Energy Changes C5

Question 1

What happens to energy during chemical reactions?

Answer 1

Energy is moved around in chemical reactions. The overall amount of energy doesn’t change; it is conserved.

Question 2

What is an exothermic reaction?

Answer 2

An exothermic reaction is one which transfers energy to the surroundings, usually by heating, resulting in a rise in temperature.

Example: Burning fuels (combustion) is a common exothermic reaction.

Question 3

What are some examples of exothermic reactions?

Answer 3

Examples include combustion, neutralization reactions (acid + alkali), and many oxidation reactions, such as adding sodium to water.

Question 4

What is an endothermic reaction?

Answer 4

An endothermic reaction is one which takes in energy from the surroundings, resulting in a fall in temperature.

Example: The reaction between citric acid and sodium bicarbonate.

Question 5

What are some examples of endothermic reactions?

Answer 5

Examples include thermal decomposition (e.g., heating calcium carbonate) and certain physical processes like melting.

Question 6

How are endothermic reactions used in everyday life?

Answer 6

Endothermic reactions are used in some sports injury packs that become instant cold packs without needing to be frozen.

Question 7

What is the mnemonic to remember the difference between exothermic and endothermic?

Answer 7

Remember: ‘exo’ means exit (heat out), and ‘thermic’ relates to heat. Exothermic reactions give out heat, while endothermic reactions take in heat.

Question 8

Is the reaction exothermic or endothermic if the temperature rises from 21°C to 28.5°C?

Answer 8

The reaction is exothermic, as the temperature of the reaction mixture increases.

Question 9

What is the difference between exothermic and endothermic reactions?

Answer 9

Exothermic reactions release energy, while endothermic reactions absorb energy.

Question 10

How can energy transfer be measured in a chemical reaction?

Answer 10

You can measure the energy released by taking the temperature of the reactants, mixing them in a polystyrene cup, and measuring the final temperature of the solution.

Question 11

What is a common problem when measuring energy in reactions?

Answer 11

The biggest problem is the amount of energy lost to the surroundings.

Question 12

How can you reduce energy loss during measurements?

Answer 12

You can reduce energy loss by placing the polystyrene cup in a beaker of cotton wool for insulation and putting a lid on the cup to minimize evaporation.

Question 13

What types of reactions can the measurement method be used for?

Answer 13

This method works for neutralization reactions or reactions between metals and acids, or carbonates and acids.

Question 14

What does a reaction profile show?

Answer 14

A reaction profile shows the relative energies of the reactants and products and how energy changes over the course of the reaction.

Question 15

What does the height difference in a reaction profile represent?

Answer 15

The height difference represents the overall energy change in the reaction (energy given out or taken in) per mole.

Question 16

What is activation energy?

Answer 16

Activation energy is the minimum amount of energy the reactants need to collide with each other and react.

Question 17

What is the procedure to test the effect of acid concentration on energy released?

Answer 17

1) Prepare 25 cm³ of hydrochloric acid and sodium hydroxide in separate beakers. 2) Place them in a water bath at 25 °C. 3) Mix them in a polystyrene cup with a lid. 4) Measure the temperature every 30 seconds and record the highest temperature. 5) Repeat with different concentrations.

Question 18

What is the equation for the combustion of methane?

Answer 18

The equation is CH₄ + 2O₂ → CO₂ + 2H₂O.

Question 19

What is the relationship between bond breaking and energy?

Answer 19

Bond breaking is an endothermic process, meaning energy must be supplied to break existing bonds.

Question 20

What happens during a chemical reaction regarding bonds?

Answer 20

Old bonds are broken and new bonds are formed.

Question 21

What is the process of bond formation?

Answer 21

Bond formation is an exothermic process, where energy is released when new bonds are formed.

Question 22

How do energy changes differ in exothermic and endothermic reactions?

Answer 22

In exothermic reactions, the energy released by forming bonds is greater than the energy used to break them. In endothermic reactions, the energy used to break bonds is greater than the energy released by forming them.

Question 23

What is bond energy?

Answer 23

Every chemical bond has a particular bond energy associated with it, which varies slightly depending on the compound.

Question 24

How is the overall energy change for a reaction calculated?

Answer 24

The overall energy change is the sum of the energies needed to break bonds in the reactants minus the energy released when new bonds are formed in the products.

Question 25

What is the formula for calculating overall energy change?

Answer 25

Overall energy change = energy required to break bonds - energy released by forming bonds.

Question 26

What are the bond energies for H-H and Cl-Cl?

Answer 26

H-H: +436 kJ/mol; Cl-Cl: +242 kJ/mol.

Question 27

What is the overall energy change for the reaction H-H + Cl-Cl → H-Cl?

Answer 27

Overall energy change = 678 kJ/mol - 862 kJ/mol = -184 kJ/mol.

Question 28

What is the bond energy for N≡N, H-H, and N-H?

Answer 28

N≡N: 941 kJ/mol; H-H: 436 kJ/mol; N-H: 391 kJ/mol.

Question 29

What is the effect of bond strength on energy release in reactions?

Answer 29

Weaker bonds require less energy to break but also release less energy when new bonds form.