How Fast? The Rate Of Chemical Change

விளையாடிக் கற்றுக்கொள்ளுங்கள்

ஆற்றல் சம்பாதிக்க இந்த கேள்விகளுக்குப் பதிலளியுங்கள், பின்னர் மீன் பிடித்து ஆராயுங்கள். கணக்கு தேவையில்லை.

கல்வியாளர்களுக்கு: How Fast? The Rate Of Chemical Change (Chemistry, HL)-க்கான தயாரான பாட ஸ்லைடுகள், திருப்புதல் குறிப்புகள் — உங்கள் பாடத்தில் அவற்றைப் பயன்படுத்தவும், அல்லது கற்பவர்கள் நேரலை விளையாட்டாக விளையாடும் ஊடாடும் வகுப்பு செயல்பாடாக தலைப்பை இயக்கவும்.

பாட குறிப்புகள்

Rate of Reaction

Rate of reaction is the change in concentration of a reactant or product per unit time.
Units of rate are typically mol dm⁻³ s⁻¹.
Rate can be calculated using: 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 sign is changed to give a positive rate.
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

Rate can be measured by monitoring a property that changes over time, such as colour, mass, volume, or conductivity.
Colorimetry measures light absorbance or transmission; it works if a coloured species is used up or formed, but not for coloured precipitates.
Mass loss is used when a gas escapes; the gas must be dense enough to produce a measurable change (e.g., CO₂ but not H₂).
Gas volume can be measured with a gas syringe or by displacement over water (for gases with low solubility).
Titration with quenching can measure concentration changes, but it is not continuous.
Conductivity changes if the concentration of ions changes during the reaction.
Clock reactions measure the time to reach a visible endpoint, such as 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; it increases with concentration, pressure, temperature, and surface area.
Collision energy is the combined energy of colliding particles; only collisions with energy ≥ activation energy are successful.
Activation energy (Ea) is the minimum energy required for a reaction to occur.
A successful collision occurs when particles collide with the correct orientation and with energy ≥ Ea.
An unsuccessful collision occurs when particles have insufficient energy or incorrect orientation.

A successful collision

Factors Affecting Rates of Reaction

Concentration: increasing concentration increases the number of particles per unit volume, increasing collision frequency and rate.
Pressure: increasing pressure in gaseous reactions has the same effect as increasing concentration, increasing collision frequency and rate.
Temperature: increasing temperature increases both the frequency of collisions and the proportion of particles with energy ≥ Ea, increasing rate.
Surface area: increasing surface area (e.g., powder vs. large pieces) exposes more particles to collisions, increasing rate.
Catalysts: provide an alternative reaction pathway with lower activation energy, increasing the proportion of successful collisions and rate.
A catalyst is chemically unchanged at the end of the reaction.

Concentration and reaction rate

Activation Energy

Activation energy (Ea) is the minimum energy that colliding particles must have to react.
In exothermic reactions, reactants are higher in energy than products.
In endothermic reactions, reactants are lower in energy than products.
Even with correct orientation, particles must have energy ≥ Ea for a successful collision.
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 provides an alternative reaction pathway with a lower activation energy.
The catalyst is not used up and remains chemically unchanged.
On an energy profile, the catalysed pathway has a lower peak but the same enthalpy change (ΔH).
Homogeneous catalysts are in the same phase as the reactants.
Heterogeneous catalysts are in a different phase from the reactants.
Catalysts reduce energy requirements, reduce waste, and increase selectivity in industrial processes.
Enzymes are biological catalysts; transition metals are often used as catalysts due to variable oxidation states.

Catalysts and reaction rate

Maxwell-Boltzmann Distribution Curves

A Maxwell-Boltzmann curve shows the distribution of molecular energies at a given temperature.
The curve starts at the origin, peaks at the most probable energy (E_MP), and approaches but never touches the x-axis.
Only a small proportion of particles have energy ≥ Ea; these can react.
Increasing temperature flattens the curve and shifts the peak to the right, increasing the proportion of particles with energy ≥ Ea.
A catalyst lowers Ea, increasing the proportion of particles with sufficient energy; the total shaded area beyond Ea increases.
When drawing curves for different temperatures, the higher temperature curve has a lower, broader peak and a higher tail.

Rate Equation and Reaction Orders (HL)

The rate equation expresses rate as: Rate = k[A]^m[B]ⁿ, where m and n are orders with respect to each reactant.
Orders must be determined experimentally; they cannot be deduced from the stoichiometric equation.
Zero order: changing concentration has no effect on rate; the reactant does not appear in the rate equation.
First order: rate is directly proportional to concentration; doubling concentration doubles rate.
Second order: rate is proportional to the square of concentration; doubling concentration quadruples rate.
Overall order is the sum of individual orders (m + n).
Fractional orders are possible and indicate a multi-step mechanism.

The Rate Constant and Arrhenius Equation (HL)

The rate constant, k, is the proportionality constant in the rate equation.
Units of k depend on the overall order of the reaction.
The Arrhenius equation is k = A e^(−Ea/RT), where A is the frequency factor, Ea is activation energy, R is the gas constant, and T is temperature in Kelvin.
A plot of ln k against 1/T gives a straight line with gradient −Ea/R and intercept ln A.
Increasing temperature increases k; increasing activation energy decreases k.
The frequency factor, A, relates to the frequency of collisions with correct orientation.

Reaction Mechanisms and Molecularity (HL)

The rate-determining step is the slowest step in a reaction mechanism and determines the rate equation.
Molecularity is the number of species taking part in an elementary step.
The rate equation includes species up to and including the rate-determining step; intermediates do not appear.
A proposed mechanism must be consistent with the experimentally determined rate equation.
A one-step mechanism involving three or more species is unlikely due to low probability of simultaneous collision.
Intermediates are species formed in one step and consumed in a later step.

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இலவச முன்னோட்டம் — 64-இல் 8 கேள்விகள். அனைத்தையும் பார்க்க பதிவு செய்யவும்.

1.Which statement best describes the rate of a chemical reaction?
Easy
- AThe change in concentration of a reactant or product per unit time
- BThe total amount of product formed when the reaction is complete
- CThe time taken for the reaction to reach completion
- DThe energy released when the reaction occurs
2.Which of the following is a valid unit for the rate of reaction?
Medium
- Amol dm⁻³ s⁻¹
- Bmol dm⁻³
- Cmol s⁻¹
- Ddm³ mol⁻¹ s⁻¹
3.Which of the following methods can be used to measure the rate of a reaction? (select all that apply)
Medium
- AMeasuring mass loss as a gas escapes
- BMeasuring the volume of gas produced using a gas syringe
- CUsing a colorimeter to measure light absorbance
- DMeasuring the temperature of the reaction mixture
- EMeasuring the pH of the solution
4.Increasing the concentration of a reactant increases the rate of reaction because it increases the collision frequency.
Easy
True or false?
5.For the reaction 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g), which piece of equipment is most suitable for measuring the volume of H₂ gas produced over time?
Hard
- AGas syringe
- BBurette
- CVolumetric flask
- DThermometer
6.Which of the following is NOT a factor that affects the rate of a chemical reaction?
Medium
- AThe enthalpy change of the reaction
- BTemperature
- CConcentration of reactants
- DSurface area of a solid reactant
7.A catalyst increases the rate of a reaction by:
Medium
- AProviding an alternative pathway with lower activation energy
- BIncreasing the activation energy of the reaction
- CIncreasing the enthalpy change of the reaction
- DBeing consumed in the reaction
8.On a Maxwell-Boltzmann distribution curve, what does the area under the curve to the right of the activation energy represent?
Medium
- AThe number of particles with energy greater than or equal to the activation energy
- BThe total number of particles in the sample
- CThe average kinetic energy of the particles
- DThe most probable energy of the particles

மேலும் கேள்விகள்

9.Match each term with its correct definition.
Medium
- Activation energy
- Collision frequency
- Catalyst
- Rate of reaction
- Minimum energy required for a reaction to occur
- Number of collisions per unit time
- Substance that increases rate by lowering activation energy
- Change in concentration per unit time
10.Arrange the steps for determining the order of reaction from experimental data in the correct sequence.
Medium
- Choose two experiments where only one reactant concentration changes
- Calculate how the concentration changes
- Calculate how the rate changes
- Deduce the order with respect to that reactant
11.For the reaction CO(g) + Cl₂(g) → COCl₂(g), the rate equation is rate = k[CO][Cl₂]^½. What is the overall order of reaction?
Hard
- A1.5
- B2
- C1
- D0.5
12.Which of the following statements about the rate constant, k, is correct?
Medium
- AIt is independent of concentration but changes with temperature
- BIt is directly proportional to the concentration of reactants
- CIt is always the same for all reactions
- DIt decreases as temperature increases
13.Match each technique for measuring the rate of a reaction with the property it monitors.
Easy
- Colorimetry
- Mass loss
- Gas volume
- Conductivity
- Quenching and titration
- Change in light absorbance or intensity of a coloured species
- Decrease in mass as a gas escapes from the vessel
- Volume of gas collected in a gas syringe or by displacement
- Change in electrical conductivity as ion concentrations change
- Concentration of a sample taken at intervals, with the reaction stopped
14.A student collects gas in a syringe and plots volume against time. The reaction starts quickly, then slows, and finally stops. Which sequence describes the steepest part of the curve, the point where the reaction stops, and the overall shape?
Medium
- The steepest gradient is at the start of the reaction
- The gradient becomes less steep as reactants are used up
- The curve plateaus when the reaction stops
- The volume of gas increases with time
15.Which of the following statements about collision theory are correct? (select all that apply)
Medium
- AA successful collision requires the particles to collide with energy equal to or greater than the activation energy.
- BThe activation energy of a reaction is changed when a catalyst is added.
- CIncreasing the concentration of a solution increases the collision frequency.
- DMost collisions between reactant particles result in a chemical reaction.
- EParticles must collide with the correct orientation for a reaction to occur.
16.A Maxwell-Boltzmann distribution curve is shown for a reaction at temperature T. Which of the following changes would increase the area under the curve to the right of the activation energy? (select all that apply)
Medium
- AIncreasing the temperature
- BAdding a catalyst
- CDecreasing the temperature
- DIncreasing the activation energy
- EDecreasing the activation energy
17.For the reaction CO(g) + Cl₂(g) → COCl₂(g), the rate equation is rate = k[CO][Cl₂]^½. Which of the following statements are correct? (select all that apply)
Hard
- AThe reaction is first order with respect to CO.
- BThe overall order of reaction is 1.5.
- CThe reaction is zero order with respect to Cl₂.
- DDoubling [CO] doubles the rate.
- EDoubling [Cl₂] quadruples the rate.
18.Which of the following statements about the rate constant, k, are correct? (select all that apply)
Medium
- Ak increases as temperature increases.
- Bk has the same units for all reactions.
- Ck is independent of the concentrations of the reactants.
- Dk decreases as the activation energy increases.
- Ek changes when a catalyst is added.
19.In the reaction between sodium thiosulfate and hydrochloric acid, a yellow precipitate of sulfur forms and obscures a cross drawn on paper. What is the main limitation of this 'clock reaction' method?
Medium
- AIt only gives one piece of data for analysis.
- BIt cannot be used with a colorimeter.
- CThe precipitate dissolves as the reaction proceeds.
- DThe reaction is too fast to measure.
20.In a reaction, the concentration of a reactant decreases from 0.50 mol dm⁻³ to 0.20 mol dm⁻³ in 30 seconds. What is the average rate of reaction?
Medium
- A0.010 mol dm⁻³ s⁻¹
- B0.030 mol dm⁻³ s⁻¹
- C0.017 mol dm⁻³ s⁻¹
- D0.30 mol dm⁻³ s⁻¹
21.The rate equation for a reaction is rate = k[NO]²[O₂]. What are the units of the rate constant, k?
Medium
- Amol⁻² dm⁶ s⁻¹
- Bmol⁻¹ dm³ s⁻¹
- Cmol dm⁻³ s⁻¹
- Dmol² dm⁻⁶ s⁻¹
22.A catalyst lowers the activation energy of a reaction by providing an alternative reaction pathway.
Easy
True or false?
23.Increasing the particle size of a solid reactant increases the rate of reaction because it increases the surface area.
Easy
True or false?
24.A student measures the volume of gas produced in a reaction every 10 seconds. Which unit is most appropriate for the rate of this reaction?
Medium
- Acm³ s⁻¹
- Bcm³
- Cs
- Dmol dm⁻³
25.For the reaction between calcium carbonate and hydrochloric acid, the mass of the reaction vessel decreases as carbon dioxide escapes. Which graph correctly represents the change in mass of the vessel against time?
Medium
- AA curve that starts steep and gradually levels off at a lower mass
- BA straight line decreasing at a constant rate
- CA curve that starts shallow and becomes steeper
- DA horizontal line at constant mass
26.Which of the following statements about measuring the rate of reaction by mass loss are correct? (select all that apply)
Medium
- AThe method works best when the gas produced has a high molar mass.
- BThe method involves measuring the decrease in mass of the reaction vessel over time.
- CThe method is suitable for measuring the rate of a reaction that produces hydrogen gas.
- DThe method can be used for any reaction that produces a gas.
- EThe mass loss is measured using a balance.
27.Colorimetry can be used to monitor the formation of a coloured precipitate.
Easy
True or false?
28.The rate of hydrolysis of sucrose under acidic conditions was studied. The data are given below. Experiment | Initial [HCl] / mol dm⁻³ | Initial [sucrose] / mol dm⁻³ | Rate / mol dm⁻³ s⁻¹ 1 | 0.10 | 0.10 | 0.024 2 | 0.10 | 0.15 | 0.036 3 | 0.20 | 0.10 | 0.048 What is the order of reaction with respect to HCl?
Hard
- A0
- B1
- C2
- D3
29.Using the same data (Experiments 1–3), what is the order of reaction with respect to sucrose?
Hard
- A0
- B1
- C2
- D3
30.The reaction between nitrogen monoxide and bromine is proposed to occur via the following two-step mechanism: Step 1: NO + NO → N₂O₂ Step 2: N₂O₂ + Br₂ → 2NOBr What is the overall reaction equation?
Medium
- A2NO + Br₂ → 2NOBr
- BNO + Br₂ → NOBr₂
- C2NO + Br₂ → 2NOBr + N₂O₂
- DNO₂ + Br₂ → NOBr + Br
31.For the reaction NO₂(g) + CO(g) → NO(g) + CO₂(g), an alternative mechanism is: Step 1: NO₂ + NO₂ → NO₃ + NO (slow) Step 2: NO₃ + CO → NO₂ + CO₂ (fast) What is the rate equation for this reaction?
Hard
- Arate = k[NO₂]²
- Brate = k[NO₂][CO]
- Crate = k[NO₂][CO]²
- Drate = k[NO₂]
32.For the mechanism in the previous question (Step 1 slow: NO₂ + NO₂ → NO₃ + NO; Step 2 fast: NO₃ + CO → NO₂ + CO₂), what is the overall order of reaction and the units of the rate constant?
Hard
- AOverall order 2; units dm³ mol⁻¹ s⁻¹
- BOverall order 1; units s⁻¹
- COverall order 2; units dm⁶ mol⁻² s⁻¹
- DOverall order 3; units dm⁶ mol⁻² s⁻¹
33.The Arrhenius equation can be written as k = A e^(−Ea/RT). A graph of ln k against 1/T gives a straight line. What does the gradient of this line represent?
Hard
- A−Ea/R
- BEa/R
- C−Ea
- Dln A
34.Two reactions have different activation energies. Arrhenius plots of ln k against 1/T are drawn for both. Which plot shows the reaction with the greater activation energy?
Medium
- AThe plot with the steeper negative gradient
- BThe plot with the less steep negative gradient
- CThe plot with the higher y-intercept
- DThe plot with the lower y-intercept
35.The rate equation for a reaction is rate = k[NO]²[O₂]. What are the units of the rate constant k?
Medium
- Adm⁶ mol⁻² s⁻¹
- Bdm³ mol⁻¹ s⁻¹
- Cmol dm⁻³ s⁻¹
- Ds⁻¹
36.A catalyst provides an alternative reaction pathway with a lower activation energy and is chemically unchanged at the end of the reaction.
Easy
True or false?
37.Which of the following statements about the effect of temperature on the Maxwell-Boltzmann distribution are correct? (select all that apply)
Medium
- AIncreasing temperature shifts the peak to the right.
- BIncreasing temperature increases the area under the curve to the right of Ea.
- CIncreasing temperature lowers the activation energy.
- DIncreasing temperature makes the curve narrower and taller.
- EIncreasing temperature increases the total number of particles.
38.In a reaction between magnesium and hydrochloric acid, the volume of hydrogen gas produced is measured over time. The graph of volume against time is a curve that becomes less steep. What does the decreasing gradient indicate?
Medium
- AThe rate of reaction is decreasing.
- BThe rate of reaction is increasing.
- CThe reaction has stopped.
- DThe concentration of hydrochloric acid is increasing.
39.The rate equation for the reaction between carbon monoxide and chlorine is rate = k[CO][Cl₂]^½. What is the overall order of reaction?
Hard
- A1.5
- B2
- C0.5
- D3
40.The rate equation for a reaction is rate = k[BrO₃⁻][Br⁻][H⁺]². If the concentration of H⁺ is doubled while all other concentrations remain constant, by what factor does the rate change?
Hard
- A4
- B2
- C8
- D1
41.In a reaction, the concentration of a reactant decreases from 0.50 mol dm⁻³ to 0.20 mol dm⁻³ in 30 seconds. Calculate the average rate of reaction in mol dm⁻³ s⁻¹.
Medium
42.Complete the sentence about collision theory.
Medium
For a reaction to occur, particles must collide with sufficient ____ and correct ____.
43.Complete the sentence about kinetic theory.
Easy
According to kinetic theory, particles in a substance have a range of kinetic energies due to their ____ motion, and at a given temperature the average kinetic energy is proportional to the ____ temperature.
44.For the reaction 2N₂O₅(g) → 4NO₂(g) + O₂(g), the rate equation is rate = k[N₂O₅]. If the concentration of N₂O₅ is tripled, by what factor does the rate of reaction change?
Medium
45.State the term used to describe a collision in which particles collide with the correct orientation and with energy equal to or greater than the activation energy.
Medium
46.Complete the sentence about measuring reaction rates.
Medium
The technique that measures the amount of light absorbed by a coloured solution to monitor reaction rate is called ____.
47.In an experiment, the volume of gas produced is measured every 10 seconds. At 20 seconds, the volume is 40 cm³. At 60 seconds, the volume is 120 cm³. Calculate the average rate of reaction between 20 and 60 seconds in cm³ s⁻¹.
Medium
48.In a clock reaction, the time taken for a cross to disappear is measured. What is the main limitation of this method?
Medium
49.For the reaction A + B → C, the rate equation is rate = k[A][B]². When [A] = 0.10 mol dm⁻³ and [B] = 0.20 mol dm⁻³, the rate is 2.0 × 10⁻³ mol dm⁻³ s⁻¹. Calculate the value of the rate constant k, including units.
Hard
50.A graph of ln k against 1/T has a gradient of −5000 K. Using the gas constant R = 8.31 J K⁻¹ mol⁻¹, calculate the activation energy Ea in kJ mol⁻¹ to 3 significant figures.
Challenge
51.For the reaction 2N₂O₅(g) → 4NO₂(g) + O₂(g), the rate equation is rate = k[N₂O₅]. If the concentration of N₂O₅ is increased from 0.10 to 0.30 mol dm⁻³, by what factor does the rate increase?
Medium
52.For a first-order reaction, the rate constant k = 0.05 s⁻¹. Calculate the rate of reaction when the concentration of the reactant is 0.20 mol dm⁻³. Give your answer in mol dm⁻³ s⁻¹.
Hard
53.Describe kinetic theory in relation to energy and temperature.
Medium
54.State what is required for a collision to result in a reaction.
Medium
55.State the meaning of activation energy (Ea).
Easy
56.Explain what is meant by the order of a reaction and how it may be determined.
Medium
57.Explain why increasing the temperature increases the rate of a chemical reaction.
Medium
58.In a reaction, the concentration of a reactant decreases from 0.50 mol dm⁻³ to 0.20 mol dm⁻³ in 30 seconds. Calculate the average rate of reaction in mol dm⁻³ s⁻¹.
Medium
- A1.01 mol dm⁻³ s⁻¹
- B0.01 mol dm⁻³ s⁻¹
- C2.01 mol dm⁻³ s⁻¹
- D-0.99 mol dm⁻³ s⁻¹
59.For the reaction 2N₂O₅(g) → 4NO₂(g) + O₂(g), the rate equation is rate = k[N₂O₅]. If the concentration of N₂O₅ is tripled, by what factor does the rate of reaction change?
Medium
- A3
- B4
- C5
- D2
60.In an experiment, the volume of gas produced is measured every 10 seconds. At 20 seconds, the volume is 40 cm³. At 60 seconds, the volume is 120 cm³. Calculate the average rate of reaction between 20 and 60 seconds in cm³ s⁻¹.
Medium
- A2 cm³ s⁻¹
- B1 cm³ s⁻¹
- C4 cm³ s⁻¹
- D3 cm³ s⁻¹
61.For the reaction A + B → C, the rate equation is rate = k[A][B]². When [A] = 0.10 mol dm⁻³ and [B] = 0.20 mol dm⁻³, the rate is 2.0 × 10⁻³ mol dm⁻³ s⁻¹. Calculate the value of the rate constant k, including units.
Hard
- A2.5 dm⁶ mol⁻² s⁻¹
- B0.5 dm⁶ mol⁻² s⁻¹
- C1.5 dm⁶ mol⁻² s⁻¹
- D-0.5 dm⁶ mol⁻² s⁻¹
62.A graph of ln k against 1/T has a gradient of −5000 K. Using the gas constant R = 8.31 J K⁻¹ mol⁻¹, calculate the activation energy Ea in kJ mol⁻¹ to 3 significant figures.
Challenge
- A45.8 kJ mol⁻¹
- B41.6 kJ mol⁻¹
- C37.4 kJ mol⁻¹
- D50 kJ mol⁻¹
63.For the reaction 2N₂O₅(g) → 4NO₂(g) + O₂(g), the rate equation is rate = k[N₂O₅]. If the concentration of N₂O₅ is increased from 0.10 to 0.30 mol dm⁻³, by what factor does the rate increase?
Medium
- A5
- B3
- C2
- D4
64.For a first-order reaction, the rate constant k = 0.05 s⁻¹. Calculate the rate of reaction when the concentration of the reactant is 0.20 mol dm⁻³. Give your answer in mol dm⁻³ s⁻¹.
Hard
- A-0.99 mol dm⁻³ s⁻¹
- B1.01 mol dm⁻³ s⁻¹
- C2.01 mol dm⁻³ s⁻¹
- D0.01 mol dm⁻³ s⁻¹

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