Energy Changes Flashcards
Describe the Movement of Energy in Chemical Reactions
-Chemicals store a certain amount of energy- and different chemicals store different amounts.
-If the products of a reaction store more energy than the original reactants, then they must have taken in the difference in energy between the products and reactants from the surroundings during the reaction.
-But if they store less, then the excess energy was transferred to the surroundings during the reaction.
-The overall amount of energy doesn’t change. This is because energy is conserved in reactions - it can’t be created or destroyed.
Describe what is Meant by an Exothermic Reaction
-Exothermic reactions give out energy to the surroundings so the temperature of the surroundings increases.
-Examples include: combustion, neutralisation and oxidation reactions.
-Exothermic reactions also have everyday uses. For example, some hand warmers use the oxidation reaction of iron in air (with a salt solution catalyst) to release energy.
-Self heating cans of hot chocolate and coffee also rely on exothermic reactions between chemicals in their bases.
Describe what is Meant by an Endothermic Reaction
-Endothermic reactions take in energy from the surroundings so the temperature of the surroundings decreases.
-Examples include: thermal decomposition and the reaction between citric acid and sodium hydrogen carbonate.
-Endothermic reactions also have everyday uses. For example, they can be used in sprots injury packs- the chemical reaction allows the pack to become instantly cooler without having to be put in the freezer.
Describe how to Draw the Energy Profile for an Exothermic Reaction
-In an exothermic reaction, the line for the reactants should be higher than the line for the products.
-The overall energy change is an arrow facing downwards from the reactants to the products.
-The activation energy is a short arrow from the reactants to the top of the bump.
-The arrows on the energy profile should reach the entire distance that they are showing.
Describe how to Draw the Energy Profile for an Endothermic Reaction
-In an endothermic reaction, the line for the reactants should be lower than the line for the products.
-The overall energy change is an arrow facing upwards from the reactants to the products.
-The activation energy is a long arrow from the reactants to the top of the bump.
-The arrows on the energy profile should reach the entire distance that they are showing.
Describe a Method for the Temperature Changes Required Practical
-First, measure 30cm^3 of hydrochloric acid into a polystyrene cup. Stand the cup inside a beaker to make it stable.
-Next, use a thermometer to measure the temperature of the acid. Record this temperature in a results table.
-Use a measuring cylinder to add 5cm^3 of sodium hydroxide solution. Fit the lid and gently stir the solution through a hole in the lid.
-Look carefully at the temperature rise on the thermometer. When the reading on the thermometer stops changing, record the highest temperature reached into the table.
-Carry out the experiment again, increasing the volume of sodium hydroxide by 5cm^3 each time. Repeat until a maximum of 40cm^3 has been added.
-Then repeat the entire experiment and calculate mean values for the temperature increases.
Explain why a Measuring Cylinder is Better for measuring Volume than a Beaker in the Temperature Changes Required Practical
A beaker is not designed to measure volumes accurately. Scientists say that a beaker is not volumetric. However, a measuring cylinder is designed to measure volumes accurately.
Explain why it is Important to Str the Solution in the Temperature Changes Required Practical
Stirring the reaction mixes the chemicals. This ensures that all of the acid and alkali react so that we get an accurate temperature reading.
Explain why a Polystyrene Cup With a Lid is Used Instead of a Glass Beaker in the Temperature Changes Required Practical
-In this reaction, we are measuring a temperature change.
-We want to reduce heat losses as much as possible. We do not want heat to pass through the walls of the container as this would produce an inaccurate reading.
-Polystyrene is an insulator so this reduces any heat losses through the walls.
-Using a glass beaker would give an inaccurate reading as heat energy can pass through glass more easily than through polystyrene.
-Using a lid reduces heat loss to the atmosphere.
Describe the Conclusions from the Temperature Changes Required Practical
-As the volume of sodium hydroxide solution increases, the mean temperature increase also increases.
-However, if we add more than 30cm^3 of sodium hydroxide solution, the temperature increases by a smaller amount each time.
Explain why the Temperature Increase Falls After 30cm^3 of Sodium Hydroxide is Added in the Temperature Changes Required Practical
-Initially the sodium hydroxide is the limiting reactant. As we add more sodium hydroxide, we have more neutralisation reactions taking place. This reaction is exothermic so the temperature increase is greater.
-However, after we have added 30cm3 of sodium hydroxide solution, all of the hydrochloric acid has reacted.
-At this stage, as we add an increasing volume of sodium hydroxide solution, the energy released by the reaction is spread through a greater volume of liquid, so the temperature rise decreases.
Explain Whether Bond Breaking and Bond Forming is Endothermic or Exothermic
-During a chemical reaction, old bonds are broken and new bonds are formed.
-Energy must be supplied to break existing bonds so bond breaking is an endothermic process.
-Energy is released when new bods are formed so bond formation is an exothermic process.
Give the Calculation for Overall Energy Change
Overall Energy Change = Sum of Bonds Formed in Reactants - Sum of Bonds Formed in Products
Describe the Bond Energies in Exothermic Reactions
-In an exothermic reaction, the bond energy of the products is less than the bond energy of the reactants.
-Energy is transferred to the surroundings.
Describe the Bond Energies in Endothermic Reactions
-In an endothermic reaction, the bond energy of the products is greater than the bond energy of the reactants.
-Energy is transferred from the surroundings.
