Chemistry 2007 Sample assessment instrument

Extended experimental investigation: Reaction rate This sample has been compiled by the QSA to help teachers plan and develop assessment instruments for individual school settings. It demonstrates the following criteria: • Knowledge and conceptual understanding • Investigative processes • Evaluating and concluding

Assessment instrument The indicative response presented in this sample is in response to an assessment task. Task Develop an experiment to investigate a factor or factors that affect the rate of a chemical reaction. Present the results of your investigation as a written scientific report. Guidelines • Use the preliminary experiment to design an investigation that will focus on one or more factors affecting reaction rates. In your investigation you should consider refinements to the method already given to provide more accurate data. Other suitable reactions may be investigated. • Alternative research questions may be investigated subject to negotiation with your teacher. • Students will be assigned into groups of 3–4 to develop and perform the experiment. Analysis and reporting will be conducted individually. • Four weeks of class time will be set aside for this investigation. • Length: 1000–1500 words.

14035 R1

Raw data, risk assessment and calculations are documented in a journal not included here.

Instrument-specific criteria and standards The indicative response has been matched to instrument-specific criteria and standards; those which best describe the work in this sample are shown below. For more information about the syllabus dimensions and standards descriptors, see www.qsa.qld.edu.au/1952-assessment.html.

Standard A Knowledge and conceptual understanding

Investigative processes

Evaluating and concluding

The student work has the following characteristics: • reproduction and interpretation of complex and challenging reaction rate concepts, theories and principles • comparison and explanation of complex reaction rate concepts, processes and phenomena • linking and application of reaction rate algorithms, concepts, principles and theories to find solutions in complex and challenging reaction rate situations • formulation of justified significant hypotheses which inform effective and efficient design, refinement and management of investigations • selection and adaptation of equipment, and appropriate application of technology to gather, record and process valid data • systematic analysis of primary and secondary data to identify relationships between patterns, trends, errors and anomalies • analysis and evaluation of complex scientific interrelationships • exploration of scenarios and possible outcomes with justification of conclusions/ recommendations • discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of range of formats.

Note: Colour highlights have been used in the table to emphasise the qualities that discriminate between the standards.

2 | Chemistry 2007: Sample assessment instrument Reaction rate

Indicative response — Standard A The annotations show the match to the instrument-specific standards.

Introduction Marble is calcium carbonate, CaCO3. When reacted with hydrochloric acid (HCl), a salt calcium chloride (CaCl2), carbon dioxide (CO2) and water (H2O) are formed: reproduction and interpretation of complex and challenging reaction rate concepts, theories and principles

Equation 1:

CaCO3( s ) + 2 HCl ( aq ) → CaCl 2 ( aq ) + H 2 O(l ) + CO2 ( g ) Several steps are involved. Equation 2:

CaCO3( s ) + 2 HCl ( aq ) → CaCl 2 ( aq ) + H 2 CO3( aq ) Equation 3:

H 2CO3( aq ) → CO2 ( g ) + H 2O( l ) This reaction is suited to an investigation on reaction rates because the reaction of marble with acid generally occurs over a period of minutes. It produces gaseous carbon dioxide meaning that the rate can be determined either by measuring the volume of gas produced or, in this case, by recording the mass lost as gas is given off. It is well established that increasing the concentration of liquid or gaseous reactants will increase the rate of a reaction; increasing the surface area of a solid reactant will also increase the rate of a reaction. For any reaction, the rate is dependent on the concentration of a single reactant (R) in the relationship (Chemguide). reproduction and interpretation of complex and challenging reaction rate theories and principles

Rate ∝ [R]

n

where n is the order of the reaction

In this reaction, it is expected that as marble chips are in the solid form, and their concentration does not change, the rate will only depend upon the concentration of hydrochloric acid, and is likely to be first order with respect to HCl. If the reaction is first order with respect to HCl, then the reaction rate will be directly proportional to the HCl concentration. However, in this experiment, the concentration of HCl will not remain constant. Initially the HCl concentration is at its maximum, but as the acid is neutralised the concentration drops. This is expected to lead to a drop in the rate at which carbon dioxide is produced and so the decrease in mass will become more gradual.

Aim

formulation of justified significant hypotheses which informs the effective and efficient design, refinement and management of the investigation

To investigate the effect of reactant concentration and surface area on the rate of the reaction between marble chips and hydrochloric acid and to determine the order of reaction.

Hypothesis • • • •

As the concentration of the acid is increased then the reaction rate will increase. As the size of the marble chips is increased then the reaction rate will decrease. The rate of the reaction will decrease as HCl is consumed. The reaction rate will be directly proportional to the HCl concentration.

Queensland Studies Authority March 2014 | 3

Method The preliminary experiment was repeated three times using the following combinations of reagents: • 1.5M hydrochloric acid with large marble chips effective and efficient design, refinement and management of investigations

• 3M hydrochloric acid with large marble chips • 1.5M hydrochloric acid with small marble chips • 3M hydrochloric acid with small marble chips A reading was taken initially every 15 seconds for a minute and then every 30 seconds for the next 4 minutes. This was done as the first trial showed that in the first 2 minutes the reaction proceeded rapidly and then the amount of gas lost seemed to become quite small.

Results selection and adaptation of equipment, and appropriate application of technology to gather, record and process valid data

Table 1: Mass loss in reaction between 10mL HCl and excess CaCO3 Time (s) 0 15 30 45 60 90 120 150 180 210 240 270 300

Average mass lost (g) Small chips 1.5M HCl

Large chips 1.5M HCl

Small chips 3M HCl

Large chips 3M HCl

0.000 0.093 0.174 0.217 0.239 0.259 0.266 0.272 0.276 0.281 0.282 0.284 0.285

0.000 0.072 0.112 0.145 0.169 0.197 0.218 0.233 0.239 0.248 0.254 0.256 0.260

0.000 0.179 0.407 0.541 0.574 0.613 0.623 0.630 0.636 0.637 0.640 0.641 0.644

0.000 0.067 0.166 0.271 0.346 0.432 0.488 0.523 0.549 0.566 0.579 0.588 0.597

Results recorded on 21 June 2013. Room temperature 17°C.

discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a range of formats

Note: The masses shown in the table are the averages taken from 3 trials — the data from individual experiments are in the journal. However it is important to note that while there was variation between trials, it was relatively small (less than ±0.02g). Graph 1: Mass loss in the reaction between CaCO3 and HCl

4 | Chemistry 2007: Sample assessment instrument Reaction rate

Table 2: Average reaction rate – mass of CO2 lost per second Time interval (s) 0 – 15 15 – 30 30 – 45 45 – 60 60 – 90 90 – 120 120 – 150 150 – 180 180 – 210 210 – 240 240 – 270 270 – 300

Reaction rate (mg/s) Small chips 1.5M HCl

6.20* 5.40 2.87 1.47 0.67 0.23 0.20 0.13 0.17 0.03 0.06 0.03

Large chips 1.5M HCl

4.80 2.67 2.20 1.60 0.93 0.70 0.50 0.20 0.30 0.20 0.07 0.13

Small chips 3M HCl

11.93 15.20 8.93 2.20 1.30 0.33 0.23 0.20 0.03 0.10 0.03 0.10

Large chips 3M HCl

4.47 6.60 7.00 5.00 2.87 1.87 1.17 0.87 0.57 0.43 0.30 0.30

*See sample calculation in the Appendix discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a range of formats

Table 3: Average reaction rate vs concentration HCl using 1.5M HCl Time interval (s) 0 – 15 15 – 30 30 – 45 45 – 60 60 – 90 90 – 120 120 – 150 150 – 180 180 – 210 210 – 240 240 – 270 270 – 300

1.5M small chips Concentration HCl (M) at start of interval

1.50 1.10* 0.71 0.51 0.41 0.32 0.29 0.26 0.25 0.22 0.22 0.21

Reaction rate (mg/s)

6.20 5.40 2.87 1.47 0.67 0.23 0.20 0.13 0.17 0.03 0.06 0.03

1.5M large chips Concentration HCl (M) at start of interval

1.50 1.17 0.99 0.84 0.73 0.61 0.51 0.44 0.41 0.37 0.35 0.34

Reaction rate (mg/s)

4.80 2.67 2.20 1.60 0.93 0.70 0.50 0.20 0.30 0.20 0.07 0.13

*See sample calculation in the Appendix

Queensland Studies Authority March 2014 | 5

Table 4: Average reaction rate vs concentration HCl using 3M HCl 3M small chips Time interval (s) 0 – 15 15 – 30 30 – 45 45 – 60 60 – 90 90 – 120 120 – 150 150 – 180 180 – 210 210 – 240 240 – 270 270 – 300 discriminating selection, use and presentation of scientific data and ideas to make meaning accessible to intended audiences through innovative use of a range of formats

Concentration HCl (M) at start of interval

3M large chips

Reaction rate (mg/s)

3.00 2.19 1.15 0.54 0.39 0.21 0.17 0.14 0.11 0.10 0.09 0.09

11.93 15.20 8.93 2.20 1.30 0.33 0.23 0.20 0.03 0.10 0.03 0.10

Concentration HCl (M) at start of interval

3.00 2.70 2.25 1.77 1.43 1.04 0.78 0.62 0.51 0.43 0.37 0.33

Reaction rate (mg/s)

4.47 6.60 7.00 5.00 2.87 1.87 1.17 0.87 0.57 0.43 0.30 0.30

Graph 2: Reaction rate vs HCl concentration

Graph 3: Reaction rate vs concentration HCl for 3M HCl and small chips (omitting highest concentration)

6 | Chemistry 2007: Sample assessment instrument Reaction rate

Discussion

comparison and explanation of complex reaction rate concepts, processes and phenomena

systematic analysis of primary and secondary data to identify relationships between patterns, trends, errors and anomalies

Graph 1 shows the mass loss due to carbon dioxide gas production. In each case, the pattern was the same. Initially mass was lost quite rapidly. After about a minute, the rate of the reaction (indicated by the slope of the graph) decreased until it eventually levelled out. This confirms the hypothesis that the rate of the reaction would decrease as HCl was consumed. None of the reactions went to completion within 5 minutes; the reaction vessels continued to lose mass after the 5 minute mark. Similarly, none of the containers lost the expected mass of 0.66g for 3M acid or 0.33g for 1.5M acid (see appendix for calculations). Table 2 gives the average reaction rate for each experiment over successive time intervals. As expected, for small marble chips, the average reaction rate for the first two minutes was significantly higher for the 3M acid than for 1.5M acid. The same relationship is evident for large chips, although there is a notable exception as the initial rate for 3M acid large chips (4.47 mg/s) is slightly lower than 1.5M large (4.80 mg/s). The value of 4.47 mg/s for the 3M acid is an anomaly as it is significantly lower than expected. The overall trend is the same for all four reactions: if surface area is kept relatively constant, higher acid concentration produces a higher reaction rate. This can be seen in Graph 1 as the initial gradients are steeper for reactions containing 3M HCl than 1.5M HCl over the first 60 seconds - a steeper gradient indicates a higher reaction rate. Similarly, if concentration is kept constant it can be seen that the smaller chips (with larger surface area) generally have a faster initial reaction rate. These results confirm the first two propositions of the hypothesis.

analysis and evaluation of complex scientific interrelationships

reproduction and interpretation of complex and challenging reaction rate concepts, theories and principles systematic analysis of primary and secondary data to identify relationships between patterns, trends, errors and anomalies

Tables 3 and 4 show the reaction rates corresponding to various concentrations of acid and graph 2 illustrates these data. The graphs show approximately linear relationships for each data set up to a concentration of around 2M. This shows that as the concentration of the hydrochloric acid drops, the reaction rate drops proportionally. In Graph 2 some of the results appear to be anomalous: the highest concentration results for the 3M acid experiments are lower than expected. This may be because the reaction is exothermic. If a large amount of heat was released in the first few seconds of the reaction, this would increase the temperature of the mixture and, consequently, the rate of the reaction. However, by the time the concentration drops to around 2.5M, a relatively constant temperature should be achieved and then the rate would decrease linearly as observed. For Graph 3, the data for the experiments with the 3M acid and the small chips of marble has been used with the initial anomalous reading removed. This demonstrates the linear relationship between concentration and reaction rate predicted in the hypothesis. 2 The R value in Graph 4 is 0.986 which shows that the relationship is very close to being perfectly linear. This confirms that the reaction is first order with respect to the acid concentration.

Queensland Studies Authority March 2014 | 7

analysis and evaluation of complex scientific interrelationships

exploration of scenarios and possible outcomes with justification of conclusions/ recommendations

The concentration of acid is a potential systematic error as the same solution was used, either directly as 3M, or as a 1 in 2 dilution (1.5M). If the original solution was slightly weaker than labelled, this would explain why the observed total mass losses were lower than expected. The purity of the marble chips however may be different for each individual trial although the small variation between trials (less than 0.02g) suggests that this is not a significant error. If the marble chips contained any hydroxide salts (or bases other than carbonate) then some of the acid would have been neutralised without contributing to a mass reduction. On the other hand it is also possible that as the marble chips decompose, tiny pockets of gas trapped within the rock may have been released and if this has occurred, the mass losses given in Table 1 may be slightly inflated. Again, the small variation (less than 0.02g) suggests that none of these potential sources of error is significant. Further investigations could involve the following changes: • record pH readings as the reaction proceeds • record the temperature as the reaction proceeds in order to estimate the solubility of the carbon dioxide. Recording the change in pH would indicate what types of reactions are occurring and whether pH had any effect on the rate. Measuring the temperature change throughout the reaction would validate whether the assumption about the effect of the temperature on the first part of the reaction was correct.

Conclusion Results have shown that a higher concentration of reactants and greater surface area of calcite produced higher rates of reaction. The results confirmed that the reaction is first order with respect to the acid concentration.

Reference list Order of Reaction and Rate Equations, accessed 19 August 2013, www. chemguide.co.uk/physical/basicrates/orders.html

8 | Chemistry 2007: Sample assessment instrument Reaction rate

Appendix: Sample calculations Mole ratio 𝑪𝒂𝑪𝑶𝟑(𝒔) + 𝟐𝑯𝑪𝒍(𝒂𝒒) → 𝑪𝒂𝑪𝒍𝟐(𝒂𝒒) + 𝑯𝟐 𝑶(𝒍) + 𝑪𝑶𝟐(𝒈) 100

:

73

:

111

:

18

: 44

Rate of mass loss For 1.5M HCl with small chips in first 15s: Rate = linking and application of reaction rate algorithms, concepts, principles and theories to find solutions in complex and challenging reaction rate situations

𝒎𝒂𝒔𝒔 𝒍𝒐𝒔𝒔 𝒕𝒊𝒎𝒆

=

𝟎.𝟎𝟗𝟑𝒈 𝟏𝟓𝒔

= 0.0062g/s = 6.2 mg/s

Change in HCl concentration Considering 1.5M HCl with small chips (see Table 1): In the first 15 seconds, 0.093g CO2(g) is lost. Mass of HCl consumed = 𝟎. 𝟎𝟗𝟑 𝒈 𝑪𝑶𝟐 ×

𝟕𝟑𝒈 𝑯𝑪𝒍

𝟒𝟒𝒈 𝑪𝑶𝟐

= 0.15 g HCl

Moles of HCl consumed = 𝟎. 𝟏𝟓 𝒈 𝑯𝑪𝒍 ÷ 𝟑𝟔. 𝟓𝒈/𝒎𝒐𝒍 = 0.0041 mol HCl

In the 10mL sample, there is 0.015 mol – 0.0041 mol

= 0.011 moles in the 10mL New concentration of HCl =

0.11mol 0.01L

= 1.10M

The concentration after 15s will be 1.10M. This will be the starting concentration for the next time interval. Expected total mass loss Considering HCl as the limiting reagent: 10 mL of 1.5 M HCl gives 0.015 mol mass of CO2 produced

= 0.015𝑚𝑜𝑙 𝐻𝐶𝑙 ×

= 0.33 g CO2

1 𝑚𝑜𝑙 𝐶𝑂2 2 𝑚𝑜𝑙 𝐻𝐶𝑙

×

44𝑔

𝑚𝑜𝑙 𝐶𝑂2

Similarly, for 10 mL of 3 M HCl, expected mass loss = 0.66 g

Queensland Studies Authority March 2014 | 9