Topic 7

Question 1

Core Practical: Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by: measuring the production of a gas and observing a colour change

Answer 1

● in these experiments, you are investigating the effect on rate of changing the size of the marble chips
(smaller chips=larger surface area= faster rate)
and also the effect of changing the concentration of hydrochloric acid
(greater concentration= greater number of particles in a given volume= faster rate)

Question 2

Suggest practical methods for determining the rate of a given reaction

Answer 2

● Rates of reactions can be measured using the amount of product used, or amount of product formed over time:

​Rate​ ​of​ ​reaction​ ​=​ ​amount​ ​of​ ​reactant​ ​used/​Time
​Rate​ ​of​ ​reaction​ ​=​ ​amount​ ​of​ ​product​ ​formed/Time

Question 3

Units for rate of reaction

Answer 3

o Units of rate of reaction may be given as g/s or cm3/s

Question 4

How to measure reactant used and product formed and give the units

Answer 4

o to measure reactant used: if the product is a gas, which will be given off, you can carry out the reaction on a set of weighing scales and measure how much mass is lost

o to measure product formed: if the product is a gas, you can measure the volume of gas produced in a gas syringe

o Quantity of reactant or product can be measured by the mass in grams or by a volume in cm3

Question 5

Explain how reactions occur when particles collide and that rates of reaction are increased when the frequency and/or energy of collisions is increased

Answer 5

● Chemical reactions only occur when the reacting particles collide with enough energy, the minimum amount of energy required is called the activation energy.
● in order to increase the rate of a reaction, you need to increase the frequency/energy of collisions, so that more of them reach the activation energy
● this can be done by: increasing temperature, pressure, concentration, surface area or by using a catalyst

Question 6

Explain the effects of changes in temperature, on rates of reaction

Answer 6

● Increasing the temperature increases the rate of reaction.
●As increasing temperature increases the kinetic energy of particles, so they collide more frequently and energetically.

Question 7

Explain the effects of changes in concentration, on rates of reaction

Answer 7

● Increasing concentration of reacting solutions increases the rate of reaction, as it increases the number of particles in a given volume and so increases the frequency of collisions

Question 8

Explain the effects of changes in pressure, on rates of reaction

Answer 8

● Increasing pressure in reacting gases increases the rate of reaction, as it increases the number of particles in a given volume so increases the frequency of collisions

Question 9

Explain the effects of changes in surface area to volume ratio of a solid on rates of reaction

Answer 9

● Increasing the surface area of solid reactants increases the rate of reaction, as it increases the frequency of collisions so increases the rate of reaction

Question 10

Interpret graphs of mass, volume or concentration of reactant or
product against time

Answer 10

● To find the rate of reaction graphically
o Draw tangents to curves and use the slope of the tangent as a measure of the rate of reaction
● Gradient = rate of reaction, therefore a steeper line means a greater increase/decrease in rate
● remember to check graph is showing a product or a reactant- for a product you would be expecting mass/volume/concentration to increase, whereas for a reactant you would be expecting mass/volume/concentration to decrease

Question 11

Describe a catalyst

Answer 11

● Catalysts are substances that speed up chemical reactions without being changed or used up during the reaction. They are the same and have the same mass at the end of the reaction.

Question 12

Explain how the addition of a catalyst increases the rate of a reaction in terms of activation energy

Answer 12

● Catalysts decrease the activation energy; this increases the proportion of particles with energy to react, leading to more frequent successful collisions and so an increased rate of reaction
● Catalysts lower the activation energy by providing a different pathway for a chemical reaction that has a lower activation energy

Question 13

Recall that enzymes are biological catalysts and that enzymes are used in the production of alcoholic drinks

Answer 13

● Enzymes act as catalysts in biological systems

● Yeast is the enzyme used in the production of ethanol as it is fermented from sugars, ethanol is in alcoholic drinks

Question 14

Is salts dissolving in water exothermic or endothermic?

Answer 14

● Salts dissolving in water is either exothermic or endothermic

Question 15

Are neutralisation reactions exothermic or endothermic?

Answer 15

● Neutralisation reaction is exothermic

Question 16

Are displacement reactions exothermic or endothermic?

Answer 16

● Displacement is an exothermic or endothermic reaction

Question 17

Are precipitation reactions exothermic or endothermic?

Answer 17

● Precipitation is an exothermic reaction

Question 18

When these reactions take place in solution, how can the heat changes be reflected?

Answer 18

● if these reactions take place in a solution, you can carry them out in a polystyrene cup with a lid, and measure the temperature change using a thermometer

Question 19

Describe an exothermic change or reaction

Answer 19

● An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.
● Examples of exothermic reactions include; combustion, many oxidation reactions and neutralisation.
● Everyday examples of exothermic reactions include; self-heating cans (e.g for coffee) and hand warmers.

Question 20

Describe an endothermic change or reaction

Answer 20

● An endothermic reaction is one that takes in energy
from the surroundings so the temperature of the
surroundings decreases.
● Examples of endothermic reactions are thermal
decomposition and the reaction of citric acid and
sodium hydrogencarbonate.
● Some sports injury packs are based on endothermic
reactions.

Question 21

What type of reaction is the breaking of bonds?

Answer 21

● During a chemical reaction:

○ Energy must be SUPPLIED to BREAK bonds in the reactants SO the breaking of bonds is ENDOTHERMIC

Question 22

What type of reaction is the making of bonds?

Answer 22

Energy is RELEASED when bonds in the products are FORMED so the making of bonds is EXOTHERMIC

Question 23

When is the overall heat energy change for a reaction exothermic?

Answer 23

The overall heat energy change for a reaction is exothermic if more heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
● Energy taken in to break < energy released when formed = EXOTHERMIC (because overall energy has been released)

Question 24

When is the overall heat energy change for a reaction endothermic?

Answer 24

The overall heat energy change for a reaction is: endothermic if less heat energy is released in forming bonds in the products than is required in breaking bonds in the reactants
● Energy taken in to break > energy released when formed = ENDOTHERMIC (because overall energy has been taken in)

Question 25

Calculate the energy change in a reaction given the energies of bonds (in kJ mol-1)

Answer 25

● The energy needed to break bonds and energy released when bonds are formed can both be calculated from bond energies
● Sum of energy taken in to break bonds – sum of energy released to form bonds = overall energy change
● If the energy out > energy in, the energy change will be negative showing an exothermic reaction and if the energy out < energy in, the energy change will be
positive showing an endothermic reaction

Question 26

Explain the term activation energy

Answer 26

● Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy.
o Activation energy = minimum amount of energy that particles must have to react

Question 27

Draw and label reaction profiles for endothermic and exothermic reactions, identifying activation energy

Answer 27

● Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.
● for the exothermic diagram, the products have less energy than the reactants, because the energy has been released to the surroundings
● for the endothermic diagram, the reactants have less energy than the products, because the energy has been taken in from the surroundings