The table shows the initial and final temperatures for four different reactions.
| reaction | initial temperature/°C | final temperature/°C |
|---|---|---|
| 1 | 19 | 28 |
| 2 | 18 | 16 |
| 3 | 20 | 20 |
| 4 | 18 | 19 |
Which reactions are endothermic?
A) 1 and 4
B) 2 and 3
C) 2 only
D) 4 only
▶️ Answer/Explanation
Ans: C
An endothermic reaction absorbs heat from the surroundings, causing the temperature to decrease. Only reaction 2 shows a temperature decrease (from 18°C to 16°C). Reaction 1 shows a temperature increase (exothermic), reaction 3 shows no change (possibly no reaction or perfect insulation), and reaction 4 shows a slight increase (exothermic). Therefore, only reaction 2 is endothermic.
Which row describes the reaction pathway diagram and energy change in an exothermic reaction?
| reaction pathway diagram | energy is | |
|---|---|---|
| A | reactants higher than products | absorbed |
| B | reactants higher than products | released |
| C | reactants lower than products | absorbed |
| D | reactants lower than products | released |
▶️ Answer/Explanation
Ans: B
In an exothermic reaction, the reactants have more energy than the products, so the reaction pathway diagram shows reactants at a higher energy level than products. Energy is released to the surroundings in an exothermic reaction (often as heat). Therefore, the correct combination is: reactants higher than products and energy is released. Option A describes an endothermic reaction where energy is absorbed, and options C and D have the energy levels reversed.
The temperature of the water in two beakers, X and Y, is measured as 21.5°C.
5 g of sodium chloride is dissolved in the water in beaker X. The temperature changes to 18.0°C.
5 g of calcium oxide is dissolved in the water in beaker Y. The temperature changes to 29.4°C.
Which types of process are occurring in beakers X and Y?
| X | Y | |
|---|---|---|
| A | endothermic | endothermic |
| B | endothermic | exothermic |
| C | exothermic | endothermic |
| D | exothermic | exothermic |
▶️ Answer/Explanation
Ans: B
To determine whether the processes are endothermic or exothermic:
Beaker X (sodium chloride):
The temperature decreases from 21.5°C to 18.0°C, indicating heat is absorbed from the surroundings. This is characteristic of an endothermic process.
Beaker Y (calcium oxide):
The temperature increases from 21.5°C to 29.4°C, indicating heat is released to the surroundings. This is characteristic of an exothermic process.
This matches option B: X is endothermic and Y is exothermic.
The diagram shows a match.
By striking the match, a chemical reaction takes place.
Which row describes the chemical reaction?
▶️ Answer/Explanation
Ans: C
The burning of a match is a classic example of an exothermic reaction because:
1. Heat and light energy are released during the combustion process
2. The energy required to initiate the reaction (striking the match) is different from the energy change of the reaction itself
3. The correct reason is that energy is given out as the match burns, not that energy is used to strike it
Therefore, option C correctly identifies the reaction as exothermic with the proper explanation.