and calculate a mean.

Higher rate

• Higher rate shows as a shorter time for the cross to disappear or a greater volume of gas in the fixed time.

Section 19 of 28What is measured

Rate

• Rate is found by measuring how quickly reactants are used up or products are formed.

Changing properties

• Changing properties colour, mass or volume change during a reaction and are taken as proportional to concentration.

Method choice

• Method choice depends on the substances involved.

Section 20 of 28Effect of temperature

1

Heat dilute hydrochloric acid to a set temperature in a water bath.

2

Add it to a conical flask.

3

Add a strip of magnesium and start the stopwatch.

4

Stop timing when the magnesium disappears.

5

Repeat at several temperatures and compare.

• Result
  • the higher the temperature, the shorter the time.

Section 21 of 28Effect of a catalyst

1

Put hydrogen peroxide in a conical flask connected by a delivery tube to an inverted measuring cylinder in water.

2

Add manganese(IV) oxide and quickly replace the bung.

3

Measure the volume of oxygen collected in a fixed time.

4

Repeat without the catalyst and compare.

• Result
  • the catalyst increases the rate of reaction.

Section 22 of 28Methods of measuring rate

Method	Advantages	Disadvantages
Disappearing cross	Simple, no specialist equipment	Different people judge the cross gone at different cloudiness; easy to contaminate
Gas syringe	Works for any gas, easy to set up, all gas is from the reaction	Fragile, expensive, can stick, limited volume
Inverted measuring cylinder	Works for any gas, common equipment	Tube can pop out, upside-down scale, gas lost before bung fitted
Mass lost on a balance	Easy to set up, common equipment	Not suitable for gases of low Mr such as hydrogen

• Mass loss
  • cotton wool in the flask mouth lets gas escape but stops liquid spraying out.

Section 23 of 28Reading rate graphs

Start

• Start the rate is fastest at the start because most reactants are present, so the line is steepest.

Slowing

• Slowing reactants are used up and the line gets less steep.

Finish

• Finish the line becomes horizontal when a reactant is used up and the rate is zero.

Reactant graph

• Reactant graph if reactant is plotted the line falls steeply then levels off.

Section 24 of 28Amount of product

Limiting reactant

• Limiting reactant the amount of product depends on the amount of the limiting reactant.
• More limiting reactant gives more product.
• More of the excess reactant gives the same product.

Comparing runs

• Comparing runs several runs can be plotted on one graph to show the effect of a variable.

Section 25 of 28Rate at a particular time

Rate at a particular time

Draw a tangent to the curve at that time.

Find the change in y and the change in x along the tangent.

Rate = change in y ÷ change in x, for example in cm³/s.

• Steeper slope
  • means a faster reaction.

Section 26 of 28More from the specification

More from the specification

Turbidimetric reaction rates

• Turbidimetric reaction rates measuring the disappearance of cross marks underneath conical flasks tracks precipitation rates to determine initial reaction kinetics.

Collision Theory Conditions

• For a reaction to occur, reactant particles must collide with sufficient kinetic energy (≥ Ea) and appropriate orientation,
• forming successful collisions.

Surface Area and Reaction Rate

• Subdividing a solid reactant into smaller pieces increases its total surface area,
• increasing the frequency of particle collisions per second.

Concentration and Pressure Effects

• Increasing reactant concentration in solution or increasing gas pressure packs particles closer together, raising the rate of successful collisions.

Reaction Progress Curves and Gradient

• On a product volume versus time graph, the initial gradient is steepest when reactant concentration is highest, showing maximum rate before gradually leveling off.

Theory

• A tested hypothesis that is widely accepted.

Successful collision

• A collision with the correct orientation and enough energy to react.

Activation energy

• The minimum energy colliding particles need to react.

Collision theory

• Particles react only if they collide with at least the activation energy and the correct orientation.

Kinetic energy

• The energy of motion of a particle.

Thermal energy

• Total kinetic energy of the particles in a substance.

Particle

• A tiny portion of matter, too small to see.

Gradient

• The change in y divided by the change in x on a graph.

Pressure

• Force of gas particles colliding with a surface.

Section 27 of 28Key terms

Term	Meaning
Rate of reaction	How fast reactants are used up or products formed.
Catalyst	Substance that speeds up a reaction and is unchanged.
Activation energy	Minimum energy for a successful collision.
Collision theory	Reactions need particles to collide with enough energy.
Surface area	Total area of a solid exposed to reactants.
Enzyme	Biological catalyst.

Term	Meaning
Collision theory	Reactions occur when particles collide with enough energy.
Activation energy	Minimum energy for a successful collision.
Surface area	Total area of the exposed surface of a solid.
Catalyst	Substance that lowers activation energy and is unchanged at the end.
Downward displacement	Collecting gas in an inverted measuring cylinder full of water.
Limiting reactant	The reactant that is used up first and determines the amount of product.

Term	Meaning
Tangent	Straight line touching a curve at one point.
Gradient	Change in y divided by change in x.
Limiting reactant	Reactant that is used up first.
Gas syringe	Syringe for measuring gas volume.
Rate of reaction	Change in amount per unit time.
Concordant	Results that agree closely.

Section 28 of 28Exam technique

1

Saying a catalyst gives particles more energy — it lowers the activation energy.

2

Saying higher temperature only gives more collisions — the energy of collisions also rises.

3

Saying a catalyst changes the amount of product — only the rate changes.

4

Saying more surface area means more particles — more are exposed.

1

Saying a catalyst gives particles more energy — it lowers the activation energy.

2

Stating that higher temperature only gives more collisions — it also gives collisions more energy.

3

Changing two variables in one investigation.

4

Forgetting to start the stopwatch on adding the acid.

1

Drawing the tangent at the wrong point.

2

Using a gas syringe that leaks.

3

Assuming the rate is constant throughout.

4

Using the mass-loss method for hydrogen.