How Fast? The Rate Of Chemical Change

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Rate of Reaction

The rate of reaction is the change in concentration of a reactant or product per unit time.
The units of rate are typically mol dm⁻³ s⁻¹.
The formula is: rate = change in concentration / time.
On a concentration–time graph, the steeper the gradient, the faster the rate.
The rate at a particular time is found by drawing a tangent to the curve and calculating its gradient (Δy/Δx).
For reactants, the gradient is negative, so the rate is given as a positive value.
For products, the gradient is positive, so the rate is already positive.

Diagram showing the apparatus needed to investigate the effect of concentration on the rate of reaction

Measuring Rates of Reaction

To measure rate, track how quickly reactants are used up or products are formed.
Colorimetry measures light intensity passing through a solution; it can monitor colour changes but not coloured precipitates.
Mass loss works when a gas escapes; the gas must be dense enough to detect a mass change (e.g., CO₂ but not H₂).
Gas volume can be measured with a gas syringe or by displacement over water (only for gases with low water solubility).
Titration can measure concentration changes if samples are taken at intervals and quenched to stop the reaction.
Conductivity can be used if the concentration of ions changes during the reaction.
Clock reactions measure the time to reach a visible endpoint, e.g., the disappearing cross with sodium thiosulfate and HCl.

Gas syringe apparatus

Collision Theory

Kinetic theory explains the properties of matter in terms of particle motion and energy.
Kinetic energy is given by KE = ½mv²; at the same temperature, lighter particles move faster.
For a reaction to occur, particles must collide with sufficient energy and the correct orientation.
Collision frequency is the number of collisions per unit time.
Collision energy is the combined energy of colliding particles.
Activation energy (Ea) is the minimum energy required for a successful collision.
Most collisions are unsuccessful because they do not have enough energy, not because of orientation.

A successful collision

Factors Affecting Rates of Reaction

Concentration: higher concentration means more particles per unit volume, increasing collision frequency and rate.
Pressure: for gases, higher pressure has the same effect as higher concentration, increasing collision frequency.
Temperature: higher temperature increases both collision frequency and the proportion of particles with energy ≥ Ea.
Surface area: smaller particles (e.g., powder) expose more surface, increasing collision frequency.
Catalysts: provide an alternative pathway with lower activation energy, increasing the proportion of successful collisions.

Concentration and reaction rate

Activation Energy

Activation energy (Ea) is the minimum energy that colliding particles need to react.
In exothermic reactions, reactants are higher in energy than products.
In endothermic reactions, reactants are lower in energy than products.
For a successful collision, particles must collide with the correct orientation and with energy ≥ Ea.
The activation energy for the reverse reaction can be found from the energy profile: for exothermic, reverse Ea = ΔH + Ea(forward); for endothermic, reverse Ea = Ea(forward) – ΔH.

Reaction pathway diagrams

Energy Profiles With & Without Catalysts

A catalyst increases rate by providing an alternative reaction pathway with lower activation energy.
The catalyst is chemically unchanged at the end of the reaction.
Catalysts do not change the enthalpy change (ΔH) of the reaction.
Homogeneous catalysts are in the same phase as reactants; heterogeneous catalysts are in a different phase.
Enzymes are biological catalysts; transition metals are often catalysts due to variable oxidation states.
Catalysts reduce energy requirements, waste, and can increase selectivity in industrial processes.

Catalysts and reaction rate

Maxwell-Boltzmann Distribution Curves

A Maxwell-Boltzmann distribution shows the distribution of molecular energies at a given temperature.
The curve starts at the origin and approaches but never touches the x-axis.
The peak represents the most probable energy (Emp).
The area under the curve represents the total number of particles.
At higher temperatures, the curve flattens and the peak shifts to the right; a greater proportion of particles exceed Ea.
A catalyst lowers Ea, increasing the proportion of particles with energy ≥ Ea, but does not change the shape of the distribution.

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练习题

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1.A catalyst provides an alternative reaction pathway with a lower activation energy.
Easy
True or false?
2.Which of the following is the correct unit for the rate of reaction?
Easy
- Amol dm⁻³ s⁻¹
- Bmol dm⁻³
- Cs⁻¹
- Dmol s⁻¹
3.Which of the following factors can increase the rate of a chemical reaction? (select all that apply)
Medium
- AIncreasing the concentration of a reactant in solution
- BIncreasing the total pressure in a reaction involving gases
- CIncreasing the particle size of a solid reactant
- DRaising the temperature of the reaction mixture
- EAdding a catalyst
4.Match each term with its correct definition.
Easy
- Rate of reaction
- Activation energy
- Collision frequency
- Successful collision
- The change in concentration of a reactant or product per unit time
- The minimum energy colliding particles need to react
- The number of collisions between particles per unit time
- A collision with correct orientation and sufficient energy to react
5.Which method would be most suitable for monitoring the rate of the reaction between calcium carbonate and hydrochloric acid, which produces carbon dioxide gas?
Easy
- AMeasuring the loss in mass of the reaction vessel over time
- BMeasuring the pH of the solution continuously
- CMeasuring the colour intensity of the solution using a colorimeter
- DMeasuring the electrical conductivity of the solution
6.Increasing the temperature of a reaction mixture increases the rate of reaction only because the particles collide more frequently.
Medium
True or false?
7.On an energy profile diagram for an exothermic reaction, how is the activation energy represented?
Medium
- AThe energy difference between the reactants and the products
- BThe energy difference between the reactants and the transition state (peak)
- CThe energy difference between the products and the transition state (peak)
- DThe energy difference between the transition state and the zero energy line
8.Which of the following are valid ways to monitor the rate of a chemical reaction? (select all that apply)
Medium
- AMeasuring the volume of gas produced over time
- BMeasuring the change in mass of the reaction vessel over time
- CMeasuring the colour intensity of a solution using a colorimeter
- DMeasuring the temperature of the reaction mixture over time
- EMeasuring the pH of the reaction mixture over time

更多题目

9.Match each term with its correct description.
Medium
- Activation energy
- Collision frequency
- Collision energy
- Collision geometry
- The minimum energy colliding particles need to react
- The number of collisions per unit time
- The combined energy of two colliding particles
- The orientation or angle at which particles collide
10.Place the steps of the disappearing cross experiment in the correct order.
Easy
- Place a conical flask on a piece of paper with a cross drawn on it
- Add sodium thiosulfate solution to the flask
- Add hydrochloric acid to the flask and start the timer
- Stop the timer when the cross is no longer visible
11.Which of the following statements about catalysts are correct? (select all that apply)
Medium
- AA catalyst provides an alternative reaction pathway.
- BA catalyst lowers the activation energy of a reaction.
- CA catalyst is permanently changed chemically by the end of the reaction.
- DA catalyst increases the rate of reaction.
- EA catalyst changes the enthalpy change (ΔH) of a reaction.
12.On a Maxwell-Boltzmann distribution curve, what does the area under the curve represent?
Medium
- AThe total number of particles in the sample
- BThe average kinetic energy of the particles
- CThe activation energy of the reaction
- DThe rate of reaction
13.A catalyst lowers the activation energy of a reaction by providing an alternative reaction pathway.
Easy
True or false?
14.What does the horizontal section of a volume-time graph indicate?
Medium
- AThe reaction has stopped because a reactant is used up
- BThe reaction is proceeding at a constant rate
- CThe reaction is at equilibrium
- DThe gas is being produced at an increasing rate
15.If the experiment is repeated using the same volume of solution B at double the concentration, with solid A still in excess, how would the results compare?
Medium
- AThe initial rate would be faster, and the final volume of gas would be the same
- BThe initial rate would be faster, and the final volume of gas would be double
- CThe initial rate would be the same, but the final volume of gas would be double
- DThe initial rate would be slower, and the final volume of gas would be the same
16.A student measures the volume of carbon dioxide produced in the reaction between iodine and methanoic acid. The results are plotted as volume of gas against time. The gradient of the tangent to the curve at 20 seconds is 0.60 cm³ s⁻¹. What does this value represent?
Medium
- AThe rate of reaction at 20 seconds
- BThe total volume of gas produced after 20 seconds
- CThe average rate of reaction over the whole experiment
- DThe time taken for the reaction to reach completion
17.In the reaction between zinc powder and copper(II) sulfate solution, 3.78 g of zinc was added to 50.0 cm³ of 0.250 mol dm⁻³ copper(II) sulfate solution. Which reagent is the limiting reagent? (Ar: Zn = 65.4)
Medium
- AZinc
- BCopper(II) sulfate
- CBoth are in excess
- DNeither, they are in exact stoichiometric amounts
18.A gas syringe is used to collect hydrogen gas produced by the reaction between magnesium and hydrochloric acid. The volume of gas is recorded every 10 seconds. Which unit is most appropriate for the rate of this reaction?
Medium
- Acm³ s⁻¹
- Bmol dm⁻³ s⁻¹
- Cg s⁻¹
- Dcm³
19.Which of the following statements about a Maxwell-Boltzmann distribution curve at a fixed temperature are correct? (select all that apply)
Medium
- AThe curve starts at the origin.
- BThe area under the curve represents the total number of particles.
- CThe peak of the curve represents the activation energy.
- DThe curve approaches but never touches the x-axis.
- EThe curve is symmetrical about the most probable energy.
20.Match each method of measuring reaction rate with the property it monitors.
Medium
- Colorimetry
- Mass loss
- Gas volume
- Change in light absorbance or transmission
- Decrease in mass as gas escapes
- Volume of gas collected over time
21.The graph of volume of gas C against time shows a curve that becomes horizontal. What does the horizontal section indicate?
Medium
- AThe reaction has stopped because B has been used up.
- BThe reaction is proceeding at a constant rate.
- CThe rate of reaction is increasing.
- DThe reaction has reached equilibrium.
22.A student connects a gas syringe to a flask in which magnesium ribbon reacts with hydrochloric acid, and records the volume of hydrogen every 10 seconds. Which change to the apparatus would most improve the accuracy of the volume readings?
Medium
- AReading the syringe scale with the eye level at the plunger, and using a syringe with finer graduations
- BUsing a larger flask so that more gas can be collected
- CAdding the magnesium ribbon before the acid
- DUsing a longer piece of magnesium ribbon and 2 mol dm-3 acid
23.Hydrogen peroxide decomposes slowly to give oxygen and water. A student wants to follow the rate of this reaction by measuring the volume of oxygen produced every 30 seconds. Which combination of apparatus is most suitable?
Hard
- AConical flask, bung with delivery tube, gas syringe
- BConical flask, bung with delivery tube, inverted measuring cylinder over water
- CBeaker, thermometer, stopwatch
- DBalance, cotton wool plug, conical flask
24.In the reaction between sodium thiosulfate solution and dilute hydrochloric acid, a yellow precipitate of sulfur forms and obscures a cross drawn on paper under the flask. Which statement about this 'disappearing cross' experiment is correct?
Medium
- AThe time taken for the cross to disappear is a measure of the rate of reaction
- BThe cross disappears because the sulfur dissolves in the solution
- CThe experiment measures the volume of sulfur dioxide produced
- DThe precipitate is sodium chloride, which is white
25.Which of the following is a limitation of using the 'disappearing cross' method to study the rate of the thiosulfate-acid reaction?
Medium
- AIt gives only one piece of data (the time to a fixed end-point) for each run
- BIt cannot be used with different concentrations of thiosulfate
- CIt requires a colorimeter to detect the cross
- DThe reaction is too fast to time with a stopwatch
26.A colorimeter is used to follow a reaction in which a coloured reactant is used up. The intensity of light reaching the detector increases with time. Which statement best explains this?
Medium
- AAs the coloured reactant is used up, less light is absorbed, so more light reaches the detector
- BThe colour of the solution becomes more intense, so more light is absorbed
- CThe colorimeter measures the mass of the coloured species
- DThe reaction produces a coloured precipitate that scatters the light
27.A student follows the reaction between calcium carbonate and hydrochloric acid by measuring mass loss every 10 seconds. Which gas would be unsuitable to monitor by this method, and why?
Medium
- AHydrogen, because its molecules are too light to give a measurable mass loss
- BCarbon dioxide, because it is too dense to escape from the flask
- COxygen, because it reacts with the hydrochloric acid
- DNitrogen, because it is insoluble in water
28.In an experiment to follow the rate of a reaction by titration, samples of the reaction mixture are removed at regular intervals and the reaction in each sample is stopped (quenched). Why is quenching necessary?
Hard
- AIt freezes the reaction at that point in time so the concentration measured corresponds to that moment
- BIt speeds up the reaction so that equilibrium is reached more quickly
- CIt removes the catalyst from the sample
- DIt prevents the titration from reaching its end-point
29.The reaction between hydrochloric acid and sodium hydroxide solution can be followed by measuring the electrical conductivity of the mixture over time. Which statement explains why the conductivity changes?
Medium
- AThe concentration of ions in the solution changes as the reaction proceeds
- BThe temperature of the solution changes
- CA gas is produced that escapes from the solution
- DA coloured precipitate forms that absorbs light
30.A student investigates the rate of reaction between zinc powder and copper(II) sulfate solution by measuring the temperature rise. The reaction is exothermic. Which change would make the initial rate faster?
Hard
- AUsing the same mass of zinc as larger pieces instead of powder
- BDiluting the copper(II) sulfate solution with water
- CCooling the copper(II) sulfate solution before adding the zinc
- DUsing a larger mass of zinc powder but the same volume and concentration of copper(II) sulfate
31.Which of the following statements about the collection of a gas over water by displacement are correct? (select all that apply)
Hard
- AIt can only be used if the gas has low solubility in water
- BIt is suitable for collecting hydrogen gas
- CIt is suitable for collecting carbon dioxide gas
- DThe gas is collected in an inverted measuring cylinder or burette
- EIt requires the gas to be denser than air
32.A reaction between two aqueous solutions produces a gas. The volume of gas is measured every 10 seconds using a gas syringe. Which of the following would improve the reliability of the results? (select all that apply)
Hard
- ARepeat the experiment and take the average of the volumes at each time
- BUse the same temperature for each run
- CUse a different concentration of one reactant in each run
- DRead the syringe at eye level
- EChange the amount of catalyst between runs
33.In a reaction followed by measuring the volume of gas produced, the rate of reaction is greatest at the start and gradually decreases as the reaction proceeds.
Easy
True or false?
34.A colorimeter can be used to monitor the rate of a reaction that produces a coloured precipitate.
Medium
True or false?
35.When a reaction is followed by measuring the loss in mass, the graph of mass against time has the same shape as a graph of the amount of reactant remaining against time.
Medium
True or false?
36.Match each experimental method with the property that is monitored to follow the rate of a reaction.
Medium
- Colorimetry
- Mass loss
- Gas syringe
- Conductivity
- Intensity of light passing through the solution
- Decrease in mass as a gas escapes
- Volume of gas produced
- Change in electrical conductivity of the solution
37.Which piece of apparatus is essential for this method?
Medium
- AA balance reading to 2 or 3 decimal places
- BA gas syringe
- CA colorimeter
- DA thermometer
38.Which equation represents the decomposition of hydrogen peroxide?
Medium
- A2H2O2 → 2H2O + O2
- BH2O2 → H2 + O2
- CH2O2 → H2O + O2
- D2H2O2 → 2H2 + O2
39.Which of the following best defines the rate of reaction?
Easy
- AThe change in concentration of a reactant or product per unit of time
- BThe total amount of reactant used up during a reaction
- CThe time taken for a reaction to reach completion
- DThe change in temperature of a reaction mixture per unit of time
40.The collision frequency is the number of collisions between particles per unit time in a system.
Easy
True or false?
41.State the term for the minimum energy that colliding particles need in order to react.
Easy
42.Complete the sentence about measuring reaction rates.
Easy
In a colorimeter, the amount of light passing through a solution is measured; the light intensity is related to the ____ of the reactant or product.
43.In the reaction between iodine and methanoic acid, the volume of carbon dioxide produced is measured over time. At 20 seconds, a tangent to the curve gives a change in volume of 24 cm³ over a time interval of 40 seconds. Calculate the rate of reaction at 20 seconds in cm³ s⁻¹.
Medium
44.In a reaction, the concentration of a reactant decreases from 0.80 mol dm⁻³ to 0.20 mol dm⁻³ in 40 seconds. Calculate the average rate of reaction in mol dm⁻³ s⁻¹.
Medium
45.Complete the sentence about the Maxwell-Boltzmann distribution curve.
Medium
The most probable energy of the particles is represented by the ____ of the curve.
46.Name the method that can be used to monitor the rate of a reaction that involves a colour change, by measuring the intensity of light passing through the solution.
Medium
47.In the reaction between sodium thiosulfate and hydrochloric acid, a yellow precipitate of sulfur is produced. State the name of this clock reaction experiment.
Medium
48.In an experiment where the volume of gas produced is measured every 10 seconds, what is the most appropriate unit for the rate of reaction if the volume is measured in cm³?
Medium
49.In a reaction, the volume of gas produced increases from 0 cm³ to 60 cm³ over 30 seconds. Calculate the average rate of reaction in cm³ s⁻¹.
Medium
50.Complete the sentence about the effect of a catalyst on the Maxwell-Boltzmann distribution.
Medium
A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower ____, which means that a greater ____ of particles have enough energy to react.
51.A student measures the volume of gas produced in a reaction every 10 seconds. The volume increases from 0 cm3 to 80 cm3 in 40 seconds. Calculate the average rate of reaction in cm3 s-1.
Medium
52.Complete the sentences about monitoring the rate of a reaction by measuring the volume of gas produced.
Medium
The volume of gas produced is measured every few seconds using a ____. The graph of volume against time becomes ____ when the reaction has stopped.
53.Explain, using collision theory, why a small increase in temperature causes a large increase in the rate of reaction.
Hard
54.Explain why the gradient of the original curve decreases over time.
Medium
55.Explain why the gradient of the volume-time curve for a reaction decreases over time.
Hard
56.A reaction between two gases is carried out in a sealed container. The rate of reaction is measured at two different pressures. Explain, in terms of collision theory, why increasing the pressure increases the rate of reaction.
Hard
57.Explain why the rate of mass loss decreases as the reaction proceeds.
Hard
58.In the reaction between iodine and methanoic acid, the volume of carbon dioxide produced is measured over time. At 20 seconds, a tangent to the curve gives a change in volume of 24 cm³ over a time interval of 40 seconds. Calculate the rate of reaction at 20 seconds in cm³ s⁻¹.
Medium
- A1.6 cm³ s⁻¹
- B-0.4 cm³ s⁻¹
- C2.6 cm³ s⁻¹
- D0.6 cm³ s⁻¹
59.In a reaction, the concentration of a reactant decreases from 0.80 mol dm⁻³ to 0.20 mol dm⁻³ in 40 seconds. Calculate the average rate of reaction in mol dm⁻³ s⁻¹.
Medium
- A1.015 mol dm⁻³ s⁻¹
- B2.015 mol dm⁻³ s⁻¹
- C-0.985 mol dm⁻³ s⁻¹
- D0.015 mol dm⁻³ s⁻¹
60.In a reaction, the volume of gas produced increases from 0 cm³ to 60 cm³ over 30 seconds. Calculate the average rate of reaction in cm³ s⁻¹.
Medium
- A3 cm³ s⁻¹
- B1 cm³ s⁻¹
- C2 cm³ s⁻¹
- D4 cm³ s⁻¹
61.A student measures the volume of gas produced in a reaction every 10 seconds. The volume increases from 0 cm3 to 80 cm3 in 40 seconds. Calculate the average rate of reaction in cm3 s-1.
Medium
- A3 cm3 s-1
- B1 cm3 s-1
- C4 cm3 s-1
- D2 cm3 s-1

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