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In today's T-Level Chemistry lesson, we are going to study A twelve point one, Factors Affecting Rates of Chemical Reactions.

By the end of this lesson, you should be able to explain how surface area, concentration and pressure change rate, using collision theory, and identify control variables in a rate investigation.

Retrieve collision theory. A successful collision needs sufficient energy and the correct spatial orientation.

Reaction rate depends on how frequently those successful collisions occur per unit time.

The common idea across A twelve point one is collision frequency. If reacting particles collide more often, there are more opportunities for successful collisions, so rate increases. Energy and orientation still matter. More collisions does not mean every collision is successful.

Specification point A twelve point one point one: surface area.

For the same mass of solid, smaller pieces have a greater total exposed surface area. Only surface particles are directly available for collisions with the other reactant.

Watch the particle models. Large chips expose less surface. Small chips expose more. Powder exposes the most surface for the same mass.

The teacher PowerPoint uses two experiments, both with two point zero grams of solid. Experiment A, large chips, reaches the endpoint in ninety five seconds. Experiment B, small chips, reaches the same endpoint in forty eight seconds.

B is faster. The same mass of small chips has a greater total surface area, so collision frequency at the solid surface is higher.

The student workbook uses a third trial as well. Large chips, ninety six seconds. Small chips, fifty one seconds. Powder, twenty nine seconds. Those times are not the same as the PowerPoint pair, so keep the two datasets separate. The scientific pattern is the same: as particle size decreases, time to the same endpoint decreases.

A common error is to say that smaller particles contain more substance. For the same mass, the amount of substance is the same. What changes is the exposed surface area.

Now specification point A twelve point one point two: concentration.

Concentration is how much dissolved reactant is present in a given volume. At higher concentration there are more reactant particles per unit volume.

Temperature is unchanged, so the particles do not need to move faster. They simply meet more often because they are packed more closely.

The shared concentration dataset is: zero point five zero moles per cubic decimetre takes one hundred and twenty seconds; one point zero zero takes sixty four seconds; one point five zero takes forty three seconds; two point zero zero takes thirty one seconds.

Higher concentration means fewer seconds to the same endpoint, so rate increases.

Do not assume that doubling concentration exactly doubles rate unless the evidence says so.

Specification point A twelve point one point three: pressure, for gases.

At constant temperature, increasing pressure can put the same number of gas particles in a smaller volume. The particles are closer together, so there are more particles per unit volume and collisions become more frequent.

Do not claim that average kinetic energy has increased if temperature is unchanged. Pressure can rise by reducing volume at constant temperature. That is not the same as heating the mixture.

On a gas-volume versus time graph, a faster reaction has a steeper initial curve. If the same limiting amount reacts, the final product amount can be the same, reached sooner.

When you plan an investigation, change only the intended factor. For surface area, control mass of solid, concentration and volume of the other reactant, temperature and apparatus. For concentration, also control the surface area or mass of any solid.

Correct the misconceptions. Smaller pieces do not contain more substance. Higher concentration does not make particles faster if temperature is unchanged. Higher gas pressure does not always mean higher temperature. And more collisions does not make every collision successful.

Examination-style question.

Part a. Explain why a powdered solid reacts faster than the same mass of large pieces. Three marks.

Powder has a greater total surface area. Collision frequency at the surface increases. Successful collisions occur more frequently, so rate increases.

Part b. Explain why increasing reactant concentration increases rate. Three marks.

There are more particles per unit volume. Collision frequency increases. Successful collisions per unit time increase.

Part c. Explain why increasing the pressure of gaseous reactants increases rate. Three marks.

At constant temperature the same particles occupy a smaller volume, so they are closer together. Collision frequency increases, and successful collisions occur more frequently.

Part d. State one control variable for a surface-area investigation and explain why you control it. Two marks.

Control the mass of solid, or the concentration of the other reactant, or temperature. That isolates surface area so the comparison is valid.

Before we finish, complete the model.

Surface area, concentration and pressure all change how often particles collide. More frequent collisions give more successful collisions per unit time, so rate increases. Energy and orientation still decide which collisions succeed.

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