Chemical Reactions & Energy (WJEC GCSE Science (Double Award): Chemistry): Flashcards
Exam code: 3430
1/270Still learning
Know0
Define exothermic reaction.
Was this flashcard helpful?
Cards in this collection (27)
Define exothermic reaction.
An exothermic reaction is a reaction that releases heat energy to the surroundings, causing the temperature of the surroundings to increase. Examples include combustion and neutralisation.
True or False?
In an endothermic reaction, the temperature of the surroundings decreases.
True.
An endothermic reaction takes in heat energy from the surroundings, so the temperature of the surroundings falls.
Energy is .......... in chemical reactions because it cannot be created or destroyed — only .......... .
Energy is conserved in chemical reactions because it cannot be created or destroyed — only transferred. This is the law of conservation of energy.
A student mixes two solutions and the temperature rises from 19 °C to 25 °C. Is this reaction exothermic or endothermic?
The reaction is exothermic because the temperature of the surroundings increased, showing that heat energy was released by the reaction.
Define endothermic reaction.
An endothermic reaction is a reaction that takes in heat energy from the surroundings, causing the temperature of the surroundings to decrease. An example is the reaction of sodium hydrogen carbonate with citric acid.
How can you identify whether a reaction is exothermic or endothermic from experimental temperature data?
You compare the initial and final temperatures. A temperature increase shows the reaction is exothermic; a temperature decrease shows the reaction is endothermic.
True or False?
Combustion is an example of an endothermic reaction.
False.
Combustion is exothermic because it releases heat energy to the surroundings, causing the temperature to increase.
In an exothermic reaction, the products have .......... energy than the reactants because energy has been transferred to the .......... .
In an exothermic reaction, the products have less energy than the reactants because energy has been transferred to the surroundings.
Give two examples of exothermic reactions.
Two examples of exothermic reactions are:
Combustion
Neutralisation
Define activation energy.
Activation energy is the minimum amount of energy needed for a reaction to occur. It is represented on an energy profile diagram as the initial rise from the reactants to the peak of the curve.
On an energy profile diagram, what does the y-axis represent and what does the x-axis represent?
The y-axis represents the energy of the reactants and products. The x-axis represents the progress of the reaction.
True or False?
In an exothermic reaction energy profile, the products are at a higher energy level than the reactants.
False.
In an exothermic reaction, the products are at a lower energy level than the reactants because energy has been released to the surroundings.
In an endothermic reaction, the arrow on the energy profile points .......... because the products have .......... energy than the reactants.
In an endothermic reaction, the arrow on the energy profile points upwards because the products have more energy than the reactants.
What does a larger initial rise on an energy profile diagram tell you about a reaction?
A larger initial rise indicates a higher activation energy, meaning more energy is required to start the reaction (e.g. more heat is needed).
What is an energy profile diagram?
An energy profile diagram is a graphical representation showing the relative energies of reactants and products during a chemical reaction, including the activation energy peak and overall energy change.
True or False?
All reactions have an activation energy.
True.
All reactions have an activation energy because the chemical bonds in the reactant molecules must be broken first before new bonds can form.
In an exothermic reaction energy profile, in which direction does the arrow between reactants and products point?
The arrow points downwards because the products have lower energy than the reactants, showing that energy has been released to the surroundings.
The overall energy change of a reaction is represented on an energy profile by the difference in .......... between the reactants and the .......... .
The overall energy change of a reaction is represented on an energy profile by the difference in height between the reactants and the products.
Define bond energy.
Bond energy is the amount of energy required to break a chemical bond, or the amount of energy released when a bond is formed. Each type of bond has a specific bond energy value measured in kJ.
True or False?
Bond breaking is an exothermic process.
False.
Bond breaking is endothermic because energy must be taken in from the surroundings to break chemical bonds.
Energy change = energy .......... − energy ..........
Energy change = energy taken in − energy given out. A negative result means the reaction is exothermic. A positive result means it is endothermic.
In a bond energy calculation, H–H bond energy = 436 kJ and Cl–Cl bond energy = 242 kJ. What is the total energy taken in when one H–H bond and one Cl–Cl bond are broken?
Total energy taken in = 436 + 242 = 678 kJ.
Define bond making.
Bond making is an exothermic process in which energy is released to the surroundings as new chemical bonds form between atoms in the products.
A reaction has energy taken in = 678 kJ and energy given out = 862 kJ. What is the energy change and is the reaction exothermic or endothermic?
Energy change = 678 − 862 = −184 kJ. The reaction is exothermic because the energy change is negative, showing energy is released to the surroundings.
True or False?
If the energy released when bonds form is greater than the energy taken in to break bonds, the reaction is exothermic.
True.
When more energy is released (bond making) than absorbed (bond breaking), the overall energy change is negative and the reaction is exothermic.
To calculate the energy change for a reaction, first add the bond energies of all bonds in the .......... (energy in), then subtract the bond energies of all bonds in the .......... (energy out).
To calculate the energy change for a reaction, first add the bond energies of all bonds in the reactants (energy in), then subtract the bond energies of all bonds in the products (energy out).
(Higher Tier Only)
The decomposition of HBr has an energy change of +103 kJ. If the H-Br bond energy is 366 kJ and the H-H bond energy is 436 kJ, what is the Br-Br bond energy?
Energy in = 2 × 366 = 732 kJ
Energy out = 436 + Br-Br
+103 = 732 − (436 + Br-Br)
Br-Br = 732 − 436 − 103 = 193 kJ
Sign up to unlock flashcards
or