Rate of reaction

a. Explain why gases react together faster at higher pressure.

b. Explain why reactants in solution react faster at higher concentration.

a. The number of collisions is proportional to the number of particles present. We all know that the same amount of reactant particles are present in a smaller volume when the pressure is increased. More collisions occur every second as a result, increasing the frequency and success of collisions per second. As a result, the reaction rate rises. 

b. There are more reactant particles in a given volume when the concentration rises. More collisions occur as a result, increasing the frequency and success of collisions per second. As a result, the reaction rate rises.

a. Explain why finely divided solids react more quickly than solid lumps.

b. Explain why raising the temperature increases the rate of reaction.

a. Finely divided solids and solid lumps are related to the surface area effect. A larger surface area of particles will be exposed to the other reactant as the surface area increases. More collisions occur as a result, increasing the frequency and success of collisions per second. Consequently, the rate of reaction rises.

b. The particles have higher kinetic energy when the temperature rises. More collisions occur every second as a result, increasing the frequency and success of collisions per second. As a consequence, the reaction rate rises.

One more remark: Temperature has a more complicated effect on collisions than concentration or surface area; a slight rise in temperature results in a significant increase in rate.

According to collision theory, state two conditions that particles need for an effective collision to occur.

Two conditions that particles need for an effective collision to occur include sufficient energy and to be in the correct orientation.

The student plotted a graph of volume of O₂ against time.

Use the student's graph and tangent to find the rate of the reaction, in cm³ s^-1 , at t = 300 s.

Show your working

Volume at 200 s = 14.5 cm³

Volume at 400 s = 21.5 cm³

Difference in volume = 21.5 - 14.5 = 7 cm³

Rate = `7/200` = 0.035 cm³ s^-1

A group of students were completing a practical, investigating the factors which affect the rate of the following chemical reaction. 

A (s) + B (aq) → C (g)

The students collected the gas produced and plotted the graph shown in Figure below

State and explain what the letter R represents on the student's graph

The letter R represents the rate of reaction at this point is zero or all of reactants have been reacted.

A reaction between reactant molecules can only occur when collisions between the reactant molecules are effective. 

i. Explain what is meant by the term effective collision.

ii. State two factors that could cause an ineffective collision.

i. The term effective collision is a collision that results in a reaction or products being formed.

ii. Two factors that could cause an ineffective collision include the reactant particles may not collide in the correct orientation and may not have an energy higher than the activation energy

Why does a mixture of hydrogen gas and bromine gas react together faster at a temperature of 500 K than it does at a temperature of 400 K?

A. 1 only

B. 1 and 2

C. 3 only

D. 1, 2, and 3

The answer is B.

To make a reaction occur, the particles need to collide in the correct orientation with the right amount of energy.

the higher temperature, the greater number of particles with sufficient energy to react, the greater chance of that collision being between two particles, as such, being effective.

A is incorrect because statement 2 is correct as well

C and D are incorrect because the activation energy of the reaction does not change

What is the main reason for the increase in reaction rate with increasing temperature?

A. The activation energy decreases

B. The activation energy increases

C. The molecules collide more frequently

D. More molecules have an energy greater than the activation energy

The answer is D.

To make a reaction occur, the particles need to collide in the correct orientation with the right amount of energy.

the higher temperature, the greater number of particles with sufficient energy to react, the greater chance of that collision being between two particles, as such, being effective.

A and B are incorrect because the activation energy of the reaction does not change by a change in temperature

C is incorrect because the number of particles with energy higher than the activation energy has a much more significant effect than the kinetic energy of particles increasing

When the pressure of a fixed mass of gaseous reactants is raised at a constant temperature, the rate of reaction increases. 

Which of the following statements explain this observation?

A. 1 only

B. 1 and 2

C. 3 only

D. 1, 2 and 3

The answer is C.

To make a reaction occur, the particles need to collide in the correct orientation with the right amount of energy.

The higer pressure, the closer particles are and more likely to collide more frequently.

A, B and D are incorrect because increasing the pressure does not lower the activation energy and does not increase the energy of particles.

Consider the following reaction:

NaOH (aq) + HCl (aq) → H₂O (l) + NaCl (aq) Which species could a student change the concentration of, in order to determine the rate of reaction?

A. H⁺

B. Cl^-

C. Na⁺

D. H₂O

The answer is A.

The rate of reaction can be measured by looking at the change in the concentration of a reactant or product in a given amount of time. And we can easily measure the change in concentration of the hydrogen ions by measuring how the pH of the solution changes over time

B, C and D are incorrect because the concentrations of these species will be extremely difficult to measure practically.