Chemical Reaction Applications: Year 11 Chemistry Practice Exam

This original tutorgum practice paper assesses the Chemistry 11-12 Syllabus (2025), Year 11 Chemical reactions focus area. It focuses specifically on evaluating the impact of chemical reactions in medicine and agriculture using secondary-source evidence. This is a focused topic assessment, not a full-course Chemistry examination.

01Exam instructions

General instructions

• Show all working for calculations.
• Include appropriate units and use sensible significant figures.
• Support written responses with specific evidence from the supplied data.
• Distinguish between what the evidence demonstrates and what it cannot establish.
• Where a question asks you to evaluate a claim, provide a judgement supported by evidence and acknowledge relevant limitations.

Assessed scope

This paper uses the Chemistry 11-12 Syllabus (2025), Year 11 Chemical reactions content, specifically the investigation and evaluation of the impact of chemical reactions in medicine or agriculture.

The paper assesses:

• Connecting a specified chemical reaction with its practical application.
• Explaining how the reaction contributes to its intended purpose.
• Interpreting quantitative and qualitative evidence from secondary sources.
• Evaluating the benefits, limitations, and broader impacts of chemical applications.
• Assessing the reliability and relevance of secondary sources.
• Communicating evidence-based scientific conclusions.

This paper does not assess reaction-rate theory, energy changes, equilibrium calculations, general stoichiometric calculations, or content from other syllabus areas or editions.

Important assumptions

All numerical datasets and secondary-source summaries in this paper are fictional, original assessment materials. They have been constructed to provide realistic scientific evidence, but they are not findings from actual published studies or commercial products.

Treat the supplied equations and scientific information as the relevant chemical context for each question. No external research is required.

Where results are presented as mean ± standard deviation (SD), the SD describes the spread of repeated measurements around the mean.

02Questions

Question 1 (10 marks)

Chemical reactions in medicine: Antacid formulations

Magnesium hydroxide is used as an active ingredient in some antacid medicines. It reacts with hydrochloric acid in the stomach.

The chemical reaction is:

\[ \ce{Mg(OH)2(s) + 2HCl(aq) -> MgCl2(aq) + 2H2O(l)} \]

A research group compared two magnesium hydroxide formulations:

• Formulation A: magnesium hydroxide suspension.
• Formulation B: compressed magnesium hydroxide tablet.

An investigation was conducted using simulated gastric fluid. Each test contained 100 mL of hydrochloric acid with an initial concentration of \(0.100\text{ mol L}^{-1}\).

Each formulation supplied 0.20 g of magnesium hydroxide. The samples were maintained at \(37^\circ\text{C}\) and stirred under identical conditions.

Three trials were conducted for each condition. The mean concentration of remaining hydrochloric acid was recorded.

(a) Identify the substance in simulated gastric fluid that reacts with magnesium hydroxide and state the practical purpose of this reaction in an antacid medicine. (2 marks)

(b) Use the supplied chemical equation to explain how the reaction changes the simulated gastric fluid. Name the products formed. (3 marks)

(c) Compare the performance of the suspension and compressed tablet using the results at 5 minutes and 30 minutes. Support your response with numerical evidence. (3 marks)

(d) A product label claims that the suspension is more effective than the tablet. Evaluate this claim using the complete dataset and identify one limitation of the investigation. (2 marks)

Question 2 (12 marks)

Chemical reactions in agriculture: Treating acidic soils

Agricultural lime containing calcium carbonate is applied to acidic soils to reduce their acidity.

The main reaction responsible for reducing soil acidity is:

\[ \ce{CaCO3(s) + 2H+(aq) -> Ca^2+(aq) + CO2(g) + H2O(l)} \]

A field investigation examined how different application rates of calcium carbonate affected soil pH and wheat production.

The investigation used the following conditions:

• All plots had an initial soil pH of 4.6.
• Five different application rates were investigated.
• Four replicate plots were used for each application rate.
• Plots were randomly allocated to treatment groups.
• All plots used the same soil type and wheat cultivar.
• Fertiliser and irrigation conditions were kept consistent.
• Measurements were taken after one growing season.

The following results were reported.

(a) Explain why calcium carbonate is applied to acidic agricultural soils. Refer to the supplied chemical equation in your response. (3 marks)

(b) Describe the relationship between the lime application rate, final soil pH, and wheat grain yield in this investigation. Use numerical evidence. (3 marks)

(c) A grower argues that applying 6 t ha\(^{-1}\) of lime is clearly better than applying 4 t ha\(^{-1}\), because the higher application rate produces a higher soil pH.

Evaluate this claim if the grower’s main objective is to increase wheat grain yield. (3 marks)

(d) Identify one strength and one limitation of the supplied evidence when deciding an appropriate lime application rate for other farms. Explain your choices. (3 marks)

Question 3 (12 marks)

Chemical reactions in agriculture: Urea fertiliser

Urea is widely used as a nitrogen fertiliser in agriculture.

In moist soil, the enzyme urease catalyses the hydrolysis of urea. The reaction is represented by:

\[ \ce{CO(NH2)2 + H2O -> 2NH3 + CO2} \]

The ammonia produced can contribute to the supply of nitrogen in soil. However, some ammonia may escape from the soil surface into the atmosphere.

A field report investigated the effect of different urea application methods on wheat production and ammonia loss.

The investigation used the following conditions:

• All plots contained the same soil type.
• The same wheat cultivar was used.
• Five replicate plots were used for each treatment.
• All plots received identical irrigation.
• No additional nitrogen fertiliser was applied.
• The urea treatments each received 80 kg of nitrogen per hectare.
• Ammonia loss was measured during the first 14 days after application.
• Grain yield was measured at harvest.

The results are presented below.

(a) Explain how the hydrolysis of urea is relevant to its use as a nitrogen fertiliser. Refer to a product of the reaction in your response. (3 marks)

(b) Calculate the percentage of applied nitrogen lost as ammonia for each urea application method.

Give both answers to two significant figures. (3 marks)

(c) Using both the grain yield and ammonia-loss data, assess which application method is better supported if the objective is to maintain crop yield while reducing nitrogen loss. (4 marks)

(d) State one limitation of this investigation and identify one additional measurement or source of information that would help assess the wider impact of using urea fertiliser. (2 marks)

Question 4 (16 marks)

Evaluating secondary sources: Chemical reactions in medicine and agriculture

A student is preparing a secondary-source investigation into the impact of chemical reactions in medicine and agriculture.

The student has located four sources. The following summaries describe the information available from each source.

Source A: Medicine – University research review

A university review summarises three laboratory studies investigating magnesium hydroxide in simulated gastric fluid.

The studies report:

• The temperature and volume of simulated gastric fluid.
• The quantity of magnesium hydroxide used.
• Measurements of remaining hydrochloric acid over time.
• Experimental methods and results.

The review references the original studies. However, none of the investigations involved human participants or measured changes in patient symptoms.

Source B: Medicine – Antacid product website

A commercial website states that its antacid:

• Relieves heartburn in 60 seconds.
• Works better than all other antacid products.

The website does not provide experimental methods, numerical results, sample sizes, or references to supporting studies.

Source C: Agriculture – Agricultural extension review

An independent agricultural extension review summarises 12 field trials investigating the application of calcium carbonate lime.

The trials covered:

• Four different soil types.
• Three different crop species.
• A range of initial soil pH values.

The review reports that crop yield increased in eight trials and remained unchanged in four trials. The largest increases were reported in soils with an initial pH below 5.0.

The review identifies the original studies but does not reproduce their raw data.

Source D: Agriculture – Fertiliser retailer brochure

A fertiliser retailer states that applying urea:

• Eliminates nitrogen loss.
• Guarantees higher crop yields.

The brochure provides no experimental methods, measurements, application conditions, or supporting references.

(a) Select the most useful source for evaluating each of the following applications:

• The use of magnesium hydroxide in medicine.
• The use of calcium carbonate in agriculture.

Justify each selection using a specific feature of the source. (4 marks)

(b) Explain why Sources B and D provide insufficient evidence to support their respective claims. (3 marks)

(c) Evaluate the following two conclusions using the supplied sources. (4 marks)

i. The reaction involving magnesium hydroxide has been shown to cure heartburn in patients.

ii. Applying calcium carbonate lime will increase crop yield on every farm.

(d) A student wants to improve the reliability and quality of their secondary-source investigation.

Describe how the student should:

• Evaluate the quality of the sources and supporting evidence.
• Organise and communicate the information collected.
• Acknowledge the sources used.

(5 marks)

03Worked solutions and marking guide

Solution 1

(a) Identify the reactant and practical purpose. (2 marks)

The substance reacting with magnesium hydroxide is hydrochloric acid (\(\ce{HCl}\)).

The practical purpose is to neutralise excess stomach acid, reducing the acidity of the stomach contents.

Marking guide:

• Identifies hydrochloric acid as the reactant. (1)
• Explains that the reaction reduces excess stomach acidity. (1)

(b) Explain the chemical reaction. (3 marks)

The equation is:

\[ \ce{Mg(OH)2 + 2HCl -> MgCl2 + 2H2O} \]

Magnesium hydroxide reacts with hydrochloric acid.

The reaction produces:

• Magnesium chloride (\(\ce{MgCl2}\)).
• Water (\(\ce{H2O}\)).

The important change is that hydrochloric acid is consumed during the reaction. This reduces the amount of acid remaining in the stomach contents.

The reaction therefore provides a chemical basis for the use of magnesium hydroxide as an antacid.

Marking guide:

• Correctly explains that magnesium hydroxide reacts with hydrochloric acid, with two HCl units consumed per Mg(OH)2 unit. (1)
• Identifies magnesium chloride as a product. (1)
• Identifies water as a product and links consumption of hydrochloric acid to reduced acidity. (1)

(c) Compare the two formulations. (3 marks)

At 5 minutes, the suspension had a remaining hydrochloric acid concentration of \(0.046\text{ mol L}^{-1}\), compared with \(0.071\text{ mol L}^{-1}\) for the tablet.

The suspension therefore produced a greater reduction in remaining acid during the first 5 minutes.

However, by 30 minutes, the remaining acid concentrations were:

• Suspension: \(0.030\text{ mol L}^{-1}\).
• Tablet: \(0.031\text{ mol L}^{-1}\).

These values are very similar, and their standard deviation ranges overlap.

The evidence suggests that the suspension produces a faster initial reduction in acid concentration, but both formulations reach a similar final level under the conditions tested.

Marking guide:

• Correctly compares the 5-minute results using numerical evidence. (1)
• Correctly compares the 30-minute results using numerical evidence. (1)
• Interprets the overall pattern as a faster initial response from the suspension but a similar later result. (1)

(d) Evaluate the product claim. (2 marks)

The claim is only partly supported.

The suspension reduces the acid concentration more rapidly during the first 5 minutes. However, by 15 and 30 minutes, the two formulations produce very similar results.

The investigation also uses simulated gastric fluid rather than human participants. It measures acid concentration, not actual relief from heartburn.

Therefore, the evidence does not establish that the suspension is generally more effective as a medicine.

Marking guide:

• Provides a balanced judgement supported by the results, recognising the faster initial response but similar later results. (1)
• Identifies the absence of human clinical evidence or another relevant limitation. (1)

Total: 10 marks

Solution 2

(a) Explain the agricultural application. (3 marks)

Calcium carbonate is applied to acidic soils to reduce their acidity.

The supplied equation is:

\[ \ce{CaCO3 + 2H+ -> Ca^2+ + CO2 + H2O} \]

Calcium carbonate reacts with hydrogen ions (\(\mathrm{H}^{+}\)) in the soil.

These hydrogen ions are consumed during the reaction, producing calcium ions, carbon dioxide, and water.

As the concentration of hydrogen ions decreases, soil acidity decreases and soil pH increases.

This makes liming useful for treating acidic agricultural soils and improving conditions for crop production.

Marking guide:

• Identifies hydrogen ions as a reactant in the reaction. (1)
• Explains that hydrogen ions are consumed and identifies relevant products. (1)
• Connects the reduction in acidity and increase in pH to the agricultural purpose of liming. (1)

(b) Describe the relationship between application rate, pH, and yield. (3 marks)

As the lime application rate increases, soil pH also increases.

For example, the soil pH rises from 4.6 with no lime to 6.0 at an application rate of 6 t ha\(^{-1}\).

Grain yield also increases as the application rate increases from 0 to 4 t ha\(^{-1}\). It rises from 2.1 t ha\(^{-1}\) to 3.2 t ha\(^{-1}\).

However, increasing the application rate from 4 to 6 t ha\(^{-1}\) does not produce a higher mean grain yield. Both treatments produce 3.2 t ha\(^{-1}\).

The results therefore show an increase in yield followed by a plateau.

Marking guide:

• Identifies the increase in soil pH with increasing lime application rate. (1)
• Identifies the increase in grain yield up to 4 t ha\(^{-1}\), supported by numerical evidence. (1)
• Recognises that grain yield remains unchanged between 4 and 6 t ha\(^{-1}\). (1)

(c) Evaluate the grower’s claim. (3 marks)

The claim is not supported if the main objective is to increase grain yield.

At 4 t ha\(^{-1}\), the mean grain yield is 3.2 t ha\(^{-1}\).

At 6 t ha\(^{-1}\), the mean grain yield is also 3.2 t ha\(^{-1}\).

Although the higher application rate increases soil pH from 5.8 to 6.0, it does not produce an increase in mean grain yield.

The standard deviation ranges for the two yield measurements overlap, so there is no clear evidence of a yield advantage from the higher application rate.

Based on yield alone, applying 4 t ha\(^{-1}\) is better supported than applying 6 t ha\(^{-1}\). However, the data do not establish that 4 t ha\(^{-1}\) is the universally optimal rate. Other considerations, such as application costs and the desired soil pH, would also matter.

Marking guide:

• Correctly identifies that both application rates produce the same mean grain yield. (1)
• Uses the uncertainty or standard deviation data to support the conclusion that a clear yield improvement has not been demonstrated. (1)
• Provides a justified conclusion, distinguishing the higher soil pH from an actual increase in yield. (1)

(d) Identify a strength and limitation. (3 marks)

One strength is that the investigation uses four replicate plots for each treatment, with random allocation and consistent growing conditions.

This makes the comparisons more reliable because the results are less dependent on a single plot, and the application rate is the main planned difference between treatment groups.

One limitation is that the investigation only uses one soil type and one wheat cultivar over one growing season.

The results may not apply to other soil types, crops, climates, or growing seasons. Different farms may therefore experience different responses to the same lime application rate.

Marking guide:

• Identifies a relevant strength, such as replication, random allocation, or controlled conditions. (1)
• Identifies a relevant limitation, such as the use of only one soil type or crop. (1)
• Explains why this limitation restricts the generalisation of the results. (1)

Total: 12 marks

Solution 3

(a) Explain the application of urea hydrolysis. (3 marks)

The hydrolysis of urea produces ammonia:

\[ \ce{CO(NH2)2 + H2O -> 2NH3 + CO2} \]

Ammonia can contribute to the supply of nitrogen in soil. In moist soil, it can form ammonium ions, which are available for plant uptake.

The reaction therefore helps convert applied urea into nitrogen-containing substances that can contribute to plant growth.

However, some ammonia can escape from the soil surface, reducing the amount of nitrogen retained in the agricultural system.

Marking guide:

• Identifies ammonia as a product of urea hydrolysis. (1)
• Explains how ammonia can contribute to ammonium formation and plant nitrogen supply. (1)
• Links the reaction to the practical purpose of nitrogen fertilisation. (1)

(b) Calculate the percentage of nitrogen lost. (3 marks)

The percentage of applied nitrogen lost is calculated using:

\[ \%\text{ N lost} = \frac{\text{Mass of N lost}} {\text{Mass of N applied}} \times 100 \]

Both urea treatments received 80 kg N ha\(^{-1}\).

For surface-applied urea:

\[ \begin{aligned} \%\text{ N lost} &= \frac{8.6}{80}\times100\\ &=10.75\%\\ &\approx 11\% \end{aligned} \]

For urea incorporated into moist soil:

\[ \begin{aligned} \%\text{ N lost} &= \frac{2.1}{80}\times100\\ &=2.625\%\\ &\approx 2.6\% \end{aligned} \]

The percentage of applied nitrogen lost is therefore:

The incorporated treatment has a substantially lower percentage of nitrogen loss.

Marking guide:

• Correctly establishes the percentage calculation using nitrogen lost divided by nitrogen applied. (1)
• Correctly calculates the surface-applied result, approximately 11%. (1)
• Correctly calculates the incorporated result, approximately 2.6%. (1)

(c) Assess the two application methods. (4 marks)

The incorporated application method is better supported if the objective is to maintain crop yield while reducing nitrogen loss.

The mean grain yield for surface-applied urea is 4.1 t ha\(^{-1}\), compared with 4.3 t ha\(^{-1}\) for incorporated urea.

These yields are similar, and their standard deviation ranges overlap. Therefore, the data do not establish a clear difference in crop yield between the two methods.

However, the ammonia-N loss differs substantially:

• Surface-applied urea: 8.6 kg N ha\(^{-1}\).
• Incorporated urea: 2.1 kg N ha\(^{-1}\).

Incorporating urea into moist soil reduces the measured ammonia-N loss by 6.5 kg N ha\(^{-1}\).

This is a substantial reduction, while the mean grain yield remains similar.

Therefore, the incorporated method is better supported because it achieves a comparable grain yield with considerably less nitrogen lost as ammonia.

The evidence does not prove that incorporated urea produces a higher yield, but it does support its advantage in reducing ammonia loss under the conditions tested.

Marking guide:

• Correctly compares the mean grain yields of the two methods. (1)
• Recognises that the yield difference is not clearly established, considering the uncertainty in the results. (1)
• Correctly compares the ammonia-N losses using numerical evidence. (1)
• Reaches a justified conclusion that incorporation is better supported for maintaining yield while reducing ammonia loss. (1)

(d) Identify a limitation and further evidence. (2 marks)

One limitation is that the investigation only examines one soil type, one crop, and one growing season.

The results may not apply to other agricultural conditions.

An additional measurement could be the amount of nitrate leached from the soil. This would help assess another pathway through which nitrogen may be lost from agricultural land.

Alternatively, further field trials across different soil types and growing seasons would help establish whether the results are consistent under a wider range of conditions.

Marking guide:

• Identifies a relevant limitation of the investigation. (1)
• Identifies a suitable additional measurement or secondary source and explains how it would improve the evaluation. (1)

Total: 12 marks

Solution 4

(a) Select the most useful sources. (4 marks)

For the medical application, Source A is the most useful.

It summarises three laboratory studies and provides information about experimental conditions, including temperature, acid volume, and the quantity of magnesium hydroxide used.

It also reports measurements of remaining hydrochloric acid and references the original studies.

This makes it useful for evaluating the chemical reaction and its ability to reduce acid concentration under controlled laboratory conditions.

However, the source cannot establish whether the antacid relieves symptoms in human patients.

For the agricultural application, Source C is the most useful.

It summarises 12 field trials across four soil types and three crop species.

The review also reports variation in the outcomes, showing that yield increased in eight trials but remained unchanged in four.

This provides a broader evidence base for evaluating the agricultural impact of calcium carbonate than a single trial would provide.

Marking guide:

• Selects Source A and justifies its relevance using its laboratory evidence or reported experimental conditions. (2)
• Selects Source C and justifies its relevance using the number and diversity of field trials or its reported results. (2)

(b) Evaluate Sources B and D. (3 marks)

Sources B and D provide insufficient evidence because they make claims without supplying the information needed to evaluate them.

Neither source provides:

• Experimental methods.
• Numerical measurements.
• Sample sizes or the number of trials.
• Conditions under which the products were tested.
• References to supporting research.

Without these details, readers cannot assess whether the claims are based on reliable evidence or whether the results could be reproduced.

The use of absolute statements such as “works better than all other products” and “eliminates nitrogen loss” is also problematic because neither claim is supported by supplied evidence.

Commercial sources may provide useful information about products, but their claims should be checked against independent scientific evidence.

Marking guide:

• Identifies the absence of experimental methods and numerical evidence. (1)
• Explains the importance of missing conditions, sample sizes, or references. (1)
• Explains why unsupported absolute or commercial claims cannot establish effectiveness. (1)

(c) Evaluate the two conclusions. (4 marks)

i. The reaction involving magnesium hydroxide has been shown to cure heartburn in patients.

This conclusion is not supported by the evidence.

Source A describes laboratory investigations using simulated gastric fluid. These investigations measure the reduction in hydrochloric acid concentration.

Although this demonstrates the chemical reaction’s ability to consume acid under laboratory conditions, it does not establish that patients experience relief from heartburn.

No human participants or symptom measurements were included.

Human clinical evidence would be required to support a claim about treating or curing heartburn.

Marking guide:

• Identifies that the evidence does not establish a cure in patients. (1)
• Explains that laboratory acid-neutralisation results cannot establish clinical outcomes without human evidence. (1)

ii. Applying calcium carbonate lime will increase crop yield on every farm.

This conclusion is also not supported.

Source C reports that crop yield increased in eight of the 12 trials but remained unchanged in four.

The review also identifies a relationship between the initial soil pH and the size of the yield response. The largest responses occurred in soils with an initial pH below 5.0.

This suggests that the agricultural impact depends on the conditions of the soil and the crop being grown.

The results therefore do not justify claiming that lime will increase crop yield on every farm.

Marking guide:

• Identifies that the evidence does not support a universal increase in crop yield. (1)
• Uses the mixed trial outcomes or variation with initial soil pH to justify this conclusion. (1)

(d) Improve the secondary-source investigation. (5 marks)

A strong secondary-source investigation should use a systematic approach to collecting, evaluating, and communicating evidence.

1. Evaluate source quality

The student should compare multiple independent sources and examine their research methods, sample sizes, experimental conditions, publication dates, and supporting references.

Sources with clear methods, relevant measurements, and independently verifiable evidence are generally more useful than sources making unsupported claims.

The student should also consider whether different sources agree and whether their results apply to the specific chemical application being investigated.

2. Organise and communicate the evidence

The student should extract relevant quantitative and qualitative information from the sources.

A table could be used to compare:

• The chemical reaction being investigated.
• Its practical application.
• The benefits reported.
• Any negative impacts or limitations.
• The conditions under which the reported results were obtained.

Graphs could be used where numerical data are available and comparable.

The final report should distinguish between findings reported by the sources and conclusions drawn by the student.

3. Acknowledge sources

The student should record the details of each source and use a consistent referencing style.

This allows readers to locate the original information, check the evidence, and distinguish published findings from the student’s own interpretation.

Marking guide:

• Explains how to evaluate source quality using relevant criteria such as methods, sample size, independence, or supporting references. (2)
• Describes an appropriate way to organise and communicate evidence, such as a labelled table, graph, or structured report. (2)
• Explains the importance of consistently acknowledging sources and distinguishing evidence from personal interpretation. (1)

Total: 16 marks

04Final mark summary

End of practice paper.