C7 Energy Changes

Question 1

Endothermic Reactions

Answer 1

If the reaction is endothermic, it requires energy (gets cold).

Question 2

Exothermic Reactions

Answer 2

If the reaction is exothermic, it gives out energy (gets hotter).

Question 3

Factors affecting the rate of reactions

Answer 3

Temp - more kinetic energy, more collisions per second, greater energy of collisions
Concentration - more reactant particles, more collisions per second
Size of solid - smaller pieces, larger SA/V ratio, more collisions per second
Catalyst - lower activation energy, therefore more successful collisions
Pressure in gases - greater pressure = gas particles closer together, greater frequency of collisions

Question 4

Calculating rates

Answer 4

Rate = reactant used (amount) / time
OR
product formed / time

Question 5

Rate at a given time

Answer 5

Depends on the gradient of the line
Always do y/x axis

Question 6

Effect of catalyst on decomposition of hydrogen peroxide

Answer 6

• catalysts lower the activation energy
• needed for reaction to proceed
• speed up the reaction
• they are not used up in the reaction
Slope on the graph goes lower more quickly

Question 7

Effects of Surface Area on Rate of Reaction between Hydrochloric Acid and Marble Chips

Answer 7

Smaller pieces have a bigger surface area to volume ratio, so more collisions between acid and C + CO3 particles

Question 8

Why do we repeat results?

Answer 8

To identify and reduce the affect of random errors.

Question 9

Why do we need to keep the control variable the same?

Answer 9

To make the experiment valid/so results are valid.

Question 10

Calculating the overall energy change of a reaction

Answer 10

Energy in - energy out = energy change

Question 11

What does a - result for energy change indicate?

Answer 11

The reaction is exothermic

Question 12

What is a reaction profile?

Answer 12

Graph to show changes in energy during a reaction

Question 13

Activation energy

Answer 13

Minimum energy needed for reaction to proceed

Question 14

Change in energy (^H) (enthalpy)

Answer 14

Difference between energy in reactants and products.

Question 15

Exothermic Reaction profile and Why?

Answer 15

• Reactants are higher than the products
• More energy is given out when new bonds form in products than is needed to break the bonds of the reactants.
(Releases energy)

Question 16

Endothermic reaction profile and why?

Answer 16

Reactants lower than products
More energy is needed to break the bonds in the reactants than is released when the products form.

Question 17

Exam Q: Another example of an endothermic reaction is the reaction between sodium hydrogen carbonate and citric acid.
A student wishes to measure the energy change of this reaction. They suggest that mixing the reactants in a beaker and stirring with a thermometer would be a good method.

What improvements would you suggest to their method to increase accuracy?

Answer 17

Use a polystyrene cup - polystyrene is a good thermal insulator and will reduce how much the external temperature affects the reaction.
Place the polystyrene cup in a beaker - the added layer of air around the cup will act as an insulator with the same result.
Put a lid on the cup - placing a lid on the cup prevents heat from the surroundings entering the cup and altering the temperature measurement.
Fix the thermometer - make sure that the thermometer is fixed with its base in the reaction solution. This will give the most accurate temperature readings.
Record the minimum temperature - make sure that you wait until the temperature has reached its lowest value to ensure that the most accurate energy change is measured.

Question 18

What is Le Chatelier’s Principle?

Answer 18

The idea that if you change the conditions of a reversible reaction at equilibrium in a closed system, the system will try to counteract that change.

Question 19

What can Le Chatelier’s Principle be used to predict?

Answer 19

The effects of changing conditions on the position of an equilibrium (in a closed system).

Question 20

Describe how a reversible chemical reaction is able to reach equilibrium.

Answer 20

When the forward and reverse reactions have the same rate.

Question 21

EXAM Q: Explain the effect of increasing the pressure on the yield of ammonia.
N + 3H <=> 2NH

Answer 21

It will shift to the right because there are less moles of gas to decrease the pressure, and the yield increases.

Question 22

Pressure in Reversible Reactions

Answer 22

• changing pressure only affects an equilibrium involving only gases
• if you increase the pressure, the equilibrium tries to reduce it - moves in the direction where there are fewer molecules of gas.
• if you decrease the pressure, the equilibrium tries to increase it - moves in the direction where there are more molecules of gas
• you can use a balanced symbol equation for a reaction to see which side has more molecules of gas

Question 23

Changes to temperature in reversible reactions

Answer 23

• all reactions are exothermic in one direction and endothermic in the other
• if you decrease the temp, the equilibrium will move in the exothermic direction to produce more heat, and so you’ll get more products for the exothermic reaction and fewer products for the endothermic reaction
• if you increase the temp, the equilibrium will move in the endothermic direction to try and decrease it. You will get more products for the endothermic reaction and fewer products for the exothermic reaction.

Question 24

Effect of concentration on reversible reactions

Answer 24

• if you change the concentration of either the reactants or the products, the system will no longer be at equilibrium so it responds to bring itself back to equilibrium again.
• if you increase the concentration of the reactants the system tries to decrease it by making more products.
• if you decrease the concentration of products, the system tries to increase it again by reducing the amount of reactants.

Question 25

What is a catalyst?

Answer 25

Catalysts increase the rate of reactions by lowering the activation energy needed for a reaction without being used up, so there are more successful collisions.
• a catalyst is an enzyme

Question 26

Advantages/disadvantages of using catalysts to speed up reactions

Answer 26

• are cost effective even though some catalysts are expensive precious metals (e.g. platinum and palladium)
• because a small amount can greatly increase the rate of a reaction
• paying for a catalyst at the start may be cheaper than paying for the energy needed to increase either temperature or pressure
• environmentally friendly because we are reducing the necessary temperatures and pressures so fewer fossil fuels need to be burned

• disadvantage is toxicity - many catalysts are toxic (e.g. transition metals), and these can escape into the environment and contaminate ecosystems.

Question 27

Conservation of energy in reversible reactions

Answer 27

• energy is conserved during reactions
• so the energy released/absorbed by the forward reaction will be exactly equal to the energy released/absorbed by the backwards reaction

Question 28

How can we measure whether a reaction is endothermic or exothermic?

Answer 28

The energy transfers that take place in exothermic and endothermic reactions can be measured by monitoring the temperature of the surroundings.

Question 29

What is a closed system?

Answer 29

• A closed system is a system where reactants and products can neither be added nor removed.

Question 30

What is dynamic equilibrium?

Answer 30

At dynamic equilibrium, the rates of the forwards and backwards reactions are equal.
The equilibrium is dynamic because both the forward and backward reactions are still taking place.
The concentrations of reactants and products do not change when a reaction is at dynamic equilibrium.

Question 31

What does a catalyst do in a reversible reaction?

Answer 31

A catalyst increases the rate of a forwards and backwards reaction to the same extent and helps the system to reach equilibrium faster.