Chapter 7 - Energy Changes
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Define an exothermic reaction.
Reactions transferring energy to the surroundings.
What is an endothermic reaction?
Reactions taking in energy from the surroundings.
Give an example of an endothermic and exothermic reaction used in day to day life.
Endothermic - Cold packs Exothermic - Hand warmers
Describe an experiment to measure the maximum temperature reached using displacement reactions. (6) You will use magnesium and copper (II) sulfate solution in the experiment.
• Measure out 20cm3 of copper sulfate into a polystyrene cup. [1] • Use a thermometer to record temperature of copper sulfate solution. [1] • Add a small amount of magnesium. [1] • Record maximum temperature reached. [1] • Empty out and measure out 20cm3 of fresh copper sulfate. [1] • Repeat twice and calculate a mean. [1]
Explain an example of an exothermic reaction taking place in day to day life.
A self heating can is heated by a reaction that triggers when a button is pressed on the base of the can to start the reaction.
Define the activation energy.
Energy needed to start a reaction.
State the four main features of a reaction profile.
Reactants. Products. Energy Change. Activation Energy. (Check Figure 1 and 3 on page 116-117)
State which type of reaction bond breaking and bond making is.
Bond breaking - Endothermic Bond Making - Exothermic
What is the bond energy of a reactant?
The energy needed to break the bond between 2 atoms.
Using Table 1 on page 118, calculate the overall energy change of the equation below: 2H – H + 2O=O ——> 2 H – O – H State whether the reaction is exothermic or endothermic.
Break 2 x H – H (436) 2 x O=O (498) +1868 Make 4 x H – O (464) -1856 Change 1868 - 1856 = 12 The reaction is endothermic.
How do you recognise whether a reaction is endothermic or exothermic after completing a bond energy calculation?
If overall energy is negative = Exothermic If overall energy is positive = Endothermic
Give 3 things that determines the voltage of a battery.
• The electrolyte. • Metal in the battery. • Difference in reactivity between 2 metals.
What energy does a chemical cell and a fuel cell provide?
Electrical energy.
Give an advantage and a disadvantage of a H2 fuel cell.
Advantage - No recharge time. Disadvantage - Explosive.
What type of reaction happens in a fuel cell?
An electrochemical (redox) reaction.