C8 - Rates and Equilibrium

Question 1

On a rate of reactions graph, what does the line getting steeper show?

Answer 1

How fast the rate of reaction is happening at.

Question 2

What does a flat line on a graph of amount of products against time show?

Answer 2

A flat line shows the reaction has finished.

Question 3

What two factors, relating to the collisions between particles, influence the rate of a reaction?

Answer 3

• Collision frequency
• Energy transferred during collisions
  (A successful collision is one that ends in the particles reacting to form products)

Question 4

What are the four factors that affect the rate of a chemical reaction?

Answer 4

• Temperature
• Concentration of a solution/ Pressure of a gas
• Surface area of a solid
• Presence of a catalyst

Question 5

Why does increasing the temperature of a reaction mixture increase the rate of a reaction?

Answer 5

=> Temperature increased = all particles moving faster (heat energy transferred into kinetic energy)
=> Increases frequency of collisions
=> More energy so more successful collisions

Question 6

Other than increasing the temperature, describe two ways of increasing the rate of reaction between a solution and a solid.

Answer 6

• INCREASING CONCENTRATION OF A SOLUTION
  => More particles in the same volume of water
  => More frequent collisions
• INCREASING SURFACE AREA OF A SOLID
  => Breaking it into smaller pieces increases SA:V
  => For same volume of the solid, more SA
  => Particles around it have more surface to act on - more frequent collisions.

Question 7

What is a catalyst?

Answer 7

A catalyst is a substance that speeds up a reaction whilst remaining chemically unchanged.

Question 8

How does a catalyst increase the rate of a reaction?

Answer 8

=> They decrease the activation energy for the reaction to occur
=> Do this by providing an alternative reaction pathway with a lower activation energy.

Question 9

State the equation that could be used to calculate the mean rate of reaction.

Answer 9

Rate of Reaction = Amount of reactant used or amount of product formed / Time

Question 10

Give three possible units for the rate of a chemical reaction

Answer 10

cm³/s; g/s; mol/s

Question 11

How would you measure the rate of reaction between two clear solutions, in which the product formed was a precipitate?

Answer 11

• Observe a mark through the solution and measure how long it takes for it to disappear
• Faster the mark disappears, faster the reaction (vice versa)
• (Results are subjective, can’t plot a rate of reaction graph)

Question 12

Explain why measuring a mass change during a reaction is an accurate method of measuring rate.

Answer 12

(For a reaction that releases a gas)

• Use mass balance to measure the drop in mass (this is the gas given off)
• Quicker the reading on the balance drops, faster the reaction (vice versa)
• If measured at regular intervals = plot graph to find rate of reaction
• Not subjective - mass balance is very accurate
• But gas is released into room.

Question 13

Describe how you could use a graph to find the mean rate of reaction between two points in time.

Answer 13

=> Change in Y / Change in X

=> From the two points, draw lines across/down to the axes and calculate the change in each.

Question 14

What is a tangent?

Answer 14

A straight line that touches the curve at one point and doesn’t cross it.

Question 15

How would you use a tangent to find the gradient of a curve at a particular point?

Answer 15

=> Draw the tangent
=> Calculate the gradient of the tangent
(Rise / Run)

Question 16

Describe how you could investigate the effect of increasing HCl concentration on the rate of reaction between HCl and Mg.

Answer 16

=> Add set volume of dilute HCl to a conical flask and place on a mass balance
=> Quickly add magnesium ribbon and plug the flask with cotton wool.
=> Start stopwatch and record mass on balance. Take reading of mass at regular intervals.
=> Put data into table; calculate mass loss for each reading. Plot graph with loss of mass against time.
=> Repeat experiment with different concentrations of HCl - all other variables must stay the same.

Question 17

Which one of the following statements is true?

a) In a reaction at equilibrium, there is the same amount of products as reactants.
b) If the forward reaction in a reversible reaction is exothermic, then the reverse reaction is endothermic.
c) If the equilibrium of a system lies to the right, then the concentration of products is less than the concentration of reactants.

Answer 17

a) An equilibrium reaction doesn’t mean the amount of products and reactants are equal.
b) True
c) If the equilibrium of a system lies to the right, then the concentration of products is greater than the concentration of reactants
(If the equilibrium of lies to the left, the concentration of reactants is greater than that of the products.

Question 18

What effect will decreasing the temperature have on a reversible reaction in which the forward reaction is exothermic?

Answer 18

=> Equilibrium moves in the exothermic direction to produce more heat (counteract change)
=> More products for the exothermic reaction and fewer for the endothermic reaction.

Question 19

How can you predict the effect of changing the pressure of a gaseous reaction?

Answer 19

=> Increasing Pressure: equilibrium will move in direction of fewer molecules of gas to reduce the pressure.
=> Decreasing Pressure: equilibrium will move in direction of more molecules of gas to increase the pressure.
(The equilibrium always wants to counteract the change)

Question 20

According to Le Chatelier’s Principle, what will the effect of decreasing the concentration of the products for the forward reaction have on the reversible reaction?

Answer 20

=> If concentration of products is decreased, the system tries to increase it again by reducing the amount of reactants.
(If concentration of reactants is increased, the system tries to decrease it by making more products.

Question 21

What is Le Chatelier’s Principle?

Answer 21

• The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract that change.
• It can be used to predict the effect of any changes you make to a reaction system.

Question 22

Describe the process of a reversible reaction reaching equilibrium.

Answer 22

=> Reactants react - their concentrations fall - forward reaction slows down.
=> More and more products formed - their concentrations rise - backward reaction speeds up.
=> Eventually forward reaction reaches same rate as backward reaction - system is at equilibrium.