C3.3 Calculating and explaining energy change

What does exothermic mean?

An exothermic reaction is one which gives out energy to the surroundings. Usually in the form of heat and usually shown by a rise in temperature.

What does a catalyst do to the activation energy?

A catalyst provides a different pathway for a reaction that has a lower activation energy (so the reaction happens more easily and more quickly). This is represented by the lower curve on the diagram showing a lower activation energy. The overall energy change for the reaction, ∆H, remains the same though.

What is a fuel cell?

A fuel cell is an electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to generate electricity. Hydrogen can be used in a hydrogen-oxygen fuel cell. Fuel cells were developed in the 1960s as part of the space programme, to provide electrical power on spacecraft - they were more practical than solar cells and safer than nuclear power. Unlike a battery, a fuel cell doesn't run down or need recharging from the mains, it'll produce energy in the form of electricity and heat as long as fuel is supplied.

What does endothermic mean?

An endothermic reaction is one which takes in energy from the surroundings. Usually in the form of heat and usually shown by a fall in temperature.

What is calorimetry?

Different fuels produce different amounts of energy. To measure the amount of energy released when a fuel is burnt, you can simply burn the fuel and use the flame to heat up some water. Calorimetry uses a glass or metal container (it's usually made of copper because it conducts heat well).

When is energy supplied and released?

Energy must always be supplied to break bonds and energy is always released when bonds form.

What are the consequences of fuel providing energy?

Fuels release energy which we use in loads of ways - e.g to generate electricity and to power cars. Burning fuels has various effects on the environment. For example, burning fossil fuels releases CO₂, a greenhouse gas which causes global warming and other types of climate change. It'll be expensive to sow down these effects, and to put things right. Developing alternative energy sources (e.g. tidal power) costs money. Crude oil is running out. We use lots of fuels made from crude oil (e.g. petrol and diesel) and as it runs out it will get more expensive. This means that everything that's transported by lorry, train or plane gets more expensive too. So the price of crude oil has a big economic effect.

What are the pros and cons of burning hydrogen gas to power vehicles?

Hydrogen gas can be burnt in oxygen as a fuel in the combustion engines of vehicles. Pros: Hydrogen combines with oxygen in the air to form just water - so it's very clean. hydrogen + oxygen → water Cons: You need a special, expensive engine. Although hydrogen can be made from water, which there's plenty of, you still need to use energy from another source to make it. Also, hydrogen's hard to store safely - it's very explosive.

What does an endothermic energy level diagram look like?

In endothermic reactions ∆H is +ve. This shows an endothermic reaction because the products are at a higher energy than the reactants. The difference in height represents the energy taken in during the reaction (per mole).

What does an exothermic energy level diagram look like?

In exothermic reactions ∆H is -ve. (∆H is energy change) This shows an exothermic reaction - the products are at a low energy than the reactants. The difference in height represents the energy given out in the reaction (per mole).

What's the problem with energy measurements and how do you reduce it?

Lots of energy is lost to the surroundings. Put the polystyrene cup in a beaker of cotton wool to give more insulation, and putting a lid on the cup to reduce energy lost by evaporation.

What happens 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 bonds are formed - so bond formation is an exothermic process.

What's the method for calorimetry?

Put 50 g of water in the copper can and record its temperature. Weigh the spirit burner and lid. Put the spirit burner underneath the can, and light the wick. Heat the water, stirring constantly, until the temperature reaches about 50°C. Pu out the flame using the burner lid, and measure the final temperature of the water. Weigh the spirit burner and lid again.

What's the formula for energy transferred?

Q = mc∆T energy transferred = mass of water (g) x specific heat capacity of water (4.2) x temperature change (°C)

Why is the car industry developing fuel cells?

The car industry is developing fuel cells to replace conventional petrol/diesel engines. Fuel cell vehicles don't produce any conventional pollutants - no greenhouse gases, no nitrogen oxides, no sulfur dioxide, no carbon monoxide. The only by-products are water and heat. This would be a major advantage in cities, where air pollution from traffic is a big problem. Fuel cells could eventually help countries to become less dependent on crude oil. However, they're not likely to mean the end of either conventional power stations or our dependence on fossil fuels. That's because: ∙hydrogen is a gas so it takes up loads more space to store than liquid fuels like petrol. ∙it's very explosive so it's difficult to store safely. ∙the hydrogen fuel is often made either from hydrocarbons (from fossil fuels) or by electrolysis of water, which uses electricity (and that electricity's got to be generated somehow - usually this involves fossil fuels).

What happens in exothermic reactions?

The energy released in bond formation is greater than the energy used in breaking old bonds.

What happens in endothermic reactions?

The energy required to break old bonds is greater than the energy released when new bonds are formed.

What is activation energy?

The initial rise in the line represents the energy needed to break the old bonds. This is the activation energy. Activation energy represents the minimum energy needed by reacting particles to break their bonds.

What happens when hydrogen and oxygen react?

They react to produce water - which isn't a pollutant. The reaction between hydrogen and oxygen is exothermic - it releases energy. You can get energy by reacting hydrogen and oxygen - either in combustion engine or in a fuel cell.

How do you measure energy transfer?

You can measure the amount of energy released by a chemical reaction (in solution) by taking the temperature of the reagents (making sure they're the same), mixing them in a polystyrene cup and measuring the temperature of the solution at the end of the reaction. The method works for reactions of solids with water (e.g. dissolving ammonium nitrate in water) as well as for neutralisation reactions. The method can also be used where energy is absorbed - the temperature will fall. Example: Place 25 cm³ of dilute hydrochloric acid in a polystyrene cup and record the temperature. Put 25 cm³ of sodium hydroxide in a measuring cylinder and record its temperature. As long as the temperatures are the same add the alkali to the acid and stir. Take the temperature of the mixture every 30 seconds and record the highest temperature it reaches.