Rates of Reaction

Question 1

What is the Rate of Reaction

Answer 1

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

Question 2

How is the Rate of Reaction Calculated

Answer 2

Rate of Reaction = quantity of reactant used / time

Rate of Reaction = quantity of product formed / time

Question 3

What is the Rate of Reaction used to Calculate

Answer 3

-Rate of reaction can measure pH, volume of gas produced and change in mass.

-It can be measured in g/min, cm3/s and Mol/s

Question 4

Why is Measuring the Change in Mass or Volume of Gas Useful during a Reaction

Answer 4

The change in mass of a reactant or product can be followed during a reaction.

-This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass.

-The change in volume of a reactant or product can be followed during a reaction.

-This method is useful when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.

Question 5

Describe why the Rate of Reaction Changes over Time

Answer 5

-With any chemical reaction, the slope of the line gives us an idea of the rate.

-Initially, the graph shows a steep slope, telling us that the rate is very rapid. This is because there is a large amount of reactant molecules available to form the product.

-However, after some time, the slope of the graph becomes less steep, telling us that the rate of the reaction is not as rapid. This is because some of the reactant has been used up.

Question 6

Describe what Happens at the First Stage in a Reaction

Answer 6

-Here the amount of reactant is greatest.

-The maximum number of reactant molecules are reacting per second.

-The reaction is at its fastest.

Question 7

Describe what Happens at the Second Stage in a Reaction

Answer 7

-Here the amount of reactant is running out.

-Only a small number of reactant molecules are reacting per second.

-The reaction is slowing down

Question 8

Describe what Happens at the Third Stage in a Reaction

Answer 8

-Here all of the reactant has been used up.

-Because there is no reactant no more product forms.

-The reaction has stopped.

Question 9

What does the Gradient of a Line Show in a Reaction

Answer 9

-The gradient of the line is equal to the rate of reaction

-The steeper the line, the greater the rate of reaction

-The faster the reaction, the sooner the line plateaus (becomes horizontal)

Question 10

Describe how to Draw a Tangent on a Graph to Calculate the Rate of Reaction

Answer 10

-Identify the point on the curve where you want to find the rate of reaction.

-Place a ruler on the graph so that the space between the ruler and the curve is the same on both sides.

-Draw a line across the graph at that point and pick two points on the tangent line.

-Calculate the change in y and the change in x between those two points.

-Divide the change in y by the change in x to find the rate of reaction.

Question 11

What is Collision Theory

Answer 11

For a chemical reaction to happen:

-Reactant particles must collide with each other

-The particles must have at least the activation energy

Question 12

What is the Activation Energy

Answer 12

The activation energy is the minimum amount of energy that the particles need in order to react.

Question 13

Describe the Effect of Concentration on the Rate of Reaction

Answer 13

-As the concentration increases, the rate of reaction increases.

-This is because at a higher concentration, there are more particles per unit volume. This means the reactant particles become more crowded.

-This means the frequency of collisions between particles increases. In turn, the rate of reaction increases.

Question 14

Describe the Effect of Gas Pressure on the Rate of Reaction

Answer 14

-As gas pressure increases, the rate of reaction increases.

-This is because at a higher pressure, there are more particles per unit volume. This means the reactant particles become more crowded.

-This means the frequency of collisions between particles increases. In turn, the rate of reaction increases.

Question 15

Describe a Method for the Disappearing Cross Required Practical

Answer 15

-First, use a measuring cylinder to transfer 10cm3 of sodium thiosulfate into a clean conical flask.

-Next, place the conical flask onto a printed, black cross.

-Then, use a measuring cylinder to add 10cm3 of hydrochloric acid to the conical flask.

-Swirl the conical flask and start the stopwatch. Look down the conical flask to the black cross.

-Overtime, the solution will become cloudy. Stop timing when the flask is no longer visible.

-Repeat the experiment with different concentrations of sodium thiosulfate and calculate a mean average of the time taken for the cross to not be visible.

Question 16

Why should a Printed Black Cross be Used Rather than one Drawn by Hand in the Disappearing Cross Practical

Answer 16

-The main issue with the disappearing cross experiment is that the final result depends on whether you think you can still see the cross.

-If we allow students to draw their own crosses then we cannot be certain that the crosses are all the same.

-For example one student may draw a small faint cross, whereas other students may draw a large thicker cross.

-By giving all the students an identical printed cross, we can reduce this variability.

Question 17

What is the Purpose of Swirling the Conical Flask in the Disappearing Cross Practical

Answer 17

-By swirling the conical flask we ensure that the reactants mix thoroughly.

-If we did not swirl, then the reactants may not mix and the reaction may be slower than it should be.

Question 18

Explain why it is Difficult to Compare Results in the Disappearing Cross Practical

Answer 18

-The students judge the reaction by eye, deciding when they can no longer see the printed cross. Students with good eyesight may see the cross for longer than other students.

-This means that there will always be some variability in the results between different groups.

-One way to reduce this is to ask all the students in a group to watch the cross and write down when they can no longer see it.

-We can then take a mean of all their results. This means that the final result does not depend on one person’s eyesight.

Question 19

Describe a Method for the Measuring Production of a Gas Required Practical

Answer 19

-Use a measuring cylinder to transfer 50cm3 of hydrochloric acid to a conical flask. Fit the bung and delivery tube to the top of the conical flask.

-Fill a measuring cylinder with water and place it upturned into a bowl half filled with water. Place the mouth of the measuring cylinder over the end of the delivery tube.

-Add a 3cm strip of magnesium to the acid. Quickly replace the bung and start timing.

-The reaction produces hydrogen which will bubble into the measuring cylinder. Every ten seconds read the volume of hydrogen. Take readings until the reaction has stopped.

-Repeat these steps with different concentrations of acid.

Question 20

Describe the Effect of Surface Area on the Rate of Reaction

Answer 20

-As the surface area increases, the rate of reaction increases.

-This is because if surface area increases, more reactant particles are exposed at the surface.

-This means the frequency of collisions between particles increases. In turn, the rate of reaction increases.

Question 21

Describe Two Advantages of Using a Gas Syringe Compared to Using a Measuring Cylinder.

Answer 21

-A gas syringe is more accurate for measuring the volume of a gas than using an upturned measuring cylinder.

-Firstly, reading a measuring cylinder upside down is difficult. Secondly, the bubbles can move rapidly, disturbing the level of the water, making it very difficult to read the volume.

Also, it is easier to carry out a repeat experiment using the gas syringe.

-If we want to repeat the experiment using the measuring cylinder, we need to refill the measuring cylinder with water and submerge it in the container. This is tricky to do.

Question 22

When Using a Measuring Cylinder, the Volume of Carbon Dioxide is often Less than if A Gas Syringe was used. Explain why.

Answer 22

-When we use a measuring cylinder, the volume of carbon dioxide gas that we measure is often slightly less than if we use a gas syringe.

-This is because some of the carbon dioxide gas dissolves in the water.

Question 23

What is the Function of Cotton Wool in the Experiment to Determine the Mass of Carbon Dioxide Produced.

Answer 23

-The cotton wool allows the carbon dioxide gas to escape from the reaction but stops any acid from splashing out.

-If any acid splashed out, the mass would fall more than it should.

Question 24

Why is it Important to have a Balance with a Resolution of at Least Two Decimal Places in the Experiment to Determine the Mass of Carbon Dioxide Produced.

Answer 24

In this experiment, we are monitoring the mass of gas that leaves the reaction. Gases have a low density so we would not expect to see a very large mass decrease.

-Because of this, we need a balance with a high resolution in order to detect the change in mass.

-If we used a balance which read to the nearest gram, we may not measure any mass change at all.

Question 25

It may be Difficult to Measure a Reaction that happens Quickly. How can this be Overcome.

Answer 25

One way to overcome this problem is to video the experiments and then replay them at a slower speed.

Question 26

Describe the Effect of Temperature on the Rate of Reaction

Answer 26

-As the temperature increases, the rate of reaction increases.

-This is because at higher temperatures, particles have more kinetic energy. This means that they move faster.

-This means that more particles have at least the activation energy.

-This means the frequency of successful collisions between particles increases. In turn, the rate of reaction increases.

Question 27

Describe the Effect of Catalysts on the Rate of Reaction

Answer 27

-If a catalyst is added to a reaction, the rate of reaction increases.

-This is because a catalyst provides an alternate reaction pathway that has a lower activation energy. This does not change the frequency of collisions.

-However, it does increase the frequency of successful collisions because more particles have energy greater than the activation energy.

-In turn, the rate of reaction increases.

Question 28

Give Reasons to add catalysts to a Reaction

Answer 28

-Catalysts allow a reaction to happen quickly but at a lower temperature which saves energy.

-Catalysts save money as they are not used up in the reaction.

-Transition metals often make good catalysts.

-Uses include catalytic converters.

Question 29

What is a Reversible Reaction

Answer 29

-A reversible reaction is a reaction in which the products can re-form the products.

-Reversible reactions use a ⇌ arrow.

-If one direction is exothermic, the opposite reaction is endothermic and takes in the same amount of energy given out by the exothermic reaction.

Question 30

What is Dynamic Equilibrium

Answer 30

-Dynamic equilibrium is a reversible reaction where the rate of reaction is equal so the concentration of reactants and products is constant.

-It happens in a closed system.

Question 31

What is Le Chatelier’s Principle

Answer 31

If a system (reaction) is at equilibrium and we make a change to the conditions, the system (reaction) responds to counteract the change.

Question 32

Explain the Impact of Concentration on the Position of Equilibrium.

Answer 32

-If the concentration of a reactant (on the left) is increased, the position of equilibrium shifts in the direction away from this reactant (shifts right), and so the yield of products increases.

-If the concentration of one of the products is decreased (on the right), then the position of equilibrium shifts to the right to make more of that product.

Question 33

Explain the Impact of Temperature on the Position of Equilibrium.

Answer 33

-In a reversible reaction, if the reaction is exothermic in one direction, it is endothermic in the other direction.

-If the forward reaction is endothermic, the temperature will increase to increase the yield of products.

-This is because the position of equilibrium shifts right (the direction of the endothermic reaction) to counteract the change.

Question 34

What Symbol Shows Whether a Reaction is Endothermic or Exothermic

Answer 34

A negative energy change e.g. -45 shows a reaction is exothermic whereas a positive energy change shows a reaction is endothermic.

Question 35

Explain the Impact of Pressure on the Position of Equilibrium.

Answer 35

-If the pressure is increased in a reaction involving gases, the position of equilibrium shifts in the direction of the fewest molecules of gas, to counteract the change.