Topic 6 - The Rate and Extent of Chemical Change

Question 1

What is the rate of reaction?

Answer 1

The rate of a chemical reaction is how fast the reactants are changed into products. Some happen quickly, some slowly.

Question 2

What are the factors that affect the rate of reaction?

Answer 2

• temperature - higher temp, faster rate
• concentration (or pressure for gas) - more concentrated/higher pressure reactants are, faster rate
• surface area - larger SA, faster rate
• catalysts - reactions with catalyst = faster that those without

Question 3

What 2 things does Collision theory say rate of reaction is dependent on?

Answer 3

1. The collision frequency of reacting particles (how often they collide). The more there are in a given period, the faster the reaction is.
2. The energy transferred during a collision. Particles have to collide with enough energy to be successful.

Question 4

Increasing the frequency of collisions

Answer 4

• temperature => when temp. increased, particles move quicker they’ll collide more often and more collisions = faster rate of reaction
• concentration => means more particles of reactant knocking about between the water molecules, making collisions between important particles more likely- pressure => means particles are squashed up together, so there will be more frequent collisions and faster rate of reaction
• surface area => increased SA:VOL means for same volume of solid, the particles around it in the solution will have more area to work on, means more frequent collisions and faster rate of reaction

Question 5

Increasing the energy of collision

Answer 5

• effect of temperature can be explained in terms of how much energy the particles have when they collide
• higher temp increases the energy of the collisions as it makes all the particles move faster

Question 6

What is activation energy?

Answer 6

The minimum amount of energy required in order for the particles to react.

Question 7

What is the rate of reaction directly proportional to?

Answer 7

The frequency of successful collisions.

Question 8

What is a catalyst and how do they work?

Answer 8

A catalyst is a substance which can speed up a specific/certain reaction, without being changed or used up in the reaction.They work by decreasing the activation energy needed for a chemical reaction.

Question 9

What is an enzyme?

Answer 9

A biological catalyst. They generally only catalyse a specific reaction for a specific molecule.

Question 10

Rate of reaction calculation

Answer 10

Mean rate of reaction = quantity of reactant used or product produced/time takenunits follow trend of ‘units of amount of substance/unit of time’

Question 11

What does a line of best fit on a rate of reaction graph look like?

Answer 11

A curved line, that starts off steep, gets shallower and levels off.

Question 12

Comparing rate of reaction graphs (2)

Answer 12

• fastest reaction will be the line with the steepest slope at the beginning, and finish first (level off)
• reactions that start off with same amount of reactants will finish at same point on graph

Question 13

What is a tangent?

Answer 13

A straight line that touches the curve at a particular point without crossing it.

Question 14

How to draw a tangent?

Answer 14

1. Place a ruler on the curve at a point where it’s just touching the curve.
2. Adjust the ruler until the space between the ruler and the curve is equal on both sides of the point.
3. Draw a line along the ruler to make the tangent.

Question 15

How to calculate rate of reaction at a specific point using a tangent?

Answer 15

gradient = change in y/change in x

Question 16

What is the chemical equation for Magnesium Metal and Hydrochloric Acid required practical?

Answer 16

2HCl(aq) + Mg(s) => MgCl2(aq) + H2(g)

Question 17

What is the method for Magnesium Metal and Hydrochloric Acid required practical?

Answer 17

1. Measure out 50cm3 of dilute hydrochloric acid using a measuring cylinder, and add this to the conical flask.
2. Add some magnesium ribbon to the acid and quickly attach an empty gas syringe to the flask.
3. Start the stopwatch.
4. Take readings of the volume at regular intervals until the volume hasn’t changed for 3 consecutive readings.
5. Put the results in a table. Plot a graph with time on x-axis, and volume of gas produced on y-axis.
6. Repeat with more concentrated acid solutions. Other variables/conditions should be kept exactly the same => makes experiment a fair test.

Question 18

What is the chemical equation for Sodium Thiosulfate and Hydrochloric Acid required practical?

Answer 18

2HCl(aq) + Na2S2O3(aq) => 2NaCl(aq) + SO2(g) + S(s) + H2O(l)

Question 19

What is the method for Sodium Thiosulfate and Hydrochloric Acid required practical?

Answer 19

1. Start by adding 50cm3 of dilute sodium thioslufate solution to a flask.
2. Place the flask on a piece of paper with a black cross drawn on it. Add 10cm3 of dilute hydrochloric acid to the flask, give the flask a brief swirl to mix the reactants and start the stop watch.
3. Now watch the black cross through the cloudy sulfur and time how long it takes to disappear. Record your results in a table.
4. Repeat the reaction in exactly the same way, using the same concentrations of reactants and use results to calculate a mean time for cross to disappear (fair test).
5. Repeat the reaction with solutions of either reactant at different concentrations (one at a time). Depth of liquid kept same, and person observing the cross kept same (fair test).

Question 20

What is a reversible reaction?

Answer 20

A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants. Reversible reactions can be represented like this;A + B ⇌ C + D

Question 21

What is equilibrium?

Answer 21

When amounts of reactants and products reach a balance - their concentrations stop changing.

Question 22

What is a closed system?

Answer 22

Where none of the reactants/ products in a reaction can escape and nothing else can get in.

Question 23

What happens when equilibrium lies to the right?

Answer 23

The concentration of products are greater than that of reactants.

Question 24

What happens when equilibrium lies to the left?

Answer 24

The concentration of reactants are greater than that of products.

Question 25

Heating Hydrated Copper Sulphate:- Chemical equation?- Forwards/backwards reaction?

Answer 25

• CuSO(4).5H(2)O (s) ⇌ CuSO(4) (s) + 5H(2)O (l)
• forwards reaction = endothermic
• backwards reaction = exothermic

Question 26

Dichromate:
- Chemical equation?
- Concentration of alkaline?

Answer 26

• K(2)Cr(2)O(7) + 2NaOH ⇌ K(2)CrO(4) + Na(2)CrO(4) + HCl
• increased concentration of alkaline = favour the forwards reactiondecreased concentration of alkaline = favour the backwards reaction

Question 27

What is La Chatelier’s Principle?

Answer 27

The idea that if you change the conditions of a reversible reaction at equilibrium, the system will try to counteract the change.This means the effect of any changes to a system can be predicted => altering pressure/temp/ concentration of reactants, alters the yield of reaction - end up with more of the product you want.

Question 28

What does yield mean?

Answer 28

The amount of product you get from a reaction.

Question 29

What does the position of equilibrium do?

Answer 29

Indicates the proportion of reactant/product : ‘to the left’ ‘to the right’

Question 30

What happens if you raise the temperature of a reversible reaction?

Answer 30

The yield of the endothermic reaction (heat in) will increase and the yield of the exothermic reaction (heat out) will decrease.

Question 31

What happens if you reduce the temperature of a reversible reaction?

Answer 31

The yield of the exothermic reaction will increase and the yield of the endothermic reaction will decrease.

Question 32

Sulfur Trioxide:
- Chemical equation?
- Effect of temp?

Answer 32

• 2SO(2) g + O(2) g ⇌ 2SO(3) g
• Increase temp = endothermic reverse reaction will be favoured, higher yield of SO(2) and O(2).
• Decrease temp = exothermic forward reaction will be favoured, higher yield of SO(3) - product that you want.

Question 33

What happens when you increase pressure in a reversible reaction of gases?

Answer 33

• Raising pressure = favours reaction which produces less volume (fewest number of gas molecules)
• Lowering pressure = favours reaction which produces more volume (greatest number of gas molecules)

Question 34

Hydrogen gas:
- Chemical equation?
- Effect of pressure?

Answer 34

• CH(4) g + H(2)O g ⇌ CO g + 3H(2) g
• Increased pressure = reversed reaction is favoured - higher yield of CH(4) and H(2)O.
• Decreased pressure = forward reaction is favoured - higher yield of CO and H(2) (product that you want)

Question 35

What happens if you increase the concentration in a reversible reaction?

Answer 35

• Increased concentration of reactant = more products
• Decreased concentration of a product = reduced amount of reactants

Question 36

Ammonia:- Chemical equation?- Effect of concentration?

Answer 36

• N(2) g + 3H(2) g ⇌ 2NH(3) g
• Concentration of N(2) or H(2) is increased = more NH(3) produced.
  NH(3) removed = more NH(3) produced.