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Define the rate of a chemical reaction - Leaving Cert Chemistry - Question 9 - 2007
Question 9
Define the rate of a chemical reaction. Why does the rate of chemical reactions generally decrease with time? The rate of reaction between an excess of marble chips... show full transcript
Worked Solution & Example Answer:Define the rate of a chemical reaction - Leaving Cert Chemistry - Question 9 - 2007
Step 1
Step 2
Why does the rate of chemical reactions generally decrease with time?
Answer
The rate of chemical reactions generally decreases with time because the concentration of reactants decreases as they are consumed in the reaction. As the reactants are used up, there are fewer particles available to collide with one another, leading to a slower reaction rate.
Step 3
Plot a graph of the mass of carbon dioxide produced versus time.
Answer
To plot the graph, label the x-axis as 'Time (minutes)' and the y-axis as 'Mass of CO₂ (g)'. Plot the points from the table, connecting them to form a curve that illustrates the trend of the gas production over time. Ensure all points are plotted correctly and the curve reflects the data trend.
Step 4
(i) the instantaneous rate of reaction in grams per minute at 4.0 minutes.
Answer
To determine the instantaneous rate of reaction at 4.0 minutes, observe the slope of the tangent to the curve at this point. The change in mass of CO₂ between the intervals before and after 4.0 minutes can be used to calculate:
ext{Instantaneous Rate} = rac{ ext{Mass at 4.0 min} - ext{Mass at 3.0 min}}{4.0 - 3.0} = rac{2.18 g - 1.90 g}{1.0} = 0.28 ext{ g min}^{-1}
Step 5
(ii) the instantaneous rate of reaction at this time in moles per minute.
Answer
To convert grams to moles, use the molar mass of CO₂, which is approximately 44 g/mol. Thus, the calculation for the moles produced from 0.28 g at 4.0 minutes is:
ightarrow ext{Instantaneous Rate} = rac{0.28 ext{ g}}{44 ext{ g/mol}} imes rac{1 ext{ mol}}{1 ext{ min}} ightarrow 0.00636 ext{ mol min}^{-1}$$Step 6
Describe and explain the effect on the rate of reaction of repeating the experiment using 50 cm³ of 1.0 M hydrochloric acid.
Answer
Using 50 cm³ of 1.0 M hydrochloric acid instead of 2.0 M will lead to a slower reaction rate. The concentration decreases, resulting in fewer HCl molecules available to collide with the marble chips, thus resulting in a lower frequency of successful collisions and a slower overall reaction rate.
Step 7
Mark clearly on your graph the approximate curve you would expect to plot if the experiment were repeated using 50 cm³ of 2.0 M HCl and smaller marble chips.
Answer
On the same graph, sketch an approximate curve for the scenario with 2.0 M HCl and smaller marble chips (1-5 mm). This curve should show a steeper slope initially due to increased surface area, indicating a faster rate of reaction compared to the previous experiment with larger chips.
Step 8
Give three conditions necessary for a dust explosion to occur.
Answer
- Presence of combustible dust particles: The fine particles must be small enough to remain suspended in the air.
- Sufficient concentration: There must be a critical concentration of dust in the air to support a sustained combustion reaction.
- Ignition source: An ignition source, such as a spark or flame, must be present to initiate the explosion. Additionally, oxygen must be available to support combustion.