C3.2 - Energetics
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What is an exothermic reaction?
• A reaction in which there is an overall transfer of energy to the surroundings. • The energy transferred to the surroundings when new bonds form is more than the energy taken in from the surroundings to break the bonds.
What is an endothermic reaction?
• A reaction is which there is an overall transfer of energy from the surroundings, which cool down. • The energy transferred to the surroundings when new bonds form is less than the energy taken in from the surroundings to break the bonds.
What is the energy change of a reaction?
• The difference between the energy transferred from the surroundings to break bonds in the reactants, and the energy transferred from products when bonds form. • Energy change = Energy transferred to break bonds - Energy transferred to make bonds.
What are the steps to find out if a reaction is an endothermic reaction or an exothermic reaction?
• Measure the start temperature of a solution in an insulated container. • Add the other reactant (reacting substance) and stir. • Measure the end temperature of the mixture. • Calculate the difference between the start temperature and the end temperature. • If the temperature has increased then the reaction is exothermic and vice versa.
What would you need to check if both reactants are solutions in an investigating temperature change practical?
If both temperatures are solutions, you may need to check that their temperatures are the same before mixing them.
What is bond energy?
• The energy needed to be transferred to break 1 mol of a particular bond (kJ/mol). • Also the amount of energy transferred to the surroundings when 1 mol of these bonds form.
Why might the calculated energy change for a reaction be different from the value found by experiment?
Bond energies are usually mean values for the same bonds in different substances.
What are the surroundings in a chemical reaction?
Anything that is not a reacting particle.
When is energy transferred in a chemical reaction?
• From the surroundings to break bonds in the reactants to form separate atoms. • To the surroundings from the reacting particles when bonds form between atoms.
What is a reaction profile?
A chart that shows the energy involved in a chemical reaction.
What is activation energy?
• Energy must be transferred to the reacting particles to start most chemical reactions. • Activation energy is the minimum energy needed for a reaction to start. • It is often provided by a flame or heating, and it breaks bonds the in reactants.
Why do acid and alkalis react as soon as you mix them, without heating?
The reactions activation energy is so low that the reactants already have enough energy to react and just need to be mixed together.
Why do acid and alkalis react as soon as you mix them, without heating?
The reactions activation energy is so low that the reactants already have enough energy to react and just need to be mixed together.
Draw a reaction profile for an endothermic reaction.
Product line higher than reactants line.
Draw a reaction profile for an exothermic reaction?
Product line lower than the reactant line.