Energy in Reactions L.O: To understand how the energy released from different fuels can be measured Starter: do you remember burning biscuits from year.

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Presentation transcript:

Energy in Reactions L.O: To understand how the energy released from different fuels can be measured Starter: do you remember burning biscuits from year 7? What were we trying to find out and how did we do it?

How could I test which spirit burner is the best?

How much energy a fuel produces is measured in a bomb calorimeter This is a little tricky in the lab so a simple method is used

Measuring the energy from reactions The energy from a reaction can be worked out using the temperature rise in water Energy transferred (J) = Change in temp x mass of water x 4.2 (this is the specific heat capacity) Can you put this into a formula triangle? Different reactions have different bonds and numbers of bonds to form and so release different amounts of energy Energy in food is measured in the same way!

04/06/2016 Experimental values Step 1: Calculate the energy gained by the water: Energy gained by water = mass of water x 4.2 J/gC 0 x change in temperature Step 2: Divide this value by the mass of the alcohol used to find out the energy gained by the water per gram of alcohol Energy gained per gram = (answer to Step 1) / mass of alcohol burned

Fuel Experiment Using the balances weigh the biscuit Using the balances weigh the biscuit Measure out 10cm 3 of water and pour this into a test tube Measure out 10cm 3 of water and pour this into a test tube Attach this test tube to a clamp stand and place a thermometer inside Attach this test tube to a clamp stand and place a thermometer inside Using the tongs heat the biscuit until it catches fire, and then hold this biscuit under the test tube to heat the water Using the tongs heat the biscuit until it catches fire, and then hold this biscuit under the test tube to heat the water Record the temperature change of the water Record the temperature change of the water Now repeat this for the other biscuits (use a new test tube of water for each test) Now repeat this for the other biscuits (use a new test tube of water for each test) Calculate the energy transferred Calculate the energy transferred Can you calculate the energy per 100g? Can you calculate the energy per 100g? Energy gained by water = mass of water x 4.2J/gC 0 x change in temperature Q=mcΔT

What is in the biscuits? BiscuitEnergyProteincarbohydrateFat Nice2037kJ 485kcal 6.5g68g20.8g Cheddars2127kJ 509kcal 11.6g53.2g27.7g Digestive2040kJ 487kcal 6.9g62.8g23.1g Rich Tea1908kJ 453kcal 7.2g72.4g15g What makes one biscuit a better fuel than another?

In an experiment a burning fuel raised the temperature of 50cm 3 of water from 16°C to 41°C. How much energy was released by the fuel? (4.2J of energy will raise the temperature of 1g of water by 1 °C Write down the data: Mass of water heated: 50g Specific heat capacity of the water: 4.2 J/g°C Rise in temperature: 41°C - 16°C = 25°C Energy gained by water = mass of water x 4.2J/gC 0 x change in temperature Q=mcΔT Q = 50 x 4.2 x 25J Q= 5250J (5.25kJ) If the mass of the fuel burned was 0.2g energy released per gram= 5.25 ÷ 0.2

Can you work out the energy released per mole if the relative formula mass of the fuel is 46? Energy released per mole = (5.25 ÷0.2) x 46 = kJ/mol

04/06/2016 An example calculation While doing this experiment, Dave got the following results for methanol: Step 1: 100g x 4.18 J/gC 0 x 35 = J Step 2: / 1.45 = J/g Therefore energy given out by the alcohol = 10090J/g Mass of water Start temp End tempTemp diffStart massEnd massMass diff 100g

1. Compound A is burned in a bomb calorimeter that contains 2500g of water. If the combustion of 0.175g of this compound causes the temperature of the water to rise C, what is the energy transfer of combustion of compound A? The heat capacity of water is 4.2 J / g 0 C.

FuelMass of fuel burned (g) Volume of water (cm 3 ) Temperatu re change (°C) A B C Energy gained by water = mass of water x 4.2J/g°C x change in temperature Q=mcΔT 1. Calculate the energy released by each fuel 2.Calculate the energy released per gram 3.The relative formula mass of Fuel A is 48. What is the energy released of fuel A in kJ per mole? 1.A J (6.72kJ) B J (3.36kJ) C – 5670J (5.67kJ) 2.A – 28000J/g (28kJ/g) B – J/g (19.67kJ/g) C – 21000J/g (21kJ/g) kJ/mol

04/06/2016 Measuring Energy in Food The energy content in foods is measured in Joules and calories (1Cal = 4.2J). Foods with large carbohydrate and fat contents have lots of energy, but too much energy and too little exercise could lead to obesity.