Rate Equations and Kp

Question 1

What is the reaction rate?

Answer 1

The reaction rate is the change in the amount of reactant or product per unit time. If the reactants are in solution, the rate will be the change in concentration per second.

Question 2

How do you work out the gradient and therefore reaction rate from a curved graph?

Answer 2

You draw a tangent to the point in which you want to find out the gradient and then work it out from there.

Question 3

What do the powers mean in the rate equation?

Answer 3

They are the orders of the reaction with respect to the reactant e.g A2 is order 2 with respect to A.

Question 4

What is an order of reaction?

Answer 4

It is the power to which the concentration of its reactant is raised in the rate equation.

Question 5

How do you find the overall order of a reaction?

Answer 5

You add the orders for each component up

Question 6

How can you find the order of a reaction?

Answer 6

You can only find it through experimentation, you cannot find it from chemical equations.

Question 7

What is k?

Answer 7

k is the rate constant, which is a number that links the rate of reaction to the concentration of the reactants. The bigger the value of k is, the faster the reaction.

Question 8

Why does the rate constant increase as you increase temperature?

Answer 8

When you increase the temperature of a reaction, the rate of reaction increase, you are increasing the number of collisions between reactant molecules and also the energy of each collision. But the concentrations of the reactants and the order of the reaction stay the same. So the value of k must increase for the rate equation to balance.

Question 9

How can the initial rates method be used to work out rate equations?

Answer 9

• repeat an experiment several times using different initial concentrations of the reactants, changing one at a time.
• calculate the initial rate for each experiment
• see how the initial concentrations affect the initial rates and figure out the order for each reactant.

Question 10

How do you measure the rate of reaction via continuous monitoring?

Answer 10

You can follow a reaction all the way through to its end by recording the amount of product or reactant you have at regular time intervals. (continuous monitoring)
The results can be used to work out how the rate changes over time.

Question 11

How would you measure rate of reaction using gas volume?

Answer 11

If a gas is given off you could collect it in a gas syringe and measure volume at regular time intervals. To find the concentration of a reactant at any time use the ideal gas equation and the molar ratio.

Question 12

How would you measure rate of reaction using loss of mass?

Answer 12

If a gas is given off then the system will lose mass. You can measure this at regular intervals with a balance. Then use mole calculations to work out how much gas you have lost and therefore how many moles of reactants are left.

Question 13

How would you measure rate of reaction using colour change?

Answer 13

Use a colorimeter
- plot a calibration curve using known concentrations
- during the experiment take a small sample from your reaction solution at regular intervals to read the absorbance
- use the curve to convert the absorbance to a concentration

Question 14

What does a colorimeter do?

Answer 14

A colorimeter measures absorbance. The more concentrated the solution the higher the absorbance is.

Question 15

How would you measure rate of reaction using a change in pH?

Answer 15

If the reaction produces or uses up H+ ions, the pH of the solution will change. So you could measure the pH of the solution at regular intervals and calculate the concentration of H+

Question 16

What shape indicates zero order?

Answer 16

A straight line on a concentration time graph.
A horizontal line on a rate concentration graph

Question 17

What shape indicates first order?

Answer 17

A curved line on a concentration time graph.
A straight line through the origin on a rate concentration graph.

Question 18

What shape indicates second order?

Answer 18

A steeper curve on the concentration time graph.
A curve on the rate concentration graph.

Question 19

What is the rate determining step?

Answer 19

Mechanisms can have one step or a series of steps. In a series of steps, each step can have a different rate. The overall rate is decided by the step with the slowest rate- the rate determining step.

Question 20

How do orders or reaction provide information about the rate determining step?

Answer 20

All things included in the rate equation are part of the rate determining step. The order of a reaction with respect to a reactant shows the number of molecules of that reactant that are involved in the rate determining step.

Question 21

What does the Arrhenius equation do?

Answer 21

The Arrhenius equation shows how the rate constant varies with temperature and activation energy.
k - rate constant
Ea - activation energy
T - temperature in K
R - gas constant
A - the Arrhenius constant

Question 22

What is total pressure and what is partial pressure?

Answer 22

In a mixture of gases, each individual gas exerts its own pressure- this is called its partial pressure.
The total pressure of a gas mixture is the sum of all the partial pressures of the individual gases.

Question 23

What is the equation for mole fraction of gas in a mixture?

Answer 23

number of moles of gas/total number of moles of gas in the mixture

Question 24

What is the equation for partial pressure of a gas?

Answer 24

mole fraction of gas x total pressure of the mixture.

Question 25

What is Kp?

Answer 25

Kp is the equilibrium constant for a reversible reaction where all the reactants and products are gases.

Question 26

How do you work out Kp?

Answer 26

It is essentially the same as Kc except you use partial pressures instead of concentrations.

Question 27

What are le Chatelier’s principles regarding pressure on the position of equilibrium?

Answer 27

If you increase the pressure, the equilibrium shifts to the side with the fewer moles of gas
If you decrease the pressure, the equilibrium shifts to the side with more moles of gas.

Question 28

What are Le Chatelier’s principles regarding temperature on the position of equilibrium?

Answer 28

If you increase the temperature, the equilibrium shifts in the endothermic direction.
If you decrease the temperature, the equilibrium shifts in the exothermic direction.

Question 29

How does changing pressure affect Kp?

Answer 29

It doesn’t

Question 30

How does the addition of a catalyst affect Kp?

Answer 30

It doesn’t it just gets the system to equilibrium more quickly.

Question 31

How does changing temperature affect Kp?

Answer 31

A particular value of Kp is only valid for a give temperature. Changing the temperature changes how much product is formed at equilibrium. This changes the mole fractions of the gases present, which changes their partial pressures.