Rate of reaction

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Measuring Rates of Reaction

The rate of reaction can be measured by how fast a reactant is used up or how fast a product is made.
Rate = amount of reactant used / time taken OR Rate = amount of product made / time taken.
Time is usually measured in seconds; the amount of reactant or product may be measured in grams (g) or volume (cm³ or dm³).
The units of rate depend on the measurement: g/s for mass, cm³/s or dm³/s for volume.
Several measurements are taken during a reaction to provide sufficient data to establish a conclusion.
Measuring the amount of product forming is usually easier than measuring the amount of reactant being used up.

Methods for Measuring Rate

Mass loss: when a gas is produced, it escapes and the mass decreases; the reaction is carried out in a flask on a balance and mass is recorded over time.
Cotton wool can be placed in the neck of the flask to allow gas to escape while stopping any materials from being ejected.
The gas must be sufficiently dense for the mass change to be measurable; carbon dioxide (Mr = 44) is suitable but hydrogen (Mr = 2) is not.
Gas collection: the gas produced is trapped and its volume measured over time, using an inverted measuring cylinder (downward displacement of water) or a gas syringe.
Precipitation: a precipitate clouds the reaction mixture; the time for a cross to disappear from view is measured.
The precipitation method is subjective because different people may not agree on the exact moment the cross disappears, and it produces only one data point per experiment so a rate graph cannot be plotted.

Reaction Rates Using Moles

Rate of reaction can be expressed in mol/s; there is no direct way to measure moles, so first calculate the rate using the rate formula triangle.
To convert mass per unit time to moles per unit time: moles per unit time = mass per unit time / molar mass of the substance.
To convert volume per unit time to moles per unit time: moles per unit time = volume per unit time / molar gas volume (24 000 cm³/mol).
For example, if 6.0 g of CO₂ (Mr = 44.0) is lost in 150 s, moles = 6.0 / 44.0 = 0.137 mol, so rate = 0.137 / 150 = 9.1 × 10⁻⁴ mol/s.
If 15 cm³ of gas is collected in 360 s, moles = 15 / 24000 = 6.25 × 10⁻⁴ mol, so rate = 6.25 × 10⁻⁴ / 360 = 1.7 × 10⁻⁶ mol/s.

Rate Graphs

Time is normally plotted on the x-axis, with the concentration of reactant or product on the y-axis.
If the relationship is directly proportional (e.g. doubling concentration doubles rate), the graph is a straight line through the origin.
The gradient of the line equals the initial rate of reaction; a steeper gradient means a faster rate.
An initial rate graph for a product shows a straight line with positive correlation starting from the origin; for a reactant it shows a straight line with negative correlation starting from the y-axis.
Plotting a graph until completion shows the rate slowing as reactants are used up; the line becomes less steep and eventually horizontal, indicating the reaction has finished.
The mean rate of reaction between two points is found by calculating the total change in y divided by the total time taken.

Graph showing the effect of using a catalyst on rate of reaction

Calculating Gradients

For a curved graph, the rate of reaction is different at each point, so a tangent must be drawn to calculate the rate at a particular point.
Place a ruler on the point being studied and adjust its position so the space on either side of the point between the ruler and curve are equal.
Use a ruler and pencil to draw a perfectly straight line that just touches the curve at the chosen point, keeping the curve visible.
Extend the tangent as far as is convenient to decrease the uncertainty in the calculation.
The gradient at that point is: gradient = change in product / change in time (rise over run).
For example, if a tangent at 0 s shows 72 cm³ of product in 20 s, initial rate = 72 / 20 = 3.6 cm³/s.

Factors Affecting Rate

The rate of reaction is affected by: concentration of reactants in solution or pressure of reacting gases, temperature, surface area of solid reactants, and the presence of a catalyst.
Increasing concentration or pressure increases the rate: the graph has a steeper gradient at the start, becomes horizontal sooner, but forms the same amount of product.
Increasing temperature increases the rate: the graph has a steeper gradient at the start, becomes horizontal sooner, but forms the same amount of product.
Increasing surface area (or decreasing particle size) increases the rate: the graph has a steeper gradient at the start, becomes horizontal sooner, but forms the same amount of product.
Adding a catalyst increases the rate: the graph has a steeper gradient at the start, becomes horizontal sooner, but forms the same amount of product.
A higher rate of reaction is of economic interest as it implies a higher rate of production and a more efficient and sustainable process.

Required Practical: Investigating Concentration

Part 5a – Measuring turbidity: investigate the effect of changing concentration on the rate of reaction by measuring the formation of a precipitate.
Method: measure 50 cm³ of sodium thiosulfate solution into a conical flask, place the flask on a white paper with a cross, add 10 cm³ of dilute hydrochloric acid, start the stopwatch, and stop when the cross can no longer be seen.
The reaction produces solid sulfur which precipitates, turning the solution cloudy: Na₂S₂O₃ + 2HCl → 2NaCl + S + SO₂ + H₂O.
Repeat using different concentrations of sodium thiosulfate solution (mix with water to dilute) and repeat each experiment three times.
Part 5b – Measuring gas volume: investigate the effect of changing concentration on the rate of reaction by measuring the volume of gas given off.
Method: measure 50 cm³ of 1.0 mol/dm³ sulfuric acid into a flask, add magnesium ribbon, connect to gas collection equipment, start the stopwatch, and record the volume of gas every 10 seconds; repeat with 1.5 mol/dm³ sulfuric acid.
Magnesium and sulfuric acid react immediately, so the flask must be connected to the gas syringe straight after adding the magnesium to avoid gas escaping.

Collision Theory and Activation Energy

Chemical reactions occur only when reactant particles collide with sufficient energy to react; the minimum amount of energy needed is called the activation energy.
A successful collision occurs when particles collide with energy greater than or equal to the activation energy, leading to a rearrangement of atoms to form products.
An unsuccessful collision occurs when particles collide with less energy than the activation energy; they bounce off each other and remain unchanged.
The rate of reaction depends on the number of successful collisions per unit time.
Factors that increase the number of successful collisions: more particles per unit volume, greater frequency of collisions, greater kinetic energy of particles, and lower activation energy.
Increasing concentration or pressure: more particles in a given volume (or same particles in a smaller volume) causes more frequent successful collisions.
Increasing temperature: particles have more kinetic energy, causing more frequent and more energetic collisions; a small increase in temperature causes a large increase in rate (roughly doubles for every 10 °C rise).
Increasing surface area: a greater surface area of particles is exposed to the other reactant, causing more frequent successful collisions.

A successful collision

Catalysts

A catalyst speeds up the rate of a reaction without itself being altered or consumed; the mass of a catalyst at the beginning and end of a reaction is the same.
Catalysts provide an alternative pathway for the reaction that has a lower activation energy.
This means a higher proportion of reactant particles have energy greater than the activation energy, resulting in more successful collisions per second.
Catalysts do not form part of the chemical equation but are sometimes shown above or below the reaction arrow.
Iron is used as a catalyst in the Haber Process for the production of ammonia; iron beads increase the surface area available for catalysis.
Catalysts work by attracting reactant molecules onto their surface, providing an alternate reaction pathway of lower energy.
Catalysis is important commercially because catalysts increase the rate of reaction (and production rate) and reduce energy costs.
Transition metals are widely used as catalysts because they have variable oxidation states, allowing them to readily donate and accept electrons; enzymes act as catalysts in biological systems.

How a catalyst affects rate

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Übungsfragen

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1.Which piece of equipment is most suitable to measure the volume of hydrochloric acid?
Easy
- ABeaker
- BMeasuring cylinder
- CConical flask
- DTest tube
2.In an experiment where calcium carbonate reacts with hydrochloric acid and the volume of gas produced is measured every 30 seconds, which is the dependent variable?
Easy
- AVolume of gas collected
- BConcentration of hydrochloric acid
- CMass of calcium carbonate
- DTime
3.A student investigates the rate of reaction between calcium carbonate and hydrochloric acid. They measure the volume of gas produced every 30 seconds. Which of the following would be a suitable control variable?
Medium
- AVolume of gas produced
- BTime
- CConcentration of hydrochloric acid
- DRate of reaction
4.In the reaction between calcium carbonate and hydrochloric acid, what is the gas produced?
Medium
- AHydrogen
- BOxygen
- CCarbon dioxide
- DSulfur dioxide
5.A student reacts calcium carbonate with hydrochloric acid. They plot a graph of volume of gas produced against time. On the same axes, they sketch a curve for a higher concentration of acid. Which of the following describes the new curve?
Medium
- ASteeper gradient and higher final volume
- BSteeper gradient and same final volume
- CLess steep gradient and same final volume
- DSame gradient and higher final volume
6.In the reaction between sodium thiosulfate and hydrochloric acid, what causes the solution to turn cloudy?
Easy
- ASulfur dioxide gas
- BSulfur precipitate
- CSodium chloride
- DWater
7.At 30°C, 0.46 g of sulfur was produced in 58 seconds. Calculate the mean rate of reaction in g/s to 2 significant figures.
Medium
- A0.0079 g/s
- B0.0080 g/s
- C0.0078 g/s
- D0.008 g/s
8.As the temperature of hydrochloric acid increases, the time taken for the cross to disappear decreases. Which word completes the sentence: The rate of reaction ________ as the temperature increases.
Medium
- Aincreases
- Bdecreases
- Cremains the same
- Ddoubles

Mehr Fragen

9.Complete the sentence: A catalyst provides an alternative ________ that has a lower ________ energy.
Medium
- Apathway, activation
- Broute, kinetic
- Cmethod, potential
- Dpath, thermal
10.By the end of a reaction, the mass of the catalyst is:
Easy
- Alower
- Bhigher
- Cthe same
- Dzero
11.What is the catalyst in the breakdown of methanol in the body?
Medium
- ANucleotide
- BEster
- CEnzyme
- DAmino acid
12.The rate of a reaction was monitored by recording the volume of gas produced every 5 seconds. What is the mean rate of reaction in the first 10 seconds, in cm³/s?
Medium
- A2.9
- B3.0
- C3.5
- D2.5
13.In an experiment, 15 cm³ of gas was collected after 6 minutes. Calculate the rate of reaction in cm³/s.
Medium
- A0.042 cm³/s
- B2.5 cm³/s
- C0.25 cm³/s
- D0.0042 cm³/s
14.A reaction produces 24 cm³ of gas in 2 minutes. What is the rate of reaction in cm³/s?
Medium
- A0.2 cm³/s
- B0.4 cm³/s
- C12 cm³/s
- D0.12 cm³/s
15.A catalyst is consumed during a chemical reaction.
Easy
True or false?
16.Increasing the surface area of a solid reactant increases the rate of reaction.
Easy
True or false?
17.The rate of reaction decreases as temperature increases.
Easy
True or false?
18.The activation energy is the minimum energy that colliding particles must have for a reaction to occur.
Medium
True or false?
19.In the reaction between sodium thiosulfate and hydrochloric acid, the time taken for the cross to disappear increases as the concentration of sodium thiosulfate increases.
Medium
True or false?
20.When a gas is produced in a reaction and the mass loss is measured, the gas must have a sufficiently high density to be detected by the balance.
Medium
True or false?
21.Match each factor to its effect on the rate of reaction.
Medium
- Increasing concentration
- Increasing temperature
- Increasing surface area
- Adding a catalyst
- Increases rate by providing more particles per unit volume
- Increases rate by increasing kinetic energy of particles
- Increases rate by exposing more particles to collisions
- Increases rate by lowering activation energy
22.Match each term to its definition.
Medium
- Rate of reaction
- Activation energy
- Catalyst
- Collision theory
- The change in amount of reactant or product per unit time
- The minimum energy required for a reaction to occur
- A substance that speeds up a reaction without being consumed
- The idea that particles must collide with sufficient energy to react
23.Arrange the steps of the disappearing cross experiment in the correct order.
Medium
- Measure 50 cm³ of sodium thiosulfate solution into a conical flask.
- Place the flask on a white paper with a cross marked on it.
- Measure 10 cm³ of dilute hydrochloric acid into a measuring cylinder.
- Add the acid to the flask and start the stopwatch.
- Stop the stopwatch when the cross can no longer be seen.
24.Which of the following are valid methods for measuring the rate of a chemical reaction? (select all that apply)
Medium
- AMeasuring mass loss using a balance
- BMeasuring gas volume using a gas syringe
- CMeasuring the time for a precipitate to obscure a cross
- DMeasuring the temperature change of the reaction mixture
- EMeasuring the pH change of the reaction mixture
25.Match each variable in the reaction between calcium carbonate and hydrochloric acid to its correct description when investigating the effect of concentration on rate.
Medium
- Independent variable
- Dependent variable
- Control variable
- Concentration of hydrochloric acid
- Volume of gas produced
- Mass of calcium carbonate
26.Which of the following statements about collision theory are correct? (select all that apply)
Medium
- AA successful collision occurs when particles collide with energy greater than or equal to the activation energy.
- BIncreasing the concentration increases the frequency of collisions.
- CA catalyst lowers the activation energy of a reaction.
- DIncreasing the temperature only increases the frequency of collisions, not the energy of collisions.
- EThe rate of reaction depends only on the number of collisions, not their energy.
27.In the reaction between sodium thiosulfate and hydrochloric acid, what is the product that causes the solution to turn cloudy?
Medium
- ASulfur dioxide gas
- BSodium chloride
- CWater
- DSulfur precipitate
28.In the reaction between magnesium and hydrochloric acid, the volume of hydrogen gas produced is measured over time. Which of the following would produce a steeper initial gradient on the graph?
Medium
- AUsing the same mass of magnesium ribbon instead of powder
- BUsing a lower concentration of hydrochloric acid
- CUsing the same mass of magnesium powder instead of ribbon
- DCooling the reaction mixture
29.A student measures the volume of gas collected (in cm³) every 10 seconds for a reaction. Which equation should they use to calculate the mean rate of reaction?
Medium
- Amean rate = volume of gas collected × time taken
- Bmean rate = time taken ÷ volume of gas collected
- Cmean rate = volume of gas collected ÷ time taken
- Dmean rate = change in volume of gas ÷ time taken
30.A student monitors the rate of reaction by measuring the volume of gas produced over time. Which piece of apparatus is most suitable for measuring the volume of gas produced?
Medium
- AMeasuring cylinder
- BGas syringe
- CConical flask
- DBalance
31.The rate of a reaction was measured by recording the volume of gas produced every 5 seconds. The volume collected after 10 seconds was 30 cm³. What is the mean rate of reaction in the first 10 seconds, in cm³/s?
Medium
- A0.3
- B3.0
- C30
- D300
32.In a reaction, 0.48 g of gas escaped from a flask in 80 seconds. What is the mean rate of reaction in g/s?
Medium
- A0.006
- B0.06
- C0.6
- D6.0
33.The mass loss method is suitable for measuring the rate of a reaction that produces hydrogen gas.
Easy
True or false?
34.Which of the following statements about measuring the rate of reaction by precipitation are correct? (select all that apply)
Hard
- AThe time taken for a cross to disappear is measured.
- BIt produces a continuous set of data that can be plotted as a graph.
- CIt relies on a subjective judgement of when the cross disappears.
- DIt can be used when two clear solutions react to form a solid precipitate.
- EIt measures the volume of gas produced over time.
35.A student measures the volume of gas produced in a reaction over time. They plot a graph of volume of gas (y-axis) against time (x-axis). What does the gradient of the graph represent?
Medium
- AThe total volume of gas produced
- BThe rate of reaction
- CThe time taken for the reaction to finish
- DThe mass of gas produced
36.A graph of volume of gas produced against time is plotted for a reaction. The curve becomes horizontal after 60 seconds. What does this show?
Medium
- AThe reaction has stopped because a reactant has been used up.
- BThe rate of reaction is increasing.
- CThe reaction is at its fastest at 60 seconds.
- DThe catalyst has been used up.
37.A graph of volume of gas produced against time is plotted. To find the rate of reaction at 20 seconds, what should be drawn on the curve at that point?
Hard
- AA horizontal line
- BA tangent
- CA vertical line
- DA line of best fit through the origin
38.A graph shows the volume of gas produced against time for a reaction. Which of the following are true about the graph? (select all that apply)
Hard
- AThe initial rate of reaction is given by the gradient at the start of the curve.
- BThe curve becomes less steep over time as reactants are used up.
- CThe final volume of gas produced is the same regardless of how fast the reaction is.
- DThe gradient of the curve increases as the reaction proceeds.
- EThe reaction is complete when the curve becomes horizontal.
39.A student reacts calcium carbonate with hydrochloric acid and plots the volume of gas produced against time. They then repeat the experiment using a higher concentration of hydrochloric acid, with the acid in excess. Which statement describes the new graph compared with the original?
Medium
- AThe gradient at the start is steeper and the final volume of gas is the same.
- BThe gradient at the start is less steep and the final volume of gas is greater.
- CThe gradient at the start is steeper and the final volume of gas is greater.
- DThe gradient at the start is the same and the final volume of gas is the same.
40.A student reacts 1.0 g of magnesium ribbon with excess hydrochloric acid and plots the volume of hydrogen produced against time. They repeat the experiment using 1.0 g of magnesium powder instead. How does the graph for the powder differ?
Hard
- AThe gradient is steeper and the final volume of gas is the same.
- BThe gradient is less steep and the final volume of gas is the same.
- CThe gradient is steeper and the final volume of gas is greater.
- DThere is no difference.
41.Complete the sentence about the rate of reaction.
Easy
The rate of reaction can be calculated using the equation: rate = amount of reactant used ÷ ____.
42.Complete the sentence about collision theory.
Medium
The minimum amount of energy needed for a reaction to occur is called the ____ energy.
43.Complete the sentence about catalysts.
Medium
A catalyst provides an alternative ____ with a lower activation energy.
44.Complete the sentence about factors affecting rate.
Easy
Increasing the concentration of a reactant ____ the rate of reaction.
45.Complete the sentence about the disappearing cross experiment.
Medium
In the reaction between sodium thiosulfate and hydrochloric acid, the time taken for the cross to disappear ____ as the concentration of sodium thiosulfate increases.
46.A reaction produces 36 cm³ of gas in 90 seconds. Calculate the mean rate of reaction in cm³/s.
Medium
47.In a reaction, 0.72 g of product is formed in 120 seconds. Calculate the mean rate of reaction in g/s.
Medium
48.A reaction produces 0.24 g of gas in 2 minutes. Calculate the rate of reaction in g/s.
Hard
49.What is the name of the gas produced when calcium carbonate reacts with hydrochloric acid?
Medium
50.What is the name of the precipitate formed when sodium thiosulfate reacts with hydrochloric acid?
Medium
51.What is the name of the biological catalyst that breaks down methanol in the body?
Medium
52.In a reaction, 36 cm³ of gas is produced in 90 seconds. Calculate the mean rate of reaction in cm³/s.
Medium
53.Explain why the rate of reaction is faster at higher pressures.
Medium
54.Explain how a catalyst increases the rate of a reaction.
Medium
55.Name two factors other than concentration and the use of a catalyst that can speed up the rate of a reaction.
Medium
56.Describe how you would use the disappearing cross method to investigate the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid. Include the measurements you would take and how you would ensure a fair test.
Hard
57.Explain why increasing the temperature increases the rate of reaction.
Hard
58.A reaction produces 36 cm³ of gas in 90 seconds. Calculate the mean rate of reaction in cm³/s.
Medium
- A-0.6 cm³/s
- B1.4 cm³/s
- C2.4 cm³/s
- D0.4 cm³/s
59.In a reaction, 0.72 g of product is formed in 120 seconds. Calculate the mean rate of reaction in g/s.
Medium
- A1.006 g/s
- B0.006 g/s
- C2.006 g/s
- D-0.994 g/s
60.A reaction produces 0.24 g of gas in 2 minutes. Calculate the rate of reaction in g/s.
Hard
- A1.002 g/s
- B-0.998 g/s
- C2.002 g/s
- D0.002 g/s
61.In a reaction, 36 cm³ of gas is produced in 90 seconds. Calculate the mean rate of reaction in cm³/s.
Medium
- A0.4 cm³/s
- B2.4 cm³/s
- C-0.6 cm³/s
- D1.4 cm³/s

Karteikarten

What is the rate of reaction?

The rate of reaction is a measure of how fast a reactant is used up or how fast a product is made.

What are the three methods used to determine the rate of reaction?

Mass loss, gas collection, and precipitation.

What is the formula for calculating the rate of reaction?

Rate = amount of reactant used / time taken OR Rate = amount of product made / time taken.

What are the units for rate of reaction if mass is measured in grams?

g/s

What are the units for rate of reaction if volume is measured in cm³?

cm³/s

Describe the mass loss method for measuring rate of reaction.

The reaction is carried out in an open flask on a balance; as gas escapes, the mass decreases and is recorded over time.

Why is cotton wool placed in the neck of the flask in the mass loss method?

To allow gas to escape while preventing any materials from being ejected.

What is the gas collection method for measuring rate of reaction?

The gas produced is trapped and its volume is measured over time, e.g., using a gas syringe or inverted measuring cylinder.

What is the precipitation method for measuring rate of reaction?

The time taken for a precipitate to form and obscure a cross viewed through the solution is measured.

What are the disadvantages of the precipitation method?

It is subjective (different people may not agree on when the cross disappears) and only one data point is produced per experiment, so a rate graph cannot be plotted.

How do you calculate the mean rate of reaction from a graph?

Find the change in y (amount of product or reactant) between two points and divide by the change in x (time taken).

How do you calculate the rate of reaction at a specific point on a curved graph?

Draw a tangent to the curve at that point and calculate its gradient (change in y ÷ change in x).

What does a steeper gradient on a rate graph indicate?

A faster rate of reaction.

What are the four factors that affect the rate of reaction?

Concentration (or pressure), temperature, surface area, and presence of a catalyst.

How does increasing concentration affect the rate of reaction?

It increases the rate because there are more reactant particles in a given volume, leading to more frequent successful collisions.

How does increasing pressure affect the rate of reaction?

It increases the rate because the same number of particles occupies a smaller volume, leading to more frequent successful collisions.

How does increasing temperature affect the rate of reaction?

It increases the rate because particles have more kinetic energy, leading to more frequent and more energetic successful collisions.

How does increasing surface area affect the rate of reaction?

It increases the rate because a greater surface area of reactant is exposed, leading to more frequent successful collisions.

What is collision theory?

Chemical reactions occur only when reactant particles collide with sufficient energy (equal to or greater than the activation energy).

What is activation energy?

The minimum amount of energy that colliding particles need to react.

What is a successful collision?

A collision where the reactant particles have energy greater than or equal to the activation energy, resulting in a rearrangement of atoms to form products.

What is an unsuccessful collision?

A collision where the reactant particles have insufficient energy (less than the activation energy) and just bounce off each other without reacting.

What is a catalyst?

A substance that speeds up the rate of a reaction without being altered or consumed; its mass remains the same at the beginning and end of the reaction.

How does a catalyst increase the rate of a reaction?

It provides an alternative reaction pathway with a lower activation energy, so a higher proportion of collisions are successful.

What is the effect of a catalyst on the total amount of product formed?

The total amount of product formed remains the same; the catalyst only increases the rate at which it is formed.

In the reaction between sodium thiosulfate and hydrochloric acid, what causes the solution to turn cloudy?

Solid sulfur is produced as a precipitate, which turns the solution cloudy.

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