What is the term given to the amount of heat needed to raise the temperature of 1 unit of mass by 10c?

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COMBUSTIONCombustion reactions require 3 things- fuel, oxygen and energy.Combustion reactions are exothermic reactions in which the reactant combines with oxygen to produce an oxide. This is an oxidation reaction.Example: C3H8 (g) + 5O2(g) --> 3CO2 (g) + 4H2O (g) ΔH = -2220 kJ mol -1

COMPLETE AND INCOMPLETE COMBUSTION

The complete combustion of hydrocarbons occurs when there is sufficient oxygen for the fuel to burn.The products of complete combustion are carbon dioxide and water.Eg. CH4 (g) + 2O2(g) --> CO2 (g) + 2H2O (g)When oxygen is not plentiful, incomplete combustion occurs.The products of incomplete combustion are carbon monoxide or carbon and water.Eg. 2CH4 (g) + 3O2 (g) --> 2CO (g) + 4H2O (g)Or CH4 (g) + O2 (g) --> C (s) + 2H2O (g)

HEAT OF COMBUSTION

•The heat of combustion of a substance is the energy released when a specified amount (eg. 1 mol, 1g, 1 L) of the substance burns completely in oxygen.•The heat of combustion is usually measured at conditions 298K (25 C) and 101.3kPa. Water will be a liquid under these conditions.•Heats of combustion are measured using a calorimeter.•Energy released= n x ΔHc

SPECIFIC HEAT CAPACITY

•The amount of energy required to raise the temperature of one gram of a substance by 1°C is called the specific heat capacity of that substance.•The higher the specific heat capacity, the more effectively a material will store heat energy.Water has a very high heat capacity, which is a consequence of the hydrogen bonding between its molecules•Heat change gained or lost by a substance during a chemical reaction can be calculated by:

q = m x C x ΔT

where q is Energy (J); m is Mass (g); C is specific heat capacity (Jg-1oC-1); T is temperature (oC), Δ means change in.

Determining heat of combustion

•During combustion chemical energy is converted to thermal energy. The thermal energy released by a quantity of fuel can be used to heat a measured volume of water.•The change in the water temperature can by used to determine the approximate amount of energy released as fuel.

Example:

0.355g of methanol (CH3OH) undergoes complete combustion in a spirit burner. The heat energy released is used to heat 100mL of water. The temperature of the water rose from 20.24 oC to 37.65 oC. Calculate the heat of combustion of methanol in kJ/mol.Mass of water (m) = 100gC (water) = 4.18 J/goCΔT= 37.65- 20.24 = 17.41 oCUse q= mxCx ΔT = 100 x 4.18 x 17.41 = 7277 J = 7.277kJFind the mole of methanol, n(CH3OH) = 0.355/32.0 = 0.0111 mol.Heat of combustion = -7.277/0.0111 = -656 kJ/mol.

Heat of combustion:
Example:Calculate the amount of energy released when 3.60 kg of butane (C4H10) is burnt in an unlimited supply of oxygen.Unlimited supply of oxygen- complete combustion.n(C4H10) = m/M= 3.60 x 103 /58.0 = 62.1 mol.Energy = n x ΔHc = 62.1 x -2886 = -1.79x105 kJ. (negative since exothermic)The energy content is often expressed in units of kilojoules per gram or mega joules per tonne (106) if the quantity of fuel is great.To work this out- ΔHc /M heat of combustion/molar mass

Example:

For ethanol- heat of combustion = -1367 kJ/molMolar mass= 46.0 g/molHeat of combustion per gram = -1367/46.0 = -29.7 kJ/g

Specific heat example:

•Calculate the energy required to heat 120mL of water for a cup of coffee to boiling point if the initial water temperatuer is 20.0°C.Since the density of water is 1g mL, the mass of 120mL is 120g. q = m x c x ΔTEnergy required to raise temperature of 120g of water by 1 degree = SHC x mass 4.184 x 120 = 502.0JSince temperature rises by 80 degrees, the total energy required = J x ΔT= 502.0 x 80 = 40160J = 40.2kJ

Steps to writing thermochemical reactions:

1.Write a combustion reaction.2.Balance the equation.3.Obtain the heat of combustion from a table of values.4.Using your balanced equation look at the coefficient in front of your fuel - multiply ΔH by this value. 5.Write the value for ΔH at the end of your equation.

Aus-e-tute Heat of reaction

Heat

Heat	Heat Capacity	Specific Heat
Latent Heat	Kinetic Molecular Theory

Heat

Heat is a way of transferring energy between a system and its surroundings that often, but not always, changes the temperature of the system. Heat is not conserved, it can be either created or destroyed. In the metric system, heat is measured in units of calories, which are defined as the amount of heat required to raise the temperature of one gram of water from 14.5oC to 15.5oC.

In the SI system, the unit of heat is the joule.

Heat Capacity

The heat capacity of a substance is the amount of heat required to raise the temperature of a defined amount of pure substances by one degree (Celsius or Kelvin). The calorie was defined so that the heat capacity of water was equal to one.

Specific Heat

The specific heat of a substance is the number of calories needed to raise the temperature of one gram by 1oC. Because one degree on the Celsius scale is equal to one Kelvin, specific heats in the metric system can be reported in units of either cal/g-oC or cal/g-K. The units of specific heat in the SI system are J/g-K. Because there are 4.184 joules in a calorie, the specific heat of water is 4.184 J/g-K.

The ease with which a substance gains or loses heat can also be described in terms of its molar heat capacity, which is the heat required to raise the temperature of one mole of the substance by either 1oC or 1 K. In the metric system, the units of molar heat capacities are therefore either cal/mol-oC or cal/mol-K. In the SI system the units of molar heat capacities are J/mol-K.

Latent Heat

When ice is heated, the heat that initially enters the system is used to melt the ice. As the ice melts the temperature remains constant at 0oC. The amount of heat required to melt the icehas historically been called the latent heat of fusion. Once the ice has melted, the temperature of the water slowly increases from 0oC to 100oC. But once the water starts to boil, the heat that enters the sample is used to convert the liquid into a gas and the temperature of the sample remains constant until the liquid evaporates. The amount of heat required to boil, or vaporize, the liquid has historically been called the latent heat of vaporization.

More than 200 years ago, Joseph Black distinguished between sensible heat and latent heat. Heat that raises the temperature of the system can be sensed, but heat that results in a change in the state of the system from solid to liquid or from liquid to gas is latent. Like the latent image on photograph film that hasn't been developed or latent fingerprints that can't be seen with the naked eye, latent heat is heat that enters the system without changing the temperature of the system.

Heat and The Kinetic Molecular Theory

The system is the small portion of the universe in which we are interested, such as the water in a beaker or a gas trapped in a piston and cylinder, as shown in the figures below. The surroundings are everything elsein other words, the rest of the universe.

The system and its surroundings are separated by a boundary. Heat is transferred across the boundary between a system and its surroundings.

One of the fundamental principles of the kinetic theory is the assumption that the average kinetic energy of a collection of gas particles depends on the temperature of the gas and nothing else. A gas becomes warmer if and only if the average kinetic energy of the gas particles increases. Heat, when it raises the temperature of a system, produces an increase in the speed with which the particles of the system move, as shown in the figure below.