Core Practical: temperature rise from combustion of alcohols

Run the practical, one step at a time

Click through the stages. At each one, ask yourself: what is this measurement FOR?

1. 1Risk assessment firstYou're about to use a lit alcohol burner, so complete a risk assessment before you do anything else. That's not optional.+
2. 2Weigh the burnerMeasure the initial mass of the alcohol burner. Later, you'll subtract the final mass from this to find how much alcohol burned.+
3. 3Position the burnerPlace the burner directly below the calorimeter. Every time you swap in a different alcohol, keep the distance between the wick and the calorimeter the same.+
4. 4Add the water, take its starting temperaturePour 50 cm³ of water into the calorimeter (the same volume every time), then measure its initial temperature.+
5. 5Light it, then stop at the riseLight the wick. When the water temperature has gone up by about 30 °C, put the flame out.+
6. 6Record the finalsWrite down the final temperature of the water and the final mass of the burner.+
7. 7Repeat with another alcoholPut a different alcohol in the burner and run it all again, keeping everything else the same. Now you've got data you can compare.+

The kit

tap the kit, then add the screens and lid ↓

Screens and a lid reduce (minimise) the heat lost to the surroundings.

Tap a part to see what it's for. Then switch to the second view to add the screens and lid.

Chemistry · Core practical

Change it, measure it, or keep it the same?

Put each quantity in the box that describes its job in this practical.

5 of 5 still to sort.

Every quantity in this practical has exactly one job. Decide which, then read why.

Predict, then check

Commit to an answer before you look. It's the surprising bit that sticks.

You heat 50 cm³ of water by about 30 °C using methanol, then repeat it with butanol. Which statement do you predict is true?

Chemistry · Reaction balancer

What actually happens when an alcohol burns

Plenty of oxygen means complete combustion: carbon dioxide and water. Set the multipliers until every atom count matches on both sides.

What you need to know

• Burning an alcohol is combustion: an exothermic reaction with oxygen that releases energy.
• Calorimetry gets round the measuring problem: burn alcohol to heat an exact volume of water, and see how much alcohol that takes.
• Here the water is heated through a set rise of about 30 °C, so the mass of alcohol burned is what gets compared.
• Have a goSam says: 'We stop at about 30 °C every time, so the temperature rise is what we're comparing between alcohols.' What is Sam missing?

  The rise is the same every time. What differs is the mass of alcohol burned.

  A quantity the method fixes can't be the thing that varies between alcohols, so the mass burned is what gets compared.

• The type of alcohol is the independent variable; wick distance, water volume and starting temperature are control variables.
• Weigh the burner and take the water's temperature before and after. Forgetting the starting values is a common mistake.
• Mass burned is initial minus final burner mass; temperature change is final minus initial; divide mass by change for the mass per 1 °C.
• Have a goYour burner reads 70.0 g before and 68.2 g after, and the water rose by 30 °C. How much alcohol was needed per 1 °C?

  0.06 g per °C

  Mass burned is 70.0 − 68.2 = 1.8 g; divide by the 30 °C change. Dividing the other way up gives degrees per gram.

• If a lower mass of alcohol causes the same rise, that alcohol is releasing more energy.
• Have a goPriya burned more fuel in burner Z than in burner Y for the same rise, and says 'so Z released more energy'. Convince her otherwise.

  Z needed more fuel for the same job, so it released less energy.

  For the same rise, a lower mass means more energy released. A bigger mass burned is not a bigger energy output.

• As the number of carbon atoms increases, the mass needed decreases and the energy released increases.
• Heat lost to the surroundings is a major error, so results are often lower than theory; screens and a lid reduce it.
• Have a goA friend says: 'With draught screens and a lid on, no heat is lost.' Fix the sentence: screens and a lid ____ the heat lost.

  reduce (minimise)

  They cut the loss but can't remove it, which is why values are often still lower than theoretical ones.

• Complete combustion, in plenty of oxygen, gives carbon dioxide and water.

The big picture

To compare the energy released when different alcohols burn, you use the burning alcohol to heat a fixed volume of water through a set rise of about 30 °C, then work out how much alcohol that took. Less alcohol for the same rise means more energy released. Heat lost to the air is the big source of error.

⚠ Watch out

Thinking a bigger mass burned means more energy. It's the other way round: for the same temperature rise, a lower mass means the alcohol is releasing more energy.

Memory hook

Same job, less fuel, more energy. The longest chain does the job with the least.

Check yourself

Cover the page and explain to an imaginary friend why the temperature rise is fixed and the mass is compared, then say what 'less mass for the same rise' tells you.