Energy from burning.

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Presentation transcript:

Energy from burning

Success… In order to be successful in this lesson you must be able to: Learning Objectives: In order to be successful in this lesson you must be able to: Describe simple calorimetry experiments for reactions, such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes understand the use of ΔH to represent molar enthalpy change for exothermic and endothermic reactions Success…

Starter What are reactions that give out heat called? Can you think of how we can measure the heat given out?

Ethanol burns readily in air… But how can we tell how much heat is given off?

Burning reactions In a burning reaction it is difficult to measure the heat given out by simply sticking a thermometer in the air. Instead we allow the heat to warm something else up that we can easily measure. The simplest example is warming a beaker of water.

Calorimetry

Calorimetry You can find out how much energy is released or absorbed by a reaction in solution by taking the temperature of the liquid reagents, mixing everything together in a polystyrene cup and measuring the temperature of the solution at the end of the reaction. You can use the change in the temperature to work out the energy change for the reaction.

Measuring Energy Transfer The biggest problem with taking measurements like this is the amount of energy that’s lost to the surroundings. You can reduce it a bit by putting the polystyrene cup into a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation. This method works for reactions of solids with water (e.g. dissolving ammonium nitrate in water) as well as for reactions where you mix two solutions, like neutralisation reactions.

How does calorimetry work? 100 ml Cold water is measured into the beaker/can The starting temperature of the water is recorded The water is heated using the flame from the burning fuel The final temperature of the water is recorded The spirit burner containing the fuel is weighed before and after the experiment.

Recording results for three repeats Mass of spirit burner before (g) Mass of spirit burner after (g) Total mass of fuel burned (g) Mass of water in beaker (100ml = 100g) Initial temperature (°C) Final temperature (°C) Temperature change (°C) 100 20

Calculations The amount of energy transferred from the burning fuel to the water in the calorimeter can be calculated if you know: the mass of water heated the temperature rise

Calculations This is the equation you need: Use this equation to calculate the energy transferred for each of your experiments.

Energy per gram of fuel You can compare fuels by measuring the mass of fuel burned in the experiment. The best fuel is likely to release the most energy per gram of fuel. This is worked out using: Energy released (J/g of fuel) = Energy transferred to water (J) ÷ Mass of fuel burned (g)

Summary Follow a set of instructions to set up practical equipment safely and quickly Use the equipment provided to investigate the heat given out when a substance burns in air Use your experimental observations to calculate the energy given out per gram of fuel.