Rate Of Chemical Change (WJEC GCSE Science (Double Award): Chemistry): Flashcards
Exam code: 3430
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What is the rate of reaction?
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What is the rate of reaction?
The rate of reaction is a measure of how quickly a reactant is used up or a product is formed over time.
Rate = amount of .......... used ÷ ..........
OR
Rate = amount of .......... made ÷ ..........
Rate = amount of reactant used ÷ time taken
OR
Rate = amount of product made ÷ time taken
What are the units for rate of reaction when volume is measured in cm³?
When volume is measured in cm³ the units for rate of reaction are cm³/s.
True or False?
The mass loss method is suitable for measuring the rate of any gas-producing reaction regardless of the gas produced.
False.
The mass loss method is only suitable when the gas produced is sufficiently dense. Hydrogen (Mr = 2) has too small a relative formula mass for the change in mass to be measurable, whereas carbon dioxide (Mr = 44) is suitable.
What is the precipitation method (rate of reaction)?
The precipitation method measures rate by timing how long it takes for a solid precipitate to cloud a solution enough to hide a cross marked on paper beneath the flask.
Give one disadvantage of the disappearing cross method for measuring rate of reaction.
The method produces only one data point per experiment, so a rate-of-reaction graph cannot be plotted. It is also subjective because different observers may disagree on the exact moment the cross disappears.
In the mass loss method, the reaction is performed in a conical flask placed on a .......... to record the loss in mass as gas escapes.
In the mass loss method, the reaction is performed in a conical flask placed on a balance to record the loss in mass as gas escapes.
True or False?
In the gas collection method, gas can be captured using either an inverted measuring cylinder filled with water or a gas syringe.
True.
Both set-ups trap the gas produced and allow its volume to be measured over time.
Why is the gas collection method preferred over the mass loss method when hydrogen is the product?
Hydrogen has a very small relative formula mass (Mr = 2), so the mass loss is too small to measure accurately on a balance. The gas collection method is preferred because it measures volume directly, which gives a detectable and reliable reading.
What is concentration (and its effect on rate)?
Concentration is the amount of solute dissolved in a given volume of solution. Increasing the concentration of a reactant in solution increases the rate of reaction because there are more particles per unit volume.
The four factors that can affect the rate of a reaction are: .........., .........., .......... and the presence of a .......... .
The four factors that can affect the rate of a reaction are: concentration (or pressure), temperature, surface area and the presence of a catalyst.
True or False?
Increasing the concentration of a reactant in solution increases the rate of reaction but the total amount of product formed also increases.
False.
Increasing concentration increases the rate of the reaction, but the total amount of product formed remains the same because the amount of reactant has not changed.
How does increasing the pressure of reacting gases affect the rate of reaction?
Increasing pressure increases the rate of reaction because the same number of particles are in a smaller volume, meaning there are more particles per unit volume and more frequent collisions.
On a rate-of-reaction graph, increasing the concentration gives a .......... gradient at the start and the curve becomes horizontal .......... .
On a rate-of-reaction graph, increasing the concentration gives a steeper gradient at the start and the curve becomes horizontal sooner.
What happens to the rate of reaction when the surface area of a solid reactant is increased?
Increasing the surface area increases the rate of reaction because a greater area of solid is exposed to the other reactant, leading to more frequent collisions. Decreasing particle size has the same effect as increasing surface area.
True or False?
A cube with 2 cm sides cut into 8 equal smaller cubes has a greater total surface area than the original cube.
True.
Cutting the cube into 8 smaller cubes doubles the total surface area from 24 cm² to 48 cm², exposing more of the solid to the other reactant.
When answering exam questions about how concentration affects rate, what must you say about the number of particles rather than just stating there are 'more particles'?
There are more particles per unit volume (e.g. per cm³), not simply more particles overall.
Describe how the rate of reaction changes when the temperature is increased, using evidence from a rate-of-reaction graph.
When temperature is increased the graph shows a steeper gradient at the start and the curve becomes horizontal sooner, while the total amount of product formed stays the same. This shows that increasing temperature increases the rate of reaction.
Explain why increasing the temperature increases the rate of reaction.
Increasing temperature causes particles to gain kinetic energy and move faster, so they collide more frequently. A greater proportion of collisions also have energy equal to or greater than the activation energy, because more particles have sufficient energy to react.
What is sodium thiosulfate (in rate experiments)?
Sodium thiosulfate (Na2S2O3) is a soluble salt that reacts with hydrochloric acid to produce a sulfur precipitate, making the solution cloudy. This cloudiness is used to measure the rate of reaction.
In the sodium thiosulfate experiment, the reaction is complete when the .......... beneath the conical flask can no longer be seen.
In the sodium thiosulfate experiment, the reaction is complete when the cross beneath the conical flask can no longer be seen.
What is the word equation for the reaction between sodium thiosulfate and hydrochloric acid?
Sodium thiosulfate + hydrochloric acid → sodium chloride + water + sulfur dioxide + sulfur
True or False?
In the sodium thiosulfate and hydrochloric acid experiment, a shorter time for the cross to disappear means a faster rate of reaction.
True.
A shorter time means the sulfur precipitate forms more quickly, indicating a faster rate of reaction.
How are different temperatures of sodium thiosulfate solution made in this experiment without using a separate heater for each temperature?
Different temperatures are made by mixing volumes of hot sodium thiosulfate solution (at 60°C) with volumes of cold sodium thiosulfate solution (at 5°C) in different proportions. This keeps the total volume and concentration of the solution constant.
The independent variable in the sodium thiosulfate temperature experiment is .......... and the dependent variable is .......... .
The independent variable in the sodium thiosulfate temperature experiment is temperature and the dependent variable is time for the cross to disappear.
Give one limitation of the sodium thiosulfate disappearing cross experiment and explain why it reduces reliability.
The experiment is subjective because different observers may disagree on the exact moment the cross disappears. This means results can vary between people, reducing the reliability of the data.
True or False?
The specified practical for the sodium thiosulfate experiment includes repeats, so repeatability can be checked.
False.
The specified practical does not include repeats, so repeatability cannot be checked. However, reproducibility can be checked by comparing results with other groups.
Why must the conical flask be washed thoroughly between each temperature in the sodium thiosulfate experiment?
The flask must be washed to remove any sulfur precipitate from the previous experiment. Leftover precipitate would cause the cross to disappear sooner, giving an inaccurately shorter time and making the results unreliable.
What is a successful collision?
A successful collision is one in which reactant particles collide with sufficient energy for the atoms to rearrange and form the products of the reaction.
Increasing the concentration of a solution means there are more reactant particles in a given .........., causing more .......... per second and a faster rate of reaction.
Increasing the concentration of a solution means there are more reactant particles in a given volume, causing more collisions per second and a faster rate of reaction.
Explain why increasing temperature increases the rate of reaction, using particle theory.
Increasing temperature gives the particles more kinetic energy, so they move faster. This causes more frequent collisions per second and a greater proportion of those collisions have energy greater than or equal to the activation energy, leading to more successful collisions and a faster rate of reaction.
True or False?
If the concentration of a reactant is doubled, the number of collisions per second also doubles.
True.
The number of collisions is proportional to the number of particles present, so doubling the concentration doubles the collision frequency.
What is activation energy? (Higher Tier Only)
Activation energy is the minimum amount of energy that colliding particles must have for a reaction to occur. (Higher Tier Only)
Explain why increasing surface area of a solid reactant increases the rate of reaction.
Increasing the surface area means a greater area of solid particles is exposed to the other reactant. This causes more collisions per second, leading to more frequent successful collisions and a faster rate of reaction.
Increasing gas pressure increases the rate of reaction because the same number of particles occupy a .......... volume, causing more .......... per second.
Increasing gas pressure increases the rate of reaction because the same number of particles occupy a smaller volume, causing more collisions per second.
True or False?
Temperature is the only factor that directly affects both the number of collisions and the energy of the collisions. (Higher Tier Only)
True.
Concentration, pressure and surface area only affect the number of collisions. Temperature also increases the kinetic energy of the particles, which is the more important of the two effects. (Higher Tier Only)
What is the approximate effect on the rate of reaction of increasing the temperature by 10°C for an aqueous or gaseous system?
For every 10°C increase in temperature, the rate of reaction approximately doubles.
What is a catalyst?
A catalyst is a substance that speeds up the rate of a reaction while remaining chemically unchanged at the end. Its mass is the same before and after the reaction.
A catalyst speeds up the rate of reaction by providing an .......... reaction pathway that has a .......... activation energy. (Higher Tier Only)
A catalyst speeds up the rate of reaction by providing an alternative reaction pathway that has a lower activation energy. (Higher Tier Only)
True or False?
The same catalyst can be used to speed up any chemical reaction.
False.
The same catalyst does not work for all reactions. Different reactions require different catalysts.
What does the graph of product formed against time show when a catalyst is added to a reaction?
The graph shows a steeper gradient at the start and the curve becomes horizontal sooner when a catalyst is present. The total amount of product formed is the same because the catalyst does not change the amount of reactant.
Explain why catalysts in industry need to be replaced regularly, even though they are not chemically used up.
Industrial catalysts are affected by impurities in the reactant mixture, which reduce their activity over time. Although they are not chemically consumed, their effectiveness decreases so they must be replaced regularly.
An iron catalyst is used in the Haber process: N2 (g) + 3H2 (g) ⇌ 2NH3 (g). The catalyst is written .......... or .......... the reaction arrow.
An iron catalyst is used in the Haber process: N2 (g) + 3H2 (g) ⇌ 2NH3 (g). The catalyst is written above or below the reaction arrow.
Explain how a catalyst increases the rate of reaction by referring to activation energy. (Higher Tier Only)
A catalyst provides an alternative reaction pathway with a lower activation energy. This means a higher proportion of the reactant particles have enough energy to react, leading to more successful collisions per second and a faster rate of reaction.
True or False?
Using a catalyst increases the total amount of product formed in a reaction.
False.
A catalyst increases the rate of the reaction but does not change the total amount of product formed. The same amount of product is made whether a catalyst is used or not.
What is the gas collection method (rate of reaction)?
The gas collection method measures the rate of reaction by recording the volume of gas produced over time. The gas can be collected using an inverted measuring cylinder filled with water or a gas syringe.
In the magnesium and hydrochloric acid experiment, the gas produced is .......... and the word equation is: magnesium + hydrochloric acid → .......... + .......... .
In the magnesium and hydrochloric acid experiment, the gas produced is hydrogen and the word equation is: magnesium + hydrochloric acid → magnesium chloride + hydrogen.
In the gas collection experiment, how is 0.5 mol/dm³ hydrochloric acid made from 1 mol/dm³ acid without preparing a fresh solution?
10 cm³ of 1 mol/dm³ hydrochloric acid is mixed with 10 cm³ of water. This gives a total volume of 20 cm³ with a concentration of 0.5 mol/dm³, matching the total volume used in the more concentrated run.
True or False?
When the concentration of hydrochloric acid is doubled in the gas collection experiment, the total volume of hydrogen produced also doubles.
False.
The total volume of hydrogen produced stays the same because the amount of magnesium (the limiting reactant) does not change. Higher concentration means the gas is produced faster, not in greater quantity.
What are the independent and dependent variables in the magnesium and hydrochloric acid gas collection experiment?
The independent variable is the concentration of hydrochloric acid. The dependent variable is the volume of hydrogen gas produced. Time is the controlled measurement used to track the reaction.
In the gas collection experiment, the volume of hydrogen gas is recorded every .......... seconds and the experiment continues until .......... gas appears to be given off.
In the gas collection experiment, the volume of hydrogen gas is recorded every ten seconds and the experiment continues until no more gas appears to be given off.
Why must the conical flask be sealed with a bung and delivery tube immediately after adding the magnesium in the gas collection experiment?
The flask must be sealed immediately because magnesium and hydrochloric acid react straight away, producing hydrogen gas. Any delay means gas escapes before collection begins, giving an inaccurately low volume reading.
True or False?
On a rate-of-reaction graph for the gas collection experiment, the curve for 1 mol/dm³ acid finishes at the same final volume of hydrogen as the curve for 0.5 mol/dm³ acid.
True.
Both curves reach the same final volume because the same length of magnesium ribbon is used in each run. The higher concentration only makes the reaction reach that volume faster.
Name the two set-ups that can be used to collect gas in the magnesium and hydrochloric acid rate experiment.
An inverted measuring cylinder filled with water, where the gas displaces the water (downward displacement of water).
A gas syringe, where the gas pushes the plunger and the volume is read directly.
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