2.4 Chemical Reactions & Energy (WJEC GCSE Chemistry): Flashcards
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Define exothermic reaction.
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Define exothermic reaction.
An exothermic reaction is a reaction that transfers thermal energy to the surroundings, causing the temperature of the surroundings to increase.
What happens to the temperature of the surroundings during an endothermic reaction?
The temperature of the surroundings decreases during an endothermic reaction, because thermal energy is transferred from the surroundings to the reaction system.
True or False?
In an exothermic reaction, the temperature of the surroundings increases.
True.
Exothermic reactions transfer thermal energy to the surroundings, so the measured temperature rises. The heat content of the system falls by the same amount.
Define the law of conservation of energy.
The law of conservation of energy is the principle that energy cannot be created or destroyed, only transferred from one form or place to another.
Give two examples of exothermic reactions.
Two examples of exothermic reactions are:
Combustion
Neutralisation
Both transfer thermal energy to the surroundings, causing a temperature rise.
In an exothermic reaction, the products have .......... energy than the reactants. The energy difference is .......... to the surroundings.
In an exothermic reaction, the products have less energy than the reactants. The energy difference is transferred to the surroundings.
True or False?
Chemical reactions can create new energy from nothing.
False.
The law of conservation of energy states that energy cannot be created or destroyed. In any chemical reaction, energy is only transferred between the system and the surroundings.
A reaction starts at 20°C and finishes at 15°C. Is this reaction exothermic or endothermic? Explain your answer.
This reaction is endothermic. The temperature decreased, which shows that the reaction took heat energy in from the surroundings.
Define activation energy.
Activation energy is the minimum amount of energy required for a collision between particles to result in a chemical reaction.
What does the y-axis represent on an energy profile diagram?
The y-axis on an energy profile diagram represents the energy of the reactants and products.
True or False?
In an exothermic reaction, the products are at a higher energy level than the reactants on an energy profile.
False.
In an exothermic reaction, energy is released, so the products are at a lower energy level than the reactants. The arrow on the energy profile points downwards.
On an energy profile, what does the peak of the curve between reactants and products represent?
The peak of the curve represents the activation energy. It is the maximum energy point that must be reached for the reaction to proceed.
On an energy profile for an endothermic reaction, the arrow points .......... because the products have .......... energy than the reactants.
On an energy profile for an endothermic reaction, the arrow points upwards because the products have more energy than the reactants.
What is an energy profile?
An energy profile is a graphical representation of the relative energies of the reactants and products in a chemical reaction, showing how energy changes as the reaction progresses.
True or False?
All chemical reactions have an activation energy.
True.
Every reaction requires a minimum energy input to break existing bonds in the reactants before new bonds can form. This minimum energy is the activation energy.
On an energy profile, the overall energy change of a reaction is negative. What does this tell you about the reaction?
A negative overall energy change tells you the reaction is exothermic. The products have less energy than the reactants, so energy has been released to the surroundings.
What is bond energy?
Bond energy is the amount of energy required to break a chemical bond, or the amount of energy released when that bond is formed.
Is bond breaking an exothermic or endothermic process? Explain.
Bond breaking is endothermic. Energy must be taken in from the surroundings to break chemical bonds.
True or False?
Bond forming is an exothermic process.
True.
When new bonds form, energy is released to the surroundings. This is why bond making is always exothermic.
Energy change = energy .......... - energy ..........
If the result is negative, the reaction is ...........
Energy change = energy taken in - energy given out
If the result is negative, the reaction is exothermic.
In a reaction, bonds broken require 678 kJ and bonds formed release 862 kJ. What is the energy change and what type of reaction is this?
The energy change is -184 kJ, so the reaction is exothermic. Working: 678 - 862 = -184 kJ. More energy is released forming bonds than is taken in breaking bonds.
True or False?
In an endothermic reaction, more energy is released when forming new bonds than is absorbed when breaking existing bonds.
False.
In an endothermic reaction, more energy is absorbed breaking bonds than is released forming new bonds. This gives a positive energy change, and the products have more energy than the reactants.
What sign does the overall energy change have in an exothermic reaction?
The overall energy change is negative in an exothermic reaction. This is because the products have less energy than the reactants, and energy has been released.
When hydrogen reacts with chlorine, the energy taken in is 678 kJ and the energy given out is 862 kJ. The energy change is .......... kJ, so the reaction is ...........
When hydrogen reacts with chlorine, the energy taken in is 678 kJ and the energy given out is 862 kJ. The energy change is -184 kJ, so the reaction is exothermic.
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