Energy Changes Flashcards
(13 cards)
What are the basic rules?
- Energy can’t be created or destroyed in a chemical reaction
- Energy is transferred to or from surroundings in a chemical reaction
What is an exothermic reaction?
Exothermic reactions transfer energy from the reacting chemicals to their surroundings
The energy transferred often heats up the surroundings and you can measure the rise in temperature
Examples include burning fossil fuels eg methane burns, is oxidised and releases energy
Neutralisation between acids and alkalis is exothermic
The products of exothermic reactions have less energy content than the reactants.
What is an endothermic reaction?
Endothermic reactions transfer energy from the surroundings to the reacting chemicals
The products have more energy than the reactants gained from the surroundings
They cause a fall in temperature as they happen.
They are less common than exothermic reactions
Examples citric acid and sodium hydrogen carbonate
Thermal decomposition is endothermic eg heating calcium carbonate forms calcium oxide and carbon dioxide but you must keep heating the calcium carbonate.
The products have higher energy content than the reactants so energy is taken from the surroundings.
Sherbet dissolves in the water in your mouth taking in energy and cooling your mouth
Types of warming up - exothermic
Work once eg Hand warmers - oxidation of iron with sodium chloride catalyst.
Reused - based on crystal formation from salt solutions. Then allow cooling. To reuse, reheat and this redissolves the crystals.
Exothermic reactions can be self heating cans that make hot drinks
Calcium oxide + water -> calcium hydroxide
Problems include not heating enough and too much reactant needed
Cooling down - endothermic
Ammonium nitrate dissolves taking energy from surroundings making them colder
Good for sports injuries as ice packs reduce swelling from injuries
Ammonium nitrate and water are kept separate - break seal and they combine
Only work once
Other uses - chill cans for drinks
Outline an experiment to investigate temperature change
Acid + metal
Hydrochloric acid + magnesium
- Polystyrene cup and thermometer
- Polystyrene is good insulator do minimal energy loss to environment - improve further with plastic (polystyrene) lid
- Record initial temperature of acid
- Measure temperature every 30 seconds
- If increases it is endothermic
- If decreases it is exothermic
Explain independent variable, dependent variable and control variable when different metals react with hydrochloric acid?
Independent variable is the one change eg type of metal
Dependent variable is the factor that is measured eg temperature
Control variable is kept the same eg volume, concentration of acids and surface area of metals
What is bond energy?
The energy needed to break the bond between 2 atoms
Measured in kJ/mol
How do you work out the energy change for a chemical reaction?
- How much energy is needed to break chemical bonds in the reactants
- How much energy is released when new bonds are formed
When is bond energy exothermic and when is it endothermic?
If exothermic more energy is given out making bonds than breaking bonds (-) minus/negative number
If endothermic more energy put into breaking than making bonds (+) plus/positive number
Energy break - energy make = energy change
Examples of exothermic reactions
Neutralisation
Combustion
Displacement
What is a use of an exothermic reaction and an endothermic reaction?
Hand warmers - exothermic
Ice packs - endothermic
How do we calculate bond energy in a reaction?
Energy change = energy to break to break - energy to make bonds
Negative answer = exothermic
Positive answer= endothermic
